# DIY PAR Meter - 2015 Version



## Hoppy (Dec 24, 2005)

For the past 2+ months I have been trying to develop a better design for my PAR meters, made from lux meters. My goal was to make one that could give a reasonably accurate reading of the PAR from a LED light that uses lots of 660 nm LEDs, plus perhaps some actinic or near actinic LEDs. This, because I bought a Finnex Planted Plus light, which has lots of red LEDs, and the Apogee PAR meter spectral response is lacking in the 660 nm area.

I started by abandoning the Excelitas photodiode I have been using, since it also has very little response in the 660 nm area. Instead I decided to use the bare photodiode from the luxmeter I am modifying, minus its green filter, which leaves it as just a basic silicon photodiode, with most of its response in the IR area. To do that I first needed an IR/UV blocking filter, small enough to fit inside my sensor. Luckily Ebay had just that available, in 9.5 mm diameter!

After about 10 unsuccessful designs I think I have a good one! The problems I had were:
A different calibration was needed for each type of light.
The sensor failed to work under water.
The "cosine diffuser" was ineffective.
Light was leaking through the sensor housing.
Calibration by physical dimensional changes became too tedious.
Great difficulty finding a filter combination that would give good spectral response in both the violet and near IR areas.

With lots of good help from the folks here I solved the failure to work under water. Epoxy just doesn't work as a diffuser when submerged in water.
Switching to a black housing eliminated the light leakage problem.
With more help from the folks here I found out how to adjust the readout module to do the final calibration.
After hours and hours of trial and error I found a two filter combination that gives me adequate near UV and near IR spectral response. With that combination a calibration with a 10000K bulb also works for a 50-50 bulb and my LED light.

The cosine diffuser remains the unsolved problem, but that diffuser is only essential if the light meter is used outdoors in the sun, where the angle from the light source to the sensor varies widely throughout the day. In an aquarium the light always comes from above in a "cone" that is about 90 degrees total angle, or close enough.

The last test I did today first discouraged me: the calibration with a 10000K PC bulb didn't match the calibration with a 50-50 bulb anywhere near accurately. I spent a couple of hours on google trying to figure out how that could be. And, I found that for several years it has been known that the Apogee meter doesn't read correctly in the "electric" mode for actinic lighting. Instead, reef keepers use the "sunlight" mode to get acceptable readings. So, I used the "sunlight" mode and repeated the 50-50 calibration - exactly the same calibration as with the 10000K bulb, in the "electric" mode. Comparing the spectral response of the Apogee meter to my meter I can see why mine can do better with actinic light in the mix - I get better near UV response than the Apogee meter.

Here is the calculated spectral response for the "final" version:









Tomorrow I plan to verify again that if I limit the sunlight outdoors to about a 90 degree cone it will read close enough to be useful. And, I will do some more calibrating with my Finnex light.

Assuming that is successful I plan to start ordering parts to make about 20 or so of these.


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## ltb420 (Mar 20, 2012)

Glad to hear your still chugging along with this! I have been following along for quite a bit of time.


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## imott (Sep 27, 2014)

I am very interested in one of these. Do you have an estimated price point?


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## jeffkrol (Jun 5, 2013)

You have more patience than me..


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## kman (Dec 11, 2013)

Tagged... and definitely interested!


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## dragam21 (Mar 23, 2014)

Tagged and also very interested in one :hihi:


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## burr740 (Feb 19, 2014)

In for one!


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## brooksie321 (Jul 19, 2014)

This is like "raiders of the lost arc" for leds!! I'm in!!


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## jrill (Nov 20, 2013)

I am curious what the par reading was comparing the original diy meter based on the lux meter sold on eBay and this new model. Especially using your new planted plus as an example. What was the reading with the old one and what is it with the new one.


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## Dead2fall (Jun 4, 2014)

Hoppy: how did your planted plus numbers compare to the finnex released fugeray numbers? 

I'd also be interested in purchasing one when ready if available.


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## Hoppy (Dec 24, 2005)

Back to the drawing board:icon_frow This morning I set up the Planted Plus so I could make more accurate readings from it at a distance of about 15 inches. My PAR meter reads about 66% of the Apogee meter reading - bad! So, I set the light back on the tank and read the PAR at the substrate. My meter read 63% of the Apogee reading. I tried again with the light moved so I could get good readings in air, and put a toilet paper roll section over each meter to limit any light coming from other than directly over it. My meter read 69% of the Apogee meter. Next I tried a blue sky, but shade reading outdoors. My meter read 73% of the Apogee reading.

I figured my setting of my PAR meter might have shifted, so I again measured the PAR from a 50-50 bulb vs. the sunshine setting on the Apogee. The two readings were still identical.

My meter just isn't reading correctly when there is lots of red in the spectrum. Not good enough, at all:icon_frow

I still have enough parts to try other things, but for now I am totally stalemated, and if I can't get it to work better I will drop the idea for now. Sorry, I was so sure it was finally right last night:icon_frow


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## jeffkrol (Jun 5, 2013)

> Tavg >95% @440-620nm
> T =50% @648+-15nm
> Tavg <= 1 @ 700-1000nm
> T： < 1％ @ 1050nm


Those cut filters have too much falloff in the red region.. 633-664 
The filter is also heavily directional.. Any off axis light will be selectively sampled..
Only way to correctly account for this is a fairly deep "well"..


http://rocoes.com.tw/2008e/optical/ircut.htm




> 1. IR cut filters with 36~48 thin film layers which vacuum-deposited on substrate.
> 2. IR cut filters are designed with 0°of incidence angle, the larger incidence angles shift the characteristics to long wave side


it will always be more of a "spot meter"...










comes in 3-150mm sizes..

Rocolax 25mm is the cheapest, smallest and best I can find..You know I like the Baader..  
http://www.uvircut.com/25mm-optical-uvir-cut-filter-for-camera-lens-p-329.html


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## Hoppy (Dec 24, 2005)

Interesting information on IR blocking filters. Maybe by using a larger aperture, with a longer distance between filter and diode, it might work better. But, probably at a price.


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## Hoppy (Dec 24, 2005)

I have not been able to get acceptable accuracy, compared to the Apogee PAR meter, for either sunlight or Finnex Planted Plus light. So, I'm going to drop this effort for now, perhaps until I think of a different, and better approach. Sorry!


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## mistergreen (Dec 9, 2006)

Success comes from failure. I guess you need a good photodiode.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> Success comes from failure. I guess you need a good photodiode.


state of the art..

http://www.led-measurement.com/BTS256E-LED-Luxmeter/

If it can do LUX it can do PAR..



> *Accurate Measurements with Bi-Technology Sensor *
> The BTS256-E meter is equipped with Gigahertz-Optik´s BTS256 Bi-Technology light sensor which includes both a photometric photodiode and diode array. Any spectral mismatch error of the photopic sensor is compensated on-line using the diode array's measured spectral data. This reduces measurement uncertainty when evaluating any type of light source.
> *Photometrically Corrected Filtered Photodiode Light Detector*
> The traditional light detector employing a silicon diode and a photometric correction filter still provides best dynamic range, linearity and response time. But it cannot perform color measurements and cannot actively adjust to the target light source's spectral function. This is why the Gigahertz Optik Bi-tech sensor is equipped with a second diode array detector that works with the filtered photodiode detector.
> ...


and:
http://www.adorama.com/SKC700R.html?gclid=CMOo8JGZg8QCFY-DaQodJSAAVg

The super blue are almost perfect "as is"....
http://www.osioptoelectronics.com/s...se-photodiodes/blue-enhanced-photodiodes.aspx


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## mistergreen (Dec 9, 2006)

this photodiode looks promising if you can find it cheap
http://www.everlight.com/file/ProductFile/201407061524148948.pdf
Digikey is asking you to buy 1000 of it.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> this photodiode looks promising if you can find it cheap
> http://www.everlight.com/file/ProductFile/201407061524148948.pdf
> Digikey is asking you to buy 1000 of it.


no it still has that horrible cutoff at 650nm..









Much better:











plenty of the NOT sb's
http://www.surplussalespa.com/catal...0_371&osCsid=ba4764262a353b82974843d357bc27b9
fun w/ scrap:
Not blue enhanced..though
http://download.siliconexpert.com/pdfs/2012/6/25/8/43/9/51/tt_/manual/opr2101.pdf
https://www.verical.com/pd/optek-te...R2100-829762?gclid=CKaZmIm6g8QCFUpp7AodZRcAIQ

module:









http://www.gamma-sci.com/products/silicon-sensor/

One more:
30-82% 400-700nm
Osram BPW34-B
$11.39 flee bay..
http://www.digikey.com/product-deta...2057691-VQ2-g-VQ6-53963505795-VQ15-1t1-VQ16-c
http://www.osram-os.com/Graphics/XPic2/00131269_0.pdf
not blue enhanced, low sensitivity but fairly gentle slope from 400-700
Hamamatsu s1226 series.
Really funny caption (Chinese copy?)
http://en.agreetao.com/taobao/view/id/38700601883

These are about the best I can find that are current
http://www.mouser.com/ds/2/427/bpw20rf-109604.pdf
http://www.mouser.com/ProductDetail...id=126304432&gclid=COOqpsvJg8QCFQho7AodNhcAZQ

Better but can't find:

vishayT1116P


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## Hoppy (Dec 24, 2005)

Those are interesting photodiodes, but the perfect spectral response is equal sensitivity from 400 to 700, not a slope from 400-700, and to be useful it also needs a IR/UV blocking filter with a sharp cutoff at 400 and 700 (perhaps 410 and 690). With anything but the flat response different light sources give different readings for the same actual PAR value. I haven't yet found a gel filter combination that changes a sloping spectral response to a flat one. Of course it is possible to get such a filter, but the cost is way beyond reasonable for a DIY PAR meter. Even the IR/UV cutoff filter costs too much for a DIY meter. Remember you can get a Apogee sensor for about $150, so a DIY sensor should cost closer to $50 than $100.

As far as I can tell, all of the non-LiCor PAR meters use a simple silicon photodiode, probably a cheap one since they all have very near the same spectral response. That seems to be because there is no significantly better option. So, the whole problem basically comes down to finding the appropriate filters at a low enough cost to make it economical.

Even if that problem is solvable, another problem shows up as soon as you compete with the Apogee meter for accuracy. You have to be able to calibrate the finished product, without just comparing it to the Apogee meter. If you are lucky enough to have ready access to a LiCor PAR meter that will work fine, but I most certainly don't have that access. The Apogee PAR meter is not perfect, and it needs correction factors for most accuracy, a different factor for each light source. But, there are a huge number of LED combinations with varying spectrum outputs, so using an Apogee PAR meter with them doesn't work.

Some problems just don't have solutions.


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## jeffkrol (Jun 5, 2013)

Hoppy said:


> Those are interesting photodiodes, but the perfect spectral response is equal sensitivity from 400 to 700, not a slope from 400-700, and to be useful it also needs a IR/UV blocking filter with a sharp cutoff at 400 and 700 (perhaps 410 and 690). With anything but the flat response different light sources give different readings for the same actual PAR value. I haven't yet found a gel filter combination that changes a sloping spectral response to a flat one. Of course it is possible to get such a filter, but the cost is way beyond reasonable for a DIY PAR meter. Even the IR/UV cutoff filter costs too much for a DIY meter. Remember you can get a Apogee sensor for about $150, so a DIY sensor should cost closer to $50 than $100.
> 
> As far as I can tell, all of the non-LiCor PAR meters use a simple silicon photodiode, probably a cheap one since they all have very near the same spectral response. That seems to be because there is no significantly better option. So, the whole problem basically comes down to finding the appropriate filters at a low enough cost to make it economical.
> 
> ...



let's reassess this a bit.. 
first the conflicting Li-Cor info Flat or:


















Second the Li-Cor construction:









Accurate measurements are obtained under all natural and artificial lighting conditions because of the computer-tailored spectral response of the LI-190SA.
Colored glass filters are used to tailor the silicon photodiode response to the desired quantum response (Figure 1). An interference filter provides a sharp cutoff at 700 nm, which is critical for measurements
under vegetation where the ratio of infrared to visible light may be high. A small response in the infrared region can cause an appreciable measurement error. This sensor, developed from earlier work (3),
was pioneered by LI-COR and has become the standard for PPFD measurement in most photosynthesis-related studies.

The Apogee "problem"









Lastly "blue enhanced" photo-diodes are hard to find..
I suspect they are used by Li-Cor and not by Apogee..(just a guess)
but the point is most of the "tailoring" is filters and the filter choice by diode type.

Point is to try to make a sensor better than Apogee but possibly not quite as good as Li-Cor for about $100 or less. 

A better IR cut filter and any si photodiode would already have an edge up on the Apogee (I'm actually surprised at how poor it is)

The UV/IR cut filters are not "that" expensive and can be found cheap IF you aren't concerned w/ size as much. I suspect the apogee uses an IR cut like the ones you got.. cutoff is about the same (650)

this states 88% at 680..
http://laserstar.en.alibaba.com/product/1814026224-209612289/optical_glass_700nm_ir_cut_filter.html
That and this and your 70% done.. 









Add this and I believe you are 90% there:
Cinegel #3203: Three-Quarter Blue (3/4 CTB)


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## Hoppy (Dec 24, 2005)

Assuming you could find a 680 nm cut off filter that cost a few dollars each, and used that #3203 filter, you would have a good looking response curve above about 460 nm, but below that the response would be very low. Assuming you found a way around that, then you might have a better PAR meter than the Apogee meter, but how would you calibrate it short of getting a Li-Cor PAR meter? Without the calibration it wouldn't be better than the Apogee meter.


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## jeffkrol (Jun 5, 2013)

Hoppy said:


> Assuming you could find a 680 nm cut off filter that cost a few dollars each, and used that #3203 filter, you would have a good looking response curve above about 460 nm, but below that the response would be very low. Assuming you found a way around that, then you might have a better PAR meter than the Apogee meter, but how would you calibrate it short of getting a Li-Cor PAR meter? Without the calibration it wouldn't be better than the Apogee meter.


Calibration is always a problem.. and you would have to make a custom amplifier (actually rather simple) to boost the now flat signal.. 

Using a light that fits in the sweet spot of the Apogee and calibrate from there..

The Appogee is flat from 470 to 650 (w/ a slight over shoot) so you would need to find a light source w/ this range.. 
A green bulb would work.. 
Actually this is where colored LED's come in handy. tight spectrum's
A 590 and 565nm LED pair may work "good enough"










After that any non matching numbers would be "normal" since the new sensor is more accurate..
There also is the calibrated light source and a diffraction grating.. 
Using an Apogee when in fact your trying to improve is a bit counter productive. The point being they shouldn't match except w light in that greenish yellow range.

IF this chart is accurate.. a RGB LED strip is a fairly good calibration source..(gray trace)









Though I might shut off the blue:


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## mistergreen (Dec 9, 2006)

I have this question, if a light source is so specific in its spectrum say only 550 nm, is it really PAR? It has no benefit for plants who needs 400nm-700nm but mostly blue AND red.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> I have this question, if a light source is so specific in its spectrum say only 550 nm, is it really PAR? It has no benefit for plants who needs 400nm-700nm but mostly blue AND red.


It is only for calibration purposes... Using a source that in theory should register equal to the Apogee and the home made. Eliminating as many error points as possible..


Once that hurdle is accomplished the harder task is measuring overall sensitivity and spectrum. There is no easy nor cheap way to do this..

You would need a bulb that is plotted by spectrum, split it using a diffraction grating and running the sensor through the spectrum recording output..Then of course plotting output and matching to the source..

Once the curves show a fit then "height" needs to be calibrated..

In my above case you start w/ "height" based on an Apogee standard.

Down and dirty you could use say a 420nm LED.. Based on the Apogee falloff and the reading of your new sensor, roughly speaking, and if correct, would record "PAR" at a 30% increase over the Apogee..
Using a 660nm LED and comparing the Apogee to your sensor should show a marked increase in "PAR" Quantifying it would be harder than the blue band but it should be somewhere around an 80-90% increase..
If all the other tests pass even 50% increase would probably make it "better" than the Apogee

Both the sensor diode and the filter slope is known and consistent.. so once you have a few data points most should fall in line. You do have the angular thing w/ the IR/UV filter that is more diffraction grating than "filter"
(susceptible to angular light distortions of transmission) but that is "just" a matter of collimation. 

OF course if Apogee gets it's *act together* and changes it's red cutoff.. all of this is wasted effort..The slight overshoot in the non red/blue area and minor falloff in the blue is nothing compared to the "red problem"


As to plants again..but photosynthesis is not really the end all here. there are other light mediated systems.


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## Hoppy (Dec 24, 2005)

mistergreen said:


> I have this question, if a light source is so specific in its spectrum say only 550 nm, is it really PAR? It has no benefit for plants who needs 400nm-700nm but mostly blue AND red.


We have to keep in mind that we are not looking to make a PUR meter, but a PAR meter, and that means one that measures all light between 400 and 700 nm, with equal response to all of the spectrum in between. I have a much better appreciation for what LiCor was able to do than I had 3 years ago! And I still believe I can make a PAR meter better than the Apogee meter, but I couldn't ever calibrate it to verify that it was better. (Given my budget and my technical abilities.)


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## mistergreen (Dec 9, 2006)

Hoppy said:


> We have to keep in mind that we are not looking to make a PUR meter, but a PAR meter, and that means one that measures all light between 400 and 700 nm, with equal response to all of the spectrum in between. I have a much better appreciation for what LiCor was able to do than I had 3 years ago! And I still believe I can make a PAR meter better than the Apogee meter, but I couldn't ever calibrate it to verify that it was better. (Given my budget and my technical abilities.)


What you can do is create a 20 or so LED array of varying spectrums from 300-800nm.
Switch on one LED at a time and measure the response from your sensor... They all should be equal except <400 && >700, if you're successful. It might cost you $50 to make this LED array. 

Now, finding or making the sensor is another thing. If I see a flat spectral response photodiode, I'll let you know.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> What you can do is create a 20 or so LED array of varying spectrums from 400-700nm.
> Switch on one LED at a time and measure the response from your sensor... They all should be equal if you're successful. It might cost you $50 to make this LED array.


How could be sure they are at equal photon output???
Current doesn't work since each will have a different efficiency ect..



mistergreen said:


> Now, finding or making the sensor is another thing. If I see a flat spectral response photodiode, I'll let you know.


Doesn't need to be flat.. (and THAT may not exist btw)
Just relatively easy to deal with..
OSI has the best that I could find.. I have a few of the "non-blue" which technically aren't "bad"..
(0.25-0.45 in the 400-700 range)
see spec sheet below.
www.qualiteitems.com/images/bhd.pdf
(0.25-0.45 in the 400-700 range for blue enhanced)


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## mistergreen (Dec 9, 2006)

jeffkrol said:


> How could be sure they are at equal photon output???
> Current doesn't work since each will have a different efficiency ect..


Ah true. It makes it hard to find the right ones. They do list the wattage but it doesn't guarantee same number of photons.

Better yet, use a prism. Move the sensor through the band of light.


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## Hoppy (Dec 24, 2005)

Since this is an obsession for me, today I started thinking about the Excelitas diode again. I found (again) the spectral response curve for it, and blew it up so I could more easily read the relative sensitivity values off of it.









Then I tried to see what some possible filters would do to that, and was surprised to see how much better they work with this diode than with the nameless diode from the lux meter, minus its filter. So, I spent most of the afternoon looking for a combination that would give a nice flat response - didn't find that, but I did find a good approximation:










So, I'm tempted to make one like this, but I'm still stumped as to how best to calibrate it?


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## mistergreen (Dec 9, 2006)

Yeah, that's the one I used for my close enough meter. It works well. Try swapping out the nameless diode for the vtb8441. Adding a diffuser will further flatten that curve.

I should buy a prism and test out how good it is.

Sent from my iPad using Tapatalk HD


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## Hoppy (Dec 24, 2005)

mistergreen said:


> Yeah, that's the one I used for my close enough meter. It works well. Try swapping out the nameless diode for the vtb8441. Adding a diffuser will further flatten that curve.
> 
> I should buy a prism and test out how good it is.
> 
> Sent from my iPad using Tapatalk HD


How does a diffuser flatten out the spectral response? That response is largely determined by the basic response of a silicon diode to light.


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## mistergreen (Dec 9, 2006)

I'm not entirely sure. I read it in edu paper on constructing a par meter. My guess is evens out and blocks certain spectrum even like ir.

They didn't even bother using filters. I guess a diffuser was good enough for them using their diode.


Sent from my iPad using Tapatalk HD


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> Yeah, that's the one I used for my close enough meter. It works well. Try swapping out the nameless diode for the vtb8441. Adding a diffuser will further flatten that curve.
> 
> I should buy a prism and test out how good it is.
> 
> Sent from my iPad using Tapatalk HD


Prism is glass and may hav it's own absorbance.. 








Diffraction Grating Slides-Linear 1000 Line/mm: Science Lab Physics Classroom Supplies: Amazon.com: Industrial & Scientific
I can't think of a spectrophotometer that uses a prism any more... 
http://myspectral.com/


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## mistergreen (Dec 9, 2006)

That's fine, glass may have its own absorbance but it's only $10. 

Nice find on the arduino spectral analysis. I'll have to read further. I guess getting a meaningful PAR value from a linear ccd array is another issue. 


I found an android app a while back that analyzes the visible spectrum. You take a picture of the rainbow and it analyzes the color pixels. It's called 'learn light' but it stopped working with the latest android update.


Sent from my iPad using Tapatalk HD


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> That's fine, glass may have its own absorbance but it's only $10.


"free" spectrophotometer "guts"..
Cereal box and CD..

http://www.scienceinschool.org/2007/issue4/spectrometer



> It is very fortunate that everybody has high-quality diffraction gratings at home: compact discs (CDs). The beautiful colours that can be seen when light reflects off the surface of a CD are a clear indication that it acts like a diffraction grating. But why? The illustration above shows what a strong magnification of the surface of a CD would look like. The music is encoded in short and long pits, which are placed in a long spiral groove on the surface of the CD. The grooves, which are spaced just 1.6 µm (1600 nm) apart, act as a grating.


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## mistergreen (Dec 9, 2006)

jeffkrol said:


> "free" spectrophotometer "guts"..
> Cereal box and CD..
> 
> http://www.scienceinschool.org/2007/issue4/spectrometer


Yup, I made one for the android app but it can't be used to test out a photodiode.


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## saiko (Mar 30, 2007)

Has anyone checked this one? is it worth a try?(worse does it match the need?)

http://rohmfs.rohm.com/en/products/databook/datasheet/ic/sensor/light/bh1750fvi-e.pdf
(It is possible to detect wide range at High resolution( 1 - 65535 lx ))


spectral response on page 3


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## jeffkrol (Jun 5, 2013)

saiko said:


> Has anyone checked this one? is it worth a try?(worse does it match the need?)
> 
> http://rohmfs.rohm.com/en/products/databook/datasheet/ic/sensor/light/bh1750fvi-e.pdf
> (It is possible to detect wide range at High resolution( 1 - 65535 lx ))
> ...


At first I thought no:


> Spectral responsibility(?) is approximately human eye response


but looking st the output chart it is really no too bad UNTIL I took a closer look.
Still the "red problem"..


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## Hoppy (Dec 24, 2005)

jeffkrol said:


> At first I thought no:
> 
> but looking st the output chart it is really no too bad UNTIL I took a closer look.
> Still the "red problem"..


Both red and blue problem! It is essentially a green light detector, like our eyes are.


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## burr740 (Feb 19, 2014)

This thing is going to be five thousand dollars when you guys get through with it!

J/K  I for one really appreciate the efforts Hoppy and the rest of you guys are putting into getting this right. Hope to be able to purchase one eventually.


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## mistergreen (Dec 9, 2006)

You guys might find this sensor interesting. 
https://www.sparkfun.com/products/11195

It's an RGB sensor so it's not going to miss out on any spectrum and It will give you a LUX output. With some coding, you can change it to PAR. It won't help Hoppy's project but if you guys want to build a PAR meter, check it out. No need to build a housing as it is embedded in a water proof housing. Just slap a diffuser on it and you're good to go. It's not cheap but it's about the price of a DIY PAR sensor + meter.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> opps never mind..
> 
> I mentioned using camera CMOS sensors before.. Basically the same problems.. All it is is a cluster of "photodiodes" w/ filters..
> Main problem is still the cutoff points..
> ...


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## mistergreen (Dec 9, 2006)

I'll have to email them to get specs on their photodiodes but it sound like it's not like CMOS. It can detect beyond the visible spectrum, I think. So there is no drop off like the CMOS.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> I'll have to email them to get specs on their photodiodes but it sound like it's not like CMOS. It can detect beyond the visible spectrum, I think. So there is no drop off like the CMOS.


so can CMOS when you remove the "hot mirror" and UV filter (well most have a weak uv response anyways..)

















COS/CCD are really good sensors, just complicated to use..
One photodiode replaced by 1000's..

to be honest.. A "simple" RGB CMOS sensor, coupled to a Baader IR/UV filter is almost "the perfect" 2 part PAR sensor AND spectrum analyzer.. 
Building the circuitry or finding the Baader in a small size is the problem. 
and of course the geometry..
and cost.. 
and programming


though it will be cheaper than a Li-Cor..
ADC camera chips are a dime a dozen..

As a side note Actually the curve here :
http://rohmfs.rohm.com/en/products/databook/datasheet/ic/sensor/light/bh1750fvi-e.pdf
looks a lot like the RGB minus the red..


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## mistergreen (Dec 9, 2006)

I emailed them for a spectrum chart and what it is. It might be a cmos now that I looked at it closely. But either way, It's taking the RGB values and converting to a value that makes sense. No way to do that starting from scratch with a CMOS with heavy investment.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> I emailed them for a spectrum chart and what it is. It might be a cmos now that I looked at it closely. But either way, It's taking the RGB values and converting to a value that makes sense. No way to do that starting from scratch with a CMOS with heavy investment.


i just mentioned how to do it.. 
This and a Baader filter.. 
http://www.newegg.com/Product/Produ...gclid=CJXG7tecksQCFQ4vaQodEmIAHg&gclsrc=aw.ds

http://www.telescopes.com/telescope-accessories/filters/baaderuvircutfilter.cfm

bottom line is that each pixel gets assigned and output..you just need to manipulate the data differently..

Analog in..ADC creates a value (color data comes from the color filters, intensity from the greens.. multiple values are interpolated (demosaic) the assigned a new value based on the demosaic.. then an image..

http://www.exclusivearchitecture.com/?page_id=749

all of this is all done and cheap actually. summing total output would be a piece of cake. Summing small groups to do a composite graph of all pixels.. a "bit" harder..

I know I'm making it sound easy.. but I'm pretty sure it is for some people (just not me)

think about a digital camera output Each pixel has a color and an intensity assigned.. THAT is the data you need to just treat differently..
In the case of a PAR meter.. color data is thrown out, luminous pixels are summed.. Done.
Spectrum analysis is harder. color pixels grouped in a small matrix and luminous summed..
Skip the spectrum analysis for now..IF this was a "photo" of the light, AND you have cut off the light from >400 and >700 (no data) a PAR number would be the sum of this luminous histogram. 









hmmthinking about this a bit.. the problem comes in sensitivity.. W a camera you can restrict the light so as to not lose resolution..

Hmm.. now it doesn't sound quite right.. i'll need to think about this a bit.


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## mistergreen (Dec 9, 2006)

I got a quick response and it looks good. The photodiode is TCS3200, TCS3210
PROGRAMMABLE COLOR LIGHT-TO-FREQUENCY CONVERTER
You can get the full data sheet online.

Here's the spectrum sensitivity.








If you need it to cut off at 700nm, get a IR filter and you're set. I'll ask the guy at atlas-scientific if he has a filter on his sensor.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> I got a quick response and it looks good. The photodiode is TCS3200, TCS3210
> PROGRAMMABLE COLOR LIGHT-TO-FREQUENCY CONVERTER
> You can get the full data sheet online.
> 
> ...


I'll have to ponder it a bit.. Does look promising. Though technically it is no better then the blue enhanced photodiode BUT you can actually buy it.. 








you still need a flattening blue filter and a IR cut.. Could skip the UV side..
On a side note:
http://www.aibcusa.com/portable-led-light-spectrometer



> CMOS Linear Image Sensor


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## mistergreen (Dec 9, 2006)

jeffkrol said:


> http://www.aibcusa.com/portable-led-light-spectrometer


yeah $2000 sensor isn't practical for hobbyists. Might as well get an apogee or li-cor.

I just got a response. There is no IR filter but it sounds like it's smart enough to ignore the IR and will tell you if IR is detected.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> yeah $2000 sensor isn't practical for hobbyists. Might as well get an apogee or licor.


Point was the CMOS sensor.. 



> CCT, CRI, LUX, Lambda Peak, CIE1931, CIE 1976.


LUX is just a subset of PAR.... 



> _Could AI-MK350 measure the UV or IR Spectrum？_
> 
> A4: This handheld spectrometer AI-MK350 is designed for Vis-light only, the measurement range is 360 ~ 750nm. You will need other model spectrometer to test UV and IR spectrum.



Droool...


> _Q13: Can I test PPF value by this spectrometer?
> _
> A13: MK350S only has PPFD or PAR (uMole/M2s) function, click here for details


http://www.aibcusa.com/portable-led-light-spectrometer/faq-of-ai-mk350-spectrometer?id=115



> How to test PPFD
> How to setup testing of PPFD or PAR value (umole/M2s)
> MK350S can test PPFD value with PPFD(avarage), PPF-UV, PPF-B(Blue), PPF-G(Green), PPF-R(Red), PPF-NIR(Near IR) and fc with handheld device and software.
> Please follow below steps to set PPFD(or others) at basic test interface.


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## Hoppy (Dec 24, 2005)

A typical planted tank hobbyist is not going to be at all interested in this type of product. That typical hobbyist is going to want to be able to stick something in the light and read a number that corresponds to the light intensity in PAR units. A small subset of planted tank hobbyists will be delighted to have a more complex "meter" and be able to manipulate some data to arrive at a light intensity number, providing he/she is confident that the number is accurate. There is plenty of "room" for both groups and the type of meters they want should be available, in my opinion. I suspect the number of potential buyers isn't sufficient to encourage someone to produce and sell those products at an appropriate price. (That leaves me a nice niche to play around in!)


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## jeffkrol (Jun 5, 2013)

Hoppy said:


> A typical planted tank hobbyist is not going to be at all interested in this type of product. That typical hobbyist is going to want to be able to stick something in the light and read a number that corresponds to the light intensity in PAR units. A small subset of planted tank hobbyists will be delighted to have a more complex "meter" and be able to manipulate some data to arrive at a light intensity number, providing he/she is confident that the number is accurate. There is plenty of "room" for both groups and the type of meters they want should be available, in my opinion. I suspect the number of potential buyers isn't sufficient to encourage someone to produce and sell those products at an appropriate price. (That leaves me a nice niche to play around in!)


i understand that.. But one of the fun things in life is most products start "upscale" and then work their way down.
Point is if there is a market that $2000 meter can be downscaled to say $500.. eventually.. Then from there?
None of the tech in there is worth $2000 I can guarantee that..
Or $1000.. or $500 cost..
That is what makes this so valid..


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## jeffkrol (Jun 5, 2013)

Sensors.. Both RGB and not..

an example (not the "best" but just one of the lot)
The good thing is th higher blue sensitivity.. 









That chip subtracts ch1 from ch 0 to "approx" human vision.. Thing is it may just be good for PAR (and not as good for vision)
An IR cut filter would still help.. This is in the I2C realm.. yours.. 
http://ams.com/eng/Products/Light-Sensors


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## mistergreen (Dec 9, 2006)

Look at this, I found your sensor on adafruit. Tsl2561.

http://www.adafruit.com/products/439

It is arduino ready.

Sent from my iPad using Tapatalk HD


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> Look at this, I found your sensor on adafruit. Tsl2561.
> 
> http://www.adafruit.com/products/439
> 
> ...


Looks quite useable.. A couple of things for clarity
1)still needs a cut off filter
2)Need to see the results of the below calculations
https://learn.adafruit.com/tsl2561/use


> By default, the driver will return light in standard SI lux units, which are a result of some complex calculations based on both photo diodes on the TSL2561 (one for full spectrum and one for IR).


If it is just a standard ch0 -ch1 or much more complicated than that. I can't see it being more than that. The sensor isn't capable of profiling by wavelength.. 
In other words what math "tweaks" it to LUX and concurrently can we "tweak" it to PAR?

Where is this "driver" and is it modifiable?? Sorry getting into the 0/1 realm.. 

I do like the auto gain feature they put in.. It was required..


Ahhh "driver"

https://github.com/adafruit/Adafruit_TSL2561



> What is the Adafruit Unified Sensor Library?
> 
> The Adafruit Unified Sensor Library (Adafruit_Sensor) provides a common interface and data type for any supported sensor. It defines some basic information about the sensor (sensor limits, etc.), and returns standard SI units of a specific type and scale for each supported sensor type.
> 
> ...


Lots of wrong (SI units) and unnecessary code there.. 

TAOS def. has sensors we can use..
Even their "analog" ones.. 
Need to find a cheap and small ir cutoff filter @700nm..


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## mistergreen (Dec 9, 2006)

Yup, the 'driver' in this case is the software library used to calculate and use the photodiode.
There is a formula to convert lux to par on apogee's site.

*Lux to PPF (μmol m-2 s-1)*
Sunlight	0.0185
Cool White Fluourescent Lamps	0.0135
High Pressure Sodium Lamps	0.0122
High Pressure Metal Halide Lamps	0.0141

Multiply the Lux by the conversion factor to get PPF. For example, full sunlight is 108,000 Lux or 2000 μmol m-2 s-1 (108,000 ∗ 0.0185).


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## Hoppy (Dec 24, 2005)

Here is a fairly cheap IR Cut filter that would probably work:
http://www.optics-online.com/IRC.asp?PN=IRC40-10R









It is the turquoise line on the chart.


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## jeffkrol (Jun 5, 2013)

Hoppy said:


> Here is a fairly cheap IR Cut filter that would probably work:
> http://www.optics-online.com/IRC.asp?PN=IRC40-10R
> 
> 
> ...


Not any better than Apogee response.. Green box 400-700..


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## Hoppy (Dec 24, 2005)

That may be the IR cut filter used on the Apogee: look at the little dip at the IR end, just like the sensor response for Apogee.

Today I assembled a sensor, using the Excelitas diode, with the 6 filters I calculated the response for as shown on the chart I posted, with a "frosted" acrylic rod diffuser, and a diffuser filter glued to the diffuser, on the inside, with epoxy. It works, and it doesn't suffer the drop in output when dunked in water, but I haven't adjusted the sensitivity to match the Apogee sensor yet, so no calibration yet.


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## mistergreen (Dec 9, 2006)

I checked out Taos catalog and they make different diodes with basically the same cmos so the spectral response is basically the same but different output like digital/ i2c, frequency/ digital pulse input, and voltage/analog. So you can take your pick on what is convenient. Glue a uv ir cut on it and you're good to go. 

Sorry, nothing that will work for replacing the diode for the lux meter though. They require at least 3 connections, ground, v+, and output.

I played briefly with the excelitas photodiode and sunlight spectrum. I get responses from the different spectra which is good but I need to find a more controlled environment.

Sent from my iPad using Tapatalk HD


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> Sorry, nothing that will work for replacing the diode for the lux meter though. They require at least 3 connections, ground, v+, and output.
> 
> 
> Sent from my iPad using Tapatalk HD


That is because it is the amplifier and diode in one package.. 
The input runs the opamp circuitry.. the output is still a low voltage signal (1.5-2.5v).. IF the meter actually boosts current this is not good..IF it boost voltage (actually voltage is not as linear as the micro amp output.. ) the meter will just boost this.. Like double amplification.

As comparison I get .37v for my monitor and a normal LUX sensor..


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## Hoppy (Dec 24, 2005)

After quitting, then going back to this, I finally made one that works like I wanted it to! It reads the same, +/-5% or so, as the Apogee meter with 10,000K PC, 50-50 PC, Finnex Planted Plus, open shade, and full sun, with the Apogee in the electric mode for the bulbs, and the sun mode for shade and full sun. And, it should be repeatable because everything is controlled pretty well. The calibration looks like:










The open shade readings were taken with the sensors tilted to point at the blue sky, but it was somewhat hazy so the readings wouldn't settle down enough to get a steady reading, explaining why those didn't fall right on the line. (I would read one, read the second, read the first, and its reading would be different, etc.)

The spectral response, calculated from the published response curves for the Excelitas Diode and the Rosco filters is:










Note that there is quite a bit of UV and IR radiation included in the readings, but, since it calibrates in agreement with the Apogee meter, for sunlight, which includes a lot of both UV and IR, that is not likely to affect the readings enough to matter.

I will make a detailed sketch of the design for anyone who wants to try to make one and post it later. Next will be trying to duplicate this a few times. I still have enough parts to make a few more.


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## kman (Dec 11, 2013)

Hurray for progress!


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## PhilipS (Jan 9, 2014)

Yay! :bounce:

Someday, I'll get one.


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## jeffkrol (Jun 5, 2013)

Hoppy said:


> Note that there is quite a bit of UV and IR radiation included in the readings....


I know someone that has filters to fix that..........


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## Hoppy (Dec 24, 2005)

jeffkrol said:


> I know someone that has filters to fix that..........


Yes, and then it might not work so well! :hihi: After doing this for 3 years this is the first time I've been able to get all light sources to fit on the same calibration line. Do you think I plan to make any major change?:wink:


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## flight50 (Apr 17, 2012)

So when do the new meters go on sale Hoppy. So that I can have 2 Hoppymeters instead of one, lol.


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## mba (Jul 18, 2011)

flight50 said:


> So when do the new meters go on sale Hoppy. So that I can have 2 Hoppymeters instead of one, lol.


I have asked same questions for yrs. Hoping to get one soon. Willing to pay in advance for secured deposit


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## jeffkrol (Jun 5, 2013)

Hoppy said:


> Yes, and then it might not work so well! :hihi: After doing this for 3 years this is the first time I've been able to get all light sources to fit on the same calibration line. Do you think I plan to make any major change?:wink:


fine save it for 2.0.. 

Anyways the filter would have zero impact (low absorption) in the visible range.. as long as the light is perpinducular to the filter..


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## Hoppy (Dec 24, 2005)

When you bury the photodiode and its filters inside the sensor, with a diffuser between them and the light source, the light is always essentially perpendicular to the diode/filters. As I see it, the function of the diffuser is to locate the source of light, as seen by the diode, at the same location, directly above the diode, at the same distance from the diode, no matter what light is being measured.

The reason I lost interest in finding an IR cut filter is that the Excelitas diodes already have one installed, and they are blue enhanced diodes, which I can get a much better filter match with than with the standard diodes I was playing with. If I could have found a good filter match for those standard diodes I would save the $6 per diode cost, and would still be interested in IR cut filters.


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## mistergreen (Dec 9, 2006)

The sun is out and finally got around to to doing some readings you might find interesting.
It's for the vtb8441.










An IR cut would definitely help.


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## Hoppy (Dec 24, 2005)

Here are the details for making the PAR sensor:
I learned to make epoxy castings, and that works great for this type of project, so I used that technique to make the sensor housing. First step is making a housing out of acrylic tubing, to use to make a mold for casting the housings out of black epoxy resin.










I used platinum cured silicone to make a mold for the castings, following the directions here: http://www.tapplastics.com/product/...old_making_supplies/tap_platinum_silicone/494

The sensor assembly is:









The bottom of the sensor can be sealed with just a flat piece of epoxy, or a more useful bottom can be cast in epoxy, with a socket for a 1/4" dia rod to use as a handle for under water measurements:









Assembly steps are:
1. Clean up the body casting using sandpaper.
2. Drill a hole in one side of the housing to be a tight fit on the electric cable.
3. Install the 1/2" dia x 1/8" long acrylic tube by pushing it into place to bottom out on the step in the housing.
4. Install the diode into the 3/8" dia diode holder and evenly spread the wires to hold it centered. Use quickset epoxy to "pot" the wires in the diode holder.
5. Push the diode/diode holder into the hold in the 1/2 dia x 1/8 acrylic tube. It should slighty bottom on the step in the housing to locate the diode properly.
6. Squeeze the end of the electric cable to reduce its insulation diameter as much as you can, with your fingers, then insert it into the hole in the housing, far enough to let you strip the insulation easily.
7. Strip about 1/4" of the outer cable insulation, the shielding and the ground wire.
8. Strip about 3/16" of the black and red insulation on the electric cable wires.
9. Identify the positive wire of the diode - marked on the top face of the diode - and match that to the red insulated cable wire.
10. Pull the cable back until the stripped wires will touch the diode wires, leaving some of the cable outer insulation through the hole in the housing. Carefully bend the wires to get good contact with each diode wire.
11. Solder the wires, making sure the red insulated wire goes to the positive wire on the diode.
12. Cut off any wire ends that extend outside of the housing.
13. Use quickset epoxy to seal the inside of the electric cable to the hole in the housing, and to hold the diode in place. Don't try to fill the cavity with epoxy, but use a generous amount of epoxy.
14. Attach the bottom of the sensor with quickset epoxy, so it is waterproof. Make sure quickset epoxy has wicked around the electric cable on the outside of the housing and add a bit more if necessary.
15. Install the filters at the top of the sensor. I cut them to size by eye, shooting for a 5/16 inch square, then cutting the corners off to make 5/16 octagons. Put the colored filters in first, then add the diffusing filters, #111, on top. I make the last of those a 1/2 inch octagon, so it sits directly in contact with the frosted acrylic diffuser.
16. The sensor can now be connected to the readout module and you can check to be sure you can adjust it to read correctly.
17. When you are sure the assembly is going to work, put a small bead of quickset epoxy around the 1/2" recess and press in the frosted acrylic diffuser.
18. The sensor should now be waterproof, and able to be calibrated.


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## sumer (Feb 6, 2010)

Okay, so when can I buy one?


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## mistergreen (Dec 9, 2006)

hint hint, hoppy wants you to build your own


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## Hoppy (Dec 24, 2005)

This sensor is designed to work with a LX1010B, Chinese/Ebay/Amazon Luxmeter readout module. These luxmeters cost less than $20 from numerous "stores". To connect the sensor to the luxmeter:
1. Install the luxmeter battery and turn it on to be sure the readout module is working.
2. Turn off the luxmeter, and disassemble the luxmeter sensor by removing the two screws that hold the two halves of its housing together.
3. Cut the electric cable right next to the inner housing that holds the sensor diode, to get the maximum length of straight cable to work with.
4. Strip about 1 inch of insulation from the cable, and about 1/4 inch of insulation from the red and black wires.
5. Strip about 1 1/5 inches of cable insulation from the electric cable from the PAR sensor. 
6. Cut the red and black wires to about 3/4 inch length, leaving the ground wire the full length. That ground wire will be used to strengthen the connection between the two cables.
7. Strip about 1/4 inch of insulation from the red and black PAR sensor cable wires.
8. Tin all 4 bare wires.
9. Slip about a 3 inch length of shrink insulation tubing onto the PAR sensor cable and move it back from the joint to protect it.
10. Slip about a 2 inch length of shrink insulation tubing onto the lux meter cable, and move it back from the joint.
11. Slip 1/4 inch lengths of shrink insulation on the two wires of the lux meter cable, and push them as far back as possible.
12. Solder red to red, and black to black. The lux meter wires are too weak to be twisted to the other wires, so just lay them in contact side to side, and solder them in turn.
13. Slip the wire shrink tubes over the bare wires and heat to shrink them.
14. Slip the shorter cable insulation shrink tube over the joints so the long ground wire will be held against the lux meter cable insulation, and heat shrink it.
15. Slip the longer cable insulation shrink tube over the joint to overlap the other shrink tube at each end, and shrink it on.
16. The luxmeter will now read when the sensor is exposed to light. Turn it on and verify this.
17. Calibrate the now PAR meter by comparing the readout to a known PAR - I use an Apogee PAR meter or a well calibrated one of my own as the known PAR. To adjust the reading you need to open up the readout module to get to the adjustment screw:









18. Turn the adjustment screw with a tiny phillips screwdriver until the reading matches the known PAR value.

NOTE: It is a very good idea to do this before cementing the diffuser onto the sensor, so you can adjust the number of diffusing filters if necessary to be within the calibration range of the readout.


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## Hoppy (Dec 24, 2005)

sumer said:


> Okay, so when can I buy one?


The purpose of this DIY forum is to show what you have made, and to provide information that lets others duplicate or improve on your work. Yes, I do plan to make some to sell, but I haven't yet figured out how much I need to sell them for to break even (probably about $100+ each, but I don't really expect to break even.)

The For Sale forum is where to look for any that I make to sell.


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## mistergreen (Dec 9, 2006)

Hey, hoppy
It'll be easier on you if you sell just the parts. The buyer can assemble it themselves, maybe.


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## Hoppy (Dec 24, 2005)

Yes, it would be easier to just sell the parts, but I suspect that would just lead to a lot of disappointed buyers. Unless someone has a good working PAR meter I don't know how they can calibrate one of these. And, the assembly can be pretty hard to do, unless you are used to working with tiny parts and doing soldering.

What parts would you suggest selling? I could just make the sensors and let the buyers find a way to use them.


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## mistergreen (Dec 9, 2006)

Oh right, the calibration. Well, there goes that idea. I went through the same thing.


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## renesis (Dec 4, 2014)

Been following along, but have you guys seen this unit by Apogee?

http://reefbuilders.com/2015/03/17/handson-biotek-marine-usb-par-sensor/

Still not as cheap as DIY, but at $200, not too bad...


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## mba (Jul 18, 2011)

renesis said:


> Been following along, but have you guys seen this unit by Apogee?
> 
> http://reefbuilders.com/2015/03/17/handson-biotek-marine-usb-par-sensor/
> 
> Still not as cheap as DIY, but at $200, not too bad...


I just purchased one..last week and suppose to arrive this week. 

I'm sick of the waiting game and monitoring the sell forum....So for a little extra $$, spend it on a reliable company that has many years of experiences and if any issues that is backed up with their warranty.


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## Hoppy (Dec 24, 2005)

mba said:


> I just purchased one..last week and suppose to arrive this week.
> 
> I'm sick of the waiting game and monitoring the sell forum....So for a little extra $$, spend it on a reliable company that has many years of experiences and if any issues that is backed up with their warranty.


Of course, if you can afford it, the Apogee PAR meter is a much better device than mine, whether you buy just the sensor or the complete meter. The wait for mine is because it is very difficult to make these and do it repeatably. But, I will have a few ready in just a few days or sooner.


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## mba (Jul 18, 2011)

Hoppy said:


> Of course, if you can afford it, the Apogee PAR meter is a much better device than mine, whether you buy just the sensor or the complete meter. The wait for mine is because it is very difficult to make these and do it repeatably. But, I will have a few ready in just a few days or sooner.


I'm sure one of the lucky TPT member will be happy for your par meter at a fair price. Most users will not spend $200+ on a Par sensor. If my wife finds out she would body slam me against the wall


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## dragam21 (Mar 23, 2014)

mba said:


> If my wife finds out she would body slam me against the wall


That really made me laugh, because I think we all know that feeling!!:hihi::hihi:


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## PhilipS (Jan 9, 2014)

Hoppy said:


> Of course, if you can afford it, the Apogee PAR meter is a much better device than mine, whether you buy just the sensor or the complete meter. The wait for mine is because it is very difficult to make these and do it repeatably. But, I will have a few ready in just a few days or sooner.


I have the previous version from a friend and it's so slick.

I want my own because he lives on the other side of the city and I am still in the trial phase of ridding the tank of GHA. Army of shrimp can only do so much.


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## treyLcham (Sep 9, 2014)

How much is one of your par meters? I want to do some testing on my 60g that has 3 kessil a160 we over it at about 25~30inches from lowest point of substrate


Sent from my iPhone using Tapatalk


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## Hoppy (Dec 24, 2005)

treyLcham said:


> How much is one of your par meters? I want to do some testing on my 60g that has 3 kessil a160 we over it at about 25~30inches from lowest point of substrate
> 
> 
> Sent from my iPhone using Tapatalk


Go to the For Sale forum and search for DIY PAR Meter. I have a wait list for them right now.


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## Hoppy (Dec 24, 2005)

I have been getting strange results when I test these under water. Sometimes it works just as it does in air, other times the reading drops to about half of what it should be even with just a film of water over the sensor. I think this is why I get this:









Because the difference in refractive index between water and acrylic is less than it is between air and acrylic much of the light is being reflected back from the acrylic-air interface before it gets to the photodiode. It may be erratic because if the gel filters that are just behind the acrylic diffuser are in good contact with the diffuser there is no acrylic-air interface, but if they have even a film of air between them there is a problem.

When I let a film of liquid epoxy resin get between the diffuser and the filters it works fine, but if I am too careful and no resin gets there I get the problem. So, I think I need to intentionally get epoxy resin between the filters and the diffuser. Does this make sense?

Tomorrow I plan to assemble one sensor with the whole volume between the diffuser and the diode filled with clear epoxy. This may be a reproducible configuration, which will stop the problem for all of them. But, it may introduce a problem if the "clear" epoxy resin cures with a yellowish color.

I think Apogee pots the whole sensor head, diode and all, with clear resin.


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## Hoppy (Dec 24, 2005)

Nobody has any thoughts about this? I tried filling in the gap behind the diffuser to make the diffuser and gel filters one solid mass. That seems to work better, but now I'm thinking I need to also fill in the gap between the diode and the diffuser/filters. I may try that tomorrow. The 5 minute epoxy does set up very clear, so that potential problem isn't going to be a real problem.


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## mistergreen (Dec 9, 2006)

yeah, I have the photodiode up against the diffuser and no problem.


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## Hoppy (Dec 24, 2005)

The PAR meter works fine with the photodiode completely potted in clear epoxy! The calibration is the same as with the air pocket between the diffuser and diode. Tomorrow I will try it in water, when the epoxy is totally cured, and see if the "water problem" is eliminated.


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## Hoppy (Dec 24, 2005)

The potting of the internals did not eliminate the water problem. I still get a drop in PAR as the sensor diffuser goes from being in air to being in water. The drop in PAR distorts the PAR readings no matter where in the water it is. 

Next theory: This problem occurs because the air to acrylic interface focuses the light more than the water to air interface does, due to the greater difference in refractive index between air and acrylic versus between water and acrylic. 









This alone will reduce the PAR reading, because the light reaching the diode will be more concentrated when the sensor is in air than when it is in water. The solution is probably to change from a flat disc diffuser to a spherical segment diffuser. This is unfortunate, because making identical diffusers that have a spherical surface is a lot more difficult than making flat disc ones. I may have to go back to casting the diffuser from white epoxy. Until I resolve this I will be making no more of these.

EDIT: Now I am wondering is my frosted acrylic rod is diffusing the light well enough. If it were a perfect diffuser the refractive index shouldn't enter into the design at all. All of the light inside the diffuser would be moving in an almost infinite number of directions, so any refractive effect would be nullified. Perhaps a flat disc made from white acrylic rod would work ok, if it allows any light at all to go through??


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## Hoppy (Dec 24, 2005)

I made one sensor with a 1/16 inch thick piece of frosted rod as the diffuser, with the idea that the light rays can't be shifted much with such a thin diffuser, compared to the 3/16 inch thick one I had been using. And, I epoxied a layer of diffuser filter from Rosco to the bottom of that disc to get more diffusion in the diffuser. It took another 4 layers of diffuser filter under that to drop the reading enough to be able to adjust the readout so it reads correctly. When I tested this in water the reading dropped a few PAR as the water flowed over the top of the sensor, but I had to lower the sensor about 1/4 inch into the water before water could overcome the surface tension and flow over it, so the PAR drop included that due to a slightly longer distance from the light (about 4 inches). The reading at the substrate is about 3-5 PAR less than I had using the Apogee PAR meter. That is within the accuracy that I'm looking for. I will make a few more like this and be sure they all work equally well before finalizing on this.


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## Hoppy (Dec 24, 2005)

*Here are the instructions for making these*

Here are the details about the "final" DIY PAR meter I have been making:








This is the sensor assembly. It is intended to minimize the thickness of the acrylic Lens or diffuser, to greatly reduce the in-the-water problem I ran into. It uses a 2 piece cast black epoxy housing, 8 layers of Roscolux gel filters, two of which are just diffuser sheets that should not affect the spectrum of light that gets through, and it reads out on the luxmeter readout module in the 20000 range.

To make the housing parts I made acrylic forms with the shape I wanted, cementing the pieces together with acrylic cement. The forms are:

















These acrylic forms are used to make silicone rubber molds for casting the parts. The parts have to be opaque, so black epoxy is the best material to use.

To assemble this I started by using Loctite Quick set 5 minute epoxy to hold the diode in the diode retainer - a 1/8 inch long piece of 3/8 dia acrylic tube. While that was setting I cleaned up the cast parts, removing the little protrusions that occur, and the pieces that were in the inlet and vent holes. Then, I used a very simple wood fixture to locate the hole for the electric cable and drilled that.

With the diode/diode retainer cured, so the diode is attached to the retainer with the back end against one end of the retainer, I tinned the electric leads to the diode, bent them back towards the middle, and inserted it into the 3/8 diameter hole inside the housing, pushing it in so the bottom surface is flush with the rim of the hole in the housing. This locates the diode the same distance from the diffuser for each sensor. 

Using a 3 foot long piece of Belden 8451 electric cable, I poked one end through the hole in the housing and pulled it far enough to strip 1/4 inch of the insulation, cut off the exposed ground wire and shielding, and strip most of the insulation off the two wires. Then, tin those wires and bend them apart so they will firmly contact the diode wires when the cable is pulled back. Always look in the other end of the housing at the top of the diode to find the positive wire side of the diode. There is a cast in tiny hole or "plus" on the face of the diode case at that end. The red wire has to be soldered to that side of the diode. Hold the cable so the tinned wires are in good contact, and the cable outside insulation still extends into the inside of the housing, and solder the two connections.

Put a battery in the luxmeter readout module and verify that the meter works. (Occasionally a bad one gets shipped.) Disassemble the luxmeter sensor and cut off the cable at the diode. Strip the outer cable insulation about 3/4 inch back, and the wire insulation about 1/8 of an inch back on both wires. Strip the outer cable insulation on the end of the cable now attached to the diode, back about 1 1/2 inches. Cut the two wires back to about 3/4" long, leaving the ground wire the full length.

Push a 2 inch length of 3/16 dia shrinkable insulation tubing over the end of the cable with the diode, and shove it several inches back out of the way. Push a 3/16 inch length of 1/16 inch dia shrinkable insulation over the wires on that cable and keep them as far from the stripped end of the wires as you can. Push a 1 1/2 inch piece of 3/16 dia shrinkable insulation over the end of the cable to the readout and push it back as far as you can.

Solder red to red, black to black wires of the cables together. Don't keep them so hot that they shrink the insulation tube. Push the small insulation tubes over the soldered wire junctions, and use a hot air gun to shrink them onto the wires. Straighten the ground wire so it extends well past the soldered junctions and onto the other cable and push the shorter 3/16 dia shrink insulation over the junctions so it also covers the ground wire. Use the hot air gun to shrink that in place. Finally, push the longer 3/16 dia shrink tube over the joint, overlapping the cable at both ends and shrink it in place. Now the long ground wire helps give the connected cables more strength to resist stress on the joints.

Turn on the lux meter and check that it works - the readout should read high when light shines on the submerged diode and zero when you hold your finger over it.

Put a big drop of Loctite epoxy over the cable where it enters inside the housing, and another drop at the opposite side where the diode retainer is flush with the housing. Fill the cavity in the base of the sensor with the same epoxy, and push the housing assembly into it to seal the base to the sensor housing. Carefully hold them together and use masking tape to hold them for curing, which takes about 15-30 minutes.

Cut the gel filters to about 1/2 inch squares, cutting off the corners to get about 1/2 inch octagons - actually a bit smaller than 1/2 inches. Stack the colored filters in the large diameter recess in the housing in any order. Stack the two diffuser gel filter octagons on top, and lay the 1/2 inch diameter acrylic "lens" on top.

Test the meter, by placing the sensor in a light of known PAR. The meter should read within about 10% or a bit more of the correct reading. If it reads off more than that you can add more diffuser filters to reduce it, or remove one diffuser filter to increase it.

Remove the lens and drizzle Loctite Quick set 5 minute epoxy in the opening for the lens, covering the top gel filter and coating the ID of the recess. Carefully place the lens in place, and place a small weight on it to hold it for about 15-30 minutes while the epoxy cures.

Test the meter again, by placing it in light of a known PAR reading. If it reads off the correct reading too much to please you, you can open the readout module housing, find the sensitivity adjustment variable resistor and adjust the reading to the accuracy you want. (This is tricky because different versions of the same lux meter have different circuit boards, with that variable resistor located differently. Some have multiple variable resistors. If you change the adjustments on all of them the meter may never read correctly!)

If your sensor assembly is water tight, and it should be, you can test if it works ok in water by slowly pushing it below the water surface while watching the readout. The reading should change consistently as you go below the water surface. If it drops suddenly as the water goes over the top of the sensor you probably won't be able to get accurate underwater readings.

*I have made a few of these and sold them, but I'm finding that I am getting too old, with too much arthritis, to keep doing this. So, I have published this much detail in the hope that someone else will want to make these and fill the need for a cheap PAR meter. I have some parts, most supplies, etc. that I could send to someone who is sure they want to do this.*


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## Kindafishy (Jan 14, 2015)

Sad Kindafishy is sad. I was really looking forward to getting one of these.

Don't get me wrong, I totally understand you can only do what you can do, but I hope somebody will pick this up and run with it. I would love to be on anyone's sale list that decides to continue with your work.

Thanks for putting up the instructions, Hoppy.


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## Hoppy (Dec 24, 2005)

I have 2 left to sell, but I have already offered them to people. Someone will eventually get them, and that will leave me with a total "profit" this year of less than $100. At least I won't need to pay income tax on that:hihi:


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## africamonk (Dec 3, 2012)

Sad panda. *cries*


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## PhilipS (Jan 9, 2014)

I like mine. Thanks Hoppy!


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