# Lighting help in Lumens



## teban (Feb 2, 2006)

hi guys can anyone enlighten me regarding lux? i believe someone has told me that it is more accurate to get lux value instead of basing on the watt per gallon rule.

Given that i can see several types of lights with lux values can anyone tell me what spectrum as well as what lux ratings can we consider as low medium and high light levels?

thanks!


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## teban (Feb 2, 2006)

hey guys just found some good material from Rex's website just want to share.

http://www.rexgrigg.com/mlt.html


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## plantbrain (Dec 15, 2003)

If you are going to upgrade, may as well use PAR and then use a standard distance from the bulb(say 10 cm, and 25 cm), both in and out of the water.

Lux tells you little other than what we see, not plants.
No botantist use that unit:thumbsup: 

Regards, 
Tom Barr


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## teban (Feb 2, 2006)

hi there plantbrain could you explain further on PAR? its the first time i have heard of this one.

thanks!


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## i4x4nMore (Mar 31, 2008)

*Measuring Light with a PAR Meter*

This is a primer on PAR that I wrote a while ago in my quest for my own understanding. It's intentionally not scientific, and I hope that it is mostly correct. I appreciate any comments and suggestions (sans malice and flame). This post is a bit long... I apologize in advance if that's annoying ​ 
------------------------------​ 
*Measuring Light with a PAR Meter*
Jeremy Squires. Photos and illustrations by author.​ 


A PAR meter is a special type of light meter that measures the amount of light available to a plant for photosynthesis. PAR is an acronym for “photosynthetic active radiation.” Although this phrase can sound quite technical, what this device actually measures is quite easy to understand for those that have ever wondered if they have enough light in their planted aquarium. 


What's exciting about a PAR meter, is that it can help determine if a particular light source or setup is sufficient for growing plants – both aquatic and terrestrial. Although PAR meters have been around for a while, their cost has made them somewhat prohibitive for a majority of hobbyists. Recently, however, manufactures have been working to produce relatively low cost PAR meters to make them more available to those who need to accurately measure light for plants.


Hobbyists have typically used a variety of methods to determine if their aquarium lighting is sufficient. However, the lack of a means to precisely quantify light has led us to rely on rules-of-thumb and guidelines which make it difficult to truly evaluate aquarium lighting. As lighting types and technologies evolve, it is beneficial to have a more accurate way of determining the light intensities in our planted aquariums. PAR measurements help us do this by eliminating the guesswork when evaluating a particular light source.









_A meter offered by Apogee Instruments_
_with a water proof sensor._​ 

*How much light do I need?*


One of the most common questions people have when setting up a planted tank is, “How much light do I need?” On the surface, this might seem like a simple question but getting a straight forward answer is not that easy.


The most common reply you get is probably the _watts per gallon_ rule-of-thumb (for example, 2-5 watts/gallon). If we all used exactly the same kind of light source having the same efficiency and the same spectral output and the same mounting specifications, I imagine the watts per gallon guideline would work pretty well. I believe the watts per gallon guideline was probably established at a time when hobbyists were mostly using standard fluorescent tubes. They were the cheapest and most available lighting method for hobbyists. Over time, however, manufactures responded to the needs of aquatic gardeners by producing different bulbs and lighting technologies specifically for plants. 


Today, the lighting technologies available to hobbyists have dramatically increased to include compact fluorescent, VHO, metal halide, mercury vapor, and sodium vapor. As such, the watts per gallon guideline is tricky to use because each lighting technology produces light in a markedly different way – each having different efficiencies and distinct spectral output. One hundred watts of standard fluorescent light will not produce the same kind - or amount - of light as 100 watts of metal halide. Thus, wattage alone is not a good choice for specifying light intensity for a planted aquarium.




*What about LUX/Lumens?*


In my quest to understand lighting, the next logical progression led me to wonder why aquatic gardeners don't use lux or lumens as a means for specifying the amount of light needed in a planted aquarium. Lux is a measurement of the number of lumens that fall on a one square meter area (lumens/m2). Thus, lux is a light intensity measurement that is independent of the wattage used to produce the light. 


You will find some books that list approximate lux values for aquatic plants. However, these lux values are very dependent on the light source used when taking the measurements. If one book lists lux values that are based on sunlight, how can I extrapolate these values to metal halide or VHO lighting?


The problem with using lumens to determine light intensity for plants is the lumen itself. The lumen is a measurement of the perceived intensity of a light source _as seen by human vision_. However, human vision has almost nothing to do with how bright a light appears to plants. In other words, we need to know the perceived intensity of light from a plant's point of view... not our own. 









_Notice where the peaks are for plants versus human vision._
_Most standard light bulbs are designed to maximize human_
_vision, not __plant photosynthesis. This is why the spectrum_
_a bulb__ produces__ is important._​ 

The chlorophyll molecules of a plant perceive light much differently than human eyes. Broadly speaking, the light receptors in human eyes are more sensitive to the green portion of a white light source. However, the light receptors in plants are more sensitive to the red and blue portion of white light. As an example, if we increase the intensity of a white light source by producing more green photons, human eyes will perceive the total light as getting progressively more intense (more lumens). However, given the same light, plants will not sense this increase because they are much less sensitive to green light. This is why the lumen alone cannot be used when evaluating various lighting technologies for growing plants.


*Light, Photons, and PAR*


To accurately measure light for plants, we have to use a method that does not rely on the human eye or its perception. Our eyes are only sensitive to a small portion of the total radiation produced by the sun. The fundamental constituent of light (electromagnetic radiation) is the _photon_. A photon is a small packet of electromagnetic energy that radiates from a source. The sun, for example, produces radiating photons in a very broad range of frequencies. 


When we are presented with the full range of photon frequencies produced by the sun, human eyes are only sensitive to a small portion of those frequencies. We give names to these frequencies and call them colors. White light is formed when all the visible colors (frequencies) are added together in equal amounts. But remember that our eyes are not equally sensitive to all the visible color frequencies. The same is true for plants, but their color limitations are different than ours.


The only way to really know how intense a light source appears to a plant is to actually measure the portion of the light to which the plant is sensitive. This is essentially what a PAR meter does – it counts photons from a light source that contribute directly to plant photosynthesis. As mentioned earlier, PAR stands for “photosynthetic active radiation” but other names for these meters include: _quantum photon flux_ (QPF) meters and _photosynthetic photon flux_ (PPF) meters. 









_Here I measure the light under the water in my office tank_
_(before __planting). Water attenuates light quite a bit: _
_the values I measured for this __metal halide bulb were_
_5 times greater at the surface than at the bottom_
_of the tank; and thats just 24 inches of water._​ 

*The PAR advantage*


Using PAR measurements is a far more accurate way of quantifying light used for growing plants. It eliminates the dependency on wattage and on perceived intensity. It is a direct measure of the light frequencies that cause a plant to photosynthesize. For example, if we know that a certain plant requires 150 PAR units to grow well, then that's how much it requires regardless of the kind of light source you are using. Thus, if my choice is to use metal halide lighting, then my goal becomes providing enough of that kind of light to produce the necessary 150 PAR units... and using a PAR meter, hobbyists can measure that directly in the tank - in the water even. (Actually, _preferably_ in the water 




*Understanding PAR units*


A PAR meter gives its measurements in units of mmol/m2/s (pronounced miromoles per square meter per second). This string of symbols can be quite intimidating to a lot of people but it's quite easy to understand if you break it down symbol by symbol. 


As stated earlier, a PAR meter counts photons. Specifically, it counts the number of photons that fall on an area of one square meter each passing second. Instead of dealing with trillions upon trillions of photons, it is useful to group photons into large collections - called _moles -_ and then count the number of moles of photons. This is like counting dozens of fish instead of each fish.


A mole is simply a predetermined number of “things.” Whereas a dozen is twelve things, a mole is 6x10²³ things (that's six followed by twenty three zeros)! Thus, a mole of marbles is 6x10²³ marbles, and a mole of gold fish is 6x10²³ gold fish. That's one big fish bowl. 


Extending this idea further, a _micromole_ (mmol) of photons is simply one-millionth of a mole of photons, or 6e17 photons (6e17 = 6 times 10 to the 17 power). 

Thus, mmol/m2/s is defined as the number of micromoles of photons falling on a square meter each passing second. Using micromoles gives us numbers that are easy to deal with instead of trailing along seventeen zeros each time we take a measurement!




*PAR measurements*


On a blue sky sunny day, the sun produces about 2000 mmol/m2/s of light as measured with a PAR meter. Of course, we probably wouldn't be supplying this amount of light to a planted aquarium, but it is a good baseline for evaluating the effectiveness of an artificial light source when growing plants. When I first got my PAR meter, I was interested to see what PAR values the standard lighting in my house would produce. Measuring a variety of incandescent and halogen bulbs, it became painfully obvious to me why these sources are not good for growing plants. Measuring at a distance of twenty-four inches away from a 100 watt incandescent bulb, my readings topped out at a dismal 5 mmol/m2/s. 


To gain a better understanding of the range of PAR values found in nature, I further experimented with my meter by taking readings outdoors in areas where healthy plants were thriving. At noon on a sunny day, I found flowering plants growing in areas where the PAR readings were in the range of 800-1750 mmol/m2/s . In partially shaded areas, I measured 300-600 mmol/m2/s. And in full shade, the PAR readings were 60-200 mmol/m2/s.


In “Ecology of the Planted Aquarium”by Diana Walstad, she states that normal light intensity for many aquatic plants is around 120 mmol/m2/s. She points out that almost all aquatic plants are technically shade plants. 







 
_In this graphic, I classify the light ranges in terms of PAR. _
_These are my own groupings based on my observations_
_and comments by others. Notice how the aquatic plants_
_fall into the shade section of terrestrial plants._​ 


*How much light is too much?*


In her book, Walstad also describes how plants have a _light saturation_ point. That is to say, there is an upper bound to the amount of light that is useful for growing submerged aquatic plants. Thus, increasing the light any further than a plant's light saturation point will not help the plant grow any better. There is debate as to whether there is such a thing as “too much” light in a planted aquarium. However, for submerged aquatic plants, it is useful to know that there is a level of light beyond which adding more light won't be beneficial for photosynthesis.


To illustrate the point, Walstad recounts an experiment performed using both submerged and emergent forms of _Myirophyllum brasiliense_. In this experiment, both forms of the aquatic plant were grown in the same conditions and the rate of photosynthesis was compared between the two plants as the light levels were increased from approximately 45 mmol/m2/s (very low light) to 2000 mmol/m2/s (very high light).


One interesting result of this experiment showed that the submerged form of _M. brasiliense_ became light saturated around 200 mmol/m2/s. That is to say, the rate of photosynthesis did not increase any further as light levels were increased beyond 200 mmol/m2/s. Similar results were also found using _Myriophyllum spicatum_ and _Potamogeton amplfolius_.










_After a certain point, adding more light does not increse_
_photosynthesis for submerged plants. Aerial leaves,_
_however, can take more advantage of stronger light._​ 
*Comparing light sources*


Armed with this new knowledge of approximate PAR values for aquatic plants (normal light ~ 120 mmol/m2/s, high light ~ 200+ mmol/m2/s), I was curious to see how different light sources compared against each other using similar wattages of bulbs. Using a variety of light sources on hand, I took PAR measurements of the brightest part of each light source at distances of one foot and two feet away from the bulb. These values are shown in Table 1 below.


Looking at the values in Table 1, it becomes very evident why the watts per gallon guideline would produce different results if these particular sources were used in similar sized aquariums. The biggest shocker to me was the reading for the halogen spot light measuring 680 mmol/m2/s at 1 foot! The light it produced was highly localized and very hot, but impressive nonetheless for an off-the-shelf bulb.


I was not able to position all the light sources over my aquarium so all the measurements were taken in air instead of the aquarium water. However, for the light sources that I could reposition, the PAR values were actually higher when measured in the aquarium (due to the reflective nature of the hood and glass). For example, the 110 watt Coralife 6700K reading was 140 mmol/m2/s at 1 foot as measured in air. However, taking the same measurement in the aquarium yielded 200 mmol/m2/s. 


There are many reasons to choose one light source over another. These factors might include:

total cost
maintenance cost
life of bulb
fixture size
safety
efficiency (PAR/watt)
color rendition index
color temperature
spectral output
heat factor
light distribution pattern
Keep in mind that the light comparison in Table 1 is not presented as a competition between light sources. If you say one is better, then you have to quantify what “better” means – and that depends on the application, the requirements, and on the many factors listed above. However, when it comes to evaluating light sources, a PAR meter can be a very useful tool in helping determine if you have *enough* of the *type of light* you choose to use in your aquarium. *--*










Table 1: Compares the PAR output of different bulb
types all having similar wattages.​ 


*References:*
Diana Walstad, 1999, _Ecology of the Planted Aquarium_. Pg 146-147


William K. Purves, David Sadava, Gordon H. Orians, and H. Craig Heller, _Life: The Science of Biology, _Seventh Edition, Chapter 8, http://bcs.whfreeman.com/thelifewire/


Iacopo Giangrandi, _Sensitivity of the Human Eye_, http://www.giangrandi.ch/optics/eye/eye.shtml


*For more information:* 
Quantum meters by Apogee Instruments Inc. http://www.apogee-inst.com/bqm_spec.htm


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## lauraleellbp (Feb 3, 2008)

Excellent read and great resource- bookmarked this one for future use :thumbsup:


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