# Photo period vs light intensity



## BradC (Dec 22, 2010)

I am prob not going to say this right and im sure someone will have a better answer.

If you have say low light, 15 watt T8 on a 10 gallon, on for say 8 hours a day. Just b/c you leave it on for double that, say 16 hours a day, it still does not change the fact that you only have 15 watts. The intensity doesnt get higher the longer you have it on. Does that make sense?


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## 2wheelsx2 (Jan 27, 2006)

If you stood outside on a cloudy day for 16 hours, would you get the same tan as if you stood outside for 2 hours on a sunny day?

You need to meet the threshold for higher light.


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## lurrch (Jul 5, 2011)

> If you have say low light, 15 watt T8 on a 10 gallon, on for say 8 hours a day. Just b/c you leave it on for double that, say 16 hours a day, it still does not change the fact that you only have 15 watts. The intensity doesnt get higher the longer you have it on. Does that make sense?


I have some idea about penetrating power and how a more intense single bulb can have a greater effect than a pair of less powerful bulbs, even if the output is supposedly equivalent. In other words, I understand that two 15w bulbs are less effective than a single 30 watt bulb and that having a 15w bulb on for 10 hours generates less results than a 30w bulb on for 5. 

I suppose the purest form of my question would be "What is the difference between having 1 15w bulb on for 10 hours compared to having 2 15w bulbs on for 5 hours?" To me, that seems to be the same amount of light (10 bulb-hours) with the same amount of penetration (everything is from the same 15w bulbs).


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## MikeP_123 (Aug 31, 2008)

I'm pretty sure it has to do with the growth period. Like humans, if a weight lifter works twice as hard but doesn't give his muscles enough time to grow and rebuild, then we wont get the best results. If a human doesn't get enough sleep his growth will stunt. Something like that.


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## sajata (Aug 21, 2009)

Its not quite as simple as watt hours/per sq in. Diffrent plants have diffrent levels of efficiency of photo synthesis. The most noticeable ones are the red plants. They dial up the red/bronze to dial down the rate of photo synthesis when light is high. So plants that are used to or need very high light dont start to photo synthisis untll a certin light level. Dont reach that breakover level and you dont start photo synthesis 

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## Hoppy (Dec 24, 2005)

The measure of light that counts with plants is PAR. If one bulb produces a PAR of 10, then two closely spaced bulbs will produce a PAR of 20. "Penetration" is a meaningless term. In water, all light loses intensity at a rate proportional to one over the square of the distance from the source, until you go to over 2 feet of water, then the absorption of light in the red part of the spectrum begins to become significant.

Completely independent of PAR is the length of the photoperiod. Some plants, if not all plants, will only continue photosynthesis for a limited time. After that the light isn't making the plant grow. If that time is 6 hours, doubling the photoperiod to 12 hours is no more effective than keeping it at 6 hours.


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## lurrch (Jul 5, 2011)

> Diffrent plants have diffrent levels of efficiency of photo synthesis. The most noticeable ones are the red plants. They dial up the red/bronze to dial down the rate of photo synthesis when light is high. So plants that are used to or need very high light dont start to photo synthisis untll a certin light level. Dont reach that breakover level and you dont start photo synthesis


This explanation sounds reasonable to me. Basically what you're saying is that growth/photosynthesis does not vary linearly with light level. Doubling the light level might lead to a 5x or 10x growth. 




> Some plants, if not all plants, will only continue photosynthesis for a limited time. After that the light isn't making the plant grow. If that time is 6 hours, doubling the photoperiod to 12 hours is no more effective than keeping it at 6 hours.


This seems like another good explanation, but it leads me to even more questions:
-Suppose a plant has a 6 hour photoperiod. What happens if I set up a 6 hours on/6 hours off rotation? (Two days per day, for lack of a better description). Would they grow twice as fast?
-More broadly, what causes a photoperiod to start/stop/reset? Darkness? Waste buildup?
-Any idea on where I can find the photoperiod for various plants?
-(More philosophically) Why do plants have a photoperiod at all? Why not just use whatever light is available at any given time, assuming sufficient nutrients and co2?


Thanks in advance for tolerating newbie questions.


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## sajata (Aug 21, 2009)

Think of it more like setting somthing on fire. Paper ignition happens about 450 def F. While magnesium is 880deg F. Leaving magnesium at 500 deg F all day wont get it to auto ignite. High light plants are like magnesium they wont even start photo synthesis until a high level iof light is reached. 

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## BlueJack (Apr 15, 2011)

lurrch said:


> This explanation sounds reasonable to me. Basically what you're saying is that growth/photosynthesis does not vary linearly with light level. Doubling the light level might lead to a 5x or 10x growth.



That's right...It's not a linear relationship. 










^Aquatic plants are considered shade plants. 

You could try finding the photosynthesis rate as a function of light intensity by searching google. 

There is a maximum and minimum aquatic plants can use for photosynthesis. Somewhere around 200 umols there is no further increase in the rate of photosynthesis. Similarly there is a point at which photosynthesis and respiration are balanced and there is 0 growth. I don't know the exact amount that this happens at, and each species is different but I would guess in the single digit umols is where this happens. An example might be: trying a single T8 on a 55 gallon tank (~9umols) will not grow anything.


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## BlueJack (Apr 15, 2011)

lurrch said:


> (More philosophically) Why do plants have a photoperiod at all? Why not just use whatever light is available at any given time, assuming sufficient nutrients and co2?


Philosophical answer:

I'm guessing since the sun is directly overhead for only a few hours per day (maybe 4-6 hours?) plants evolved to photosynthesize best during this amount of time. Most light is reflected (early and late in the day) off the lake, especially glassy lakes. As the angle of incidence decreases more light is able to get through the water.


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## plantbrain (Dec 15, 2003)

BlueJack said:


> Philosophical answer:
> 
> I'm guessing since the sun is directly overhead for only a few hours per day (maybe 4-6 hours?) plants evolved to photosynthesize best during this amount of time. Most light is reflected (early and late in the day) off the lake, especially glassy lakes. As the angle of incidence decreases more light is able to get through the water.


Aquatic plants are very competitive and as you mentioned prior...........they are low light shade plants......this means the first ones that can reach the light compensation point, will get all the CO2 that is limiting, leaving less for other species.

Hydrilla is an aquatic weed that is REALLY good at this, whereas say Cabomba, is much less competitive.

Light is light, angle plays a role, but if there's no CO2..........then it does not matter when or how much or the angle of the light. This is just the same as Liebig's Law, but adding light and CO2 to that same model.
It does not take much light at all, and there's plenty long before the 4-6 hour window you suggest for photosynthesis to occur in natural systems where the plants are present.

Evolution has produced very low light, rapid stem elongation rates and high competition for CO2 within plant beds.

Plants will start photosynthezing much earlier than the 4-6 hour model suggested above, adding to this, where trees etc are present.........there are FEW submersed plants, most aquatic plants are found in natural systems without tree cover. Marshes, wetlands, shallow lakes are more typical, they grow out away from the edge a little bit and if the sediment is shallow enough, to about 20-30ft deep.

I've measured virtually absent CO2 before 9:30 am for a 12 hour day(2 hours of sunlight).

Between 10-6pm.........there's no CO2, the plants are not growing, even if the light is blasting.

The "early plant"....gets the worm .........so to speak........ when plant density is high, like in aquariums. We can also see and measure this in non CO2 planted tanks. It's why some plants do well, others.......not so well and die off slowly.


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## lurrch (Jul 5, 2011)

plantbrain said:


> Light is light, angle plays a role, but if there's no CO2..........then it does not matter when or how much or the angle of the light. This is just the same as Liebig's Law, but adding light and CO2 to that same model.
> It does not take much light at all, and there's plenty long before the 4-6 hour window you suggest for photosynthesis to occur in natural systems where the plants are present.
> 
> Evolution has produced very low light, rapid stem elongation rates and high competition for CO2 within plant beds.
> ...


If I'm reading what you're saying correctly, the photoperiod (at least in the wild) is much more dependent on CO2 supply than on light. The plants grow for as long as there is CO2 in the water.

In that case wouldn't my "half as much light for twice as long" scheme work, as long as I keep the CO2 running?


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## 150EH (Dec 6, 2004)

I've been using the 24 hour method from the PPS site with fair results using CFL. The 24 hour method goes like this if you have 3 wpg on for 8 hours = 24 or 3 x 8 = 24, or with multiple lighting units 1.75 wpg for 9 hrs = 15.75 wpgpd plus 3 wpg for 3 hrs = 9 wpgpd so the total is 24.75 wpgpd. This may not work for your type of lighting but has worked well for me as a starting point to make adjustments. But what you are asking does not work, low light for a longer period does not equal high light. Example: my CO2 is cranking away an hour before any lights come on, my low lighting comes on for 3 hours and then my high light comes on for five hours, as soon as the high light comes on the plants start to pearl. If I use the low light only with my CO2 cranking away my plants grow slow, but only grow rapidly with the high light. This may not be the best way to have a lighting set up, I do it so I can have an extented lighted period for viewing the tank, others may only have their lights on for 5 hours a day with the same result using high light and CO2.

Nature also has some added tricks like dormant periods, then growth is triggered by soil temps, water temps, and added light for longer periods. I would also Guess that plants in streams may do better then plants in ponds due to the nature of a constant flow of fresh water with new nutrients and maybe even more CO2, where a pond may gas out earlier due to heat and low O2, another guess.


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## BlueJack (Apr 15, 2011)

lurrch said:


> If I'm reading what you're saying correctly, the photoperiod (at least in the wild) is much more dependent on CO2 supply than on light. The plants grow for as long as there is CO2 in the water.
> 
> In that case wouldn't my "half as much light for twice as long" scheme work, as long as I keep the CO2 running?


Here's a good article on the relationship between light and CO2

http://www.tropica.com/advising/technical-articles/biology-of-aquatic-plants/co2-and-light.aspx

I wish I could give you a better answer but I'm not entirely sure why that happens. I just now that when I ran my low lights for long photoperiods I got more algae when compared to higher light with shorter photoperiods.


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## BlueJack (Apr 15, 2011)

150EH said:


> I've been using the 24 hour method from the PPS site with fair results using CFL. The 24 hour method goes like this if you have 3 wpg on for 8 hours = 24 or 3 x 8 = 24, or with multiple lighting units 1.75 wpg for 9 hrs = 15.75 wpgpd plus 3 wpg for 3 hrs = 9 wpgpd so the total is 24.75 wpgpd. This may not work for your type of lighting but has worked well for me as a starting point to make adjustments.


Is this the same as the 24 wpg system?

2.0 Wpg 12 hours
2.2 Wpg 11 hours
2.4 Wpg 10 hours
2.7 Wpg 9 hours
3.0 Wpg 8 hours
3.4 Wpg 7 hours
4.0 Wpg 6 hours

I'm using this also and it's definitely helped me find a photoperiod amount. I don't know how it works for all types of lighting but i'm using T8's that = 2.7 WPG.


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## BlueJack (Apr 15, 2011)

plantbrain said:


> The "early plant"....gets the worm .........so to speak........ .


Is this why lakes usually have a few species that dominate them? The plant's with the lowest LCP will grow faster and spread further. I have a pond by my house that's entirely hornwort. There's also a canal by here that seems like it's 100% filled with E. Densa.

What is the evolutionary benefit for having higher LCP's. Thanks


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## lurrch (Jul 5, 2011)

BlueJack said:


> Here's a good article on the relationship between light and CO2
> 
> http://www.tropica.com/advising/technical-articles/biology-of-aquatic-plants/co2-and-light.aspx
> 
> I wish I could give you a better answer but I'm not entirely sure why that happens. I just now that when I ran my low lights for long photoperiods I got more algae when compared to higher light with shorter photoperiods.


That's an interesting article. As I read it, the authors say that even at the same amount of light, increasing the co2 can increase growth (i.e. there's not always a single limiting growth nutrient). I'm not sure whether that supports or opposes my idea though :icon_conf




> What is the evolutionary benefit for having higher LCP's?


Presumably they can use light more efficiently when there's a lot of light. It's like desert plants vs rainforest plants. Desert plants survive at lower thresholds of water, but can't effectively use a large amount of water even if they get it.


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## plantbrain (Dec 15, 2003)

BlueJack said:


> Is this the same as the 24 wpg system?
> 
> 2.0 Wpg 12 hours
> 2.2 Wpg 11 hours
> ...


This is lousy.

We do not need this much light *ever* and w/gal is a lousy comparative parameter anyways. More light just means more troubles for folks.

I have not suggested a w/gal range since the 1990's for this very reason and if so, 2w/gal and no more.

I have long used 1.5-2w/gal for 10 hours or less, according this gibberish above, I'd be way under lighting my tanks, clearly this is NOT the case.
I use a tad under 1 w/gal on one tank.

Micromols of light is far better comparative method. MUCH wiser to use less light intensity. 

It's give a a frame work, but only if the light is all the same otherwise, which is virtually never the case IME. Leads to more myths, not advancement.

Also, plants will grow FAR FAR more efficienctly at lower light intensity, CO2 will be FAR more effective. Tropica: Ole and Troels make this point and I and many others have long made this point over and over and over.......decades now......

Why waste lighting? Why drive things so fast that they lose efficiency? These are biological system and have optimal ranges, adding more and more light and not providing subsequent more and more CO2......then more and more nutrients......is the tail wagging the dog.


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## plantbrain (Dec 15, 2003)

BlueJack said:


> Is this why lakes usually have a few species that dominate them? The plant's with the lowest LCP will grow faster and spread further. I have a pond by my house that's entirely hornwort. There's also a canal by here that seems like it's 100% filled with E. Densa.
> 
> What is the evolutionary benefit for having higher LCP's. Thanks


Yes, both are mean mofros when it comes to the LCP and CO2 uptake.
Hydrilla will do even better.

See this paper and fig 6:

http://www.plantphysiol.org/content/58/6/761.full.pdf

Also, *see figure 2* and time after sunrise for the amount of light at various depths, there's a ton of light right after sunrise in water!!!

That steer manure diagram from ADA is rubbish.
Do not try and sell myths about nature like that, then claim you it's all about detailed study of nature. Selling what meets their marketing agenda, not what is really going on.

This type of stuff gets my goat. Goat roast at Tom's place next week:icon_cool

This is a very good paper, even if it's from the 1970's. I know two of the authors, have not met Thai Van though.


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## plantbrain (Dec 15, 2003)

lurrch said:


> If I'm reading what you're saying correctly, the photoperiod (at least in the wild) is much more dependent on CO2 supply than on light. The plants grow for as long as there is CO2 in the water.
> 
> In that case wouldn't my "half as much light for twice as long" scheme work, as long as I keep the CO2 running?


It would work more efficiently and easier management, yes.

But it's not that simple as the suggestion........you can grow plants at pretty darn low light with excellent color and relatively short days, say 8-10 hours max.

Why waste light and cause more headache when it's not needed?


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## BlueJack (Apr 15, 2011)

plantbrain said:


> Micromols of light is far better comparative method.


I've been thinking of ways to measure my light on the cheap. Do you think getting a $13 lux meter and using lux-par conversions is a good idea? I know lux meters read more green light, but since plants use light from 400nm-700nm almost equally, does having a par meter that reads blue/red light better really matter? The conversion coefficients aren't exact but should give you results within a 10% st.dev. of a PAR meters results. 

This method won't be as accurate as a PAR meter, but will it be close enough? This is a very cheap way to measure lighting, if it's worth a .....

I know sylvannia has conversion coefficients for most of their fluorescent bulbs. Other manufacturers probably don't. Could you use the general .0135 for cool white bulbs, and .014 for daylight bulbs?


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## BlueJack (Apr 15, 2011)

plantbrain said:


> That steer manure diagram from ADA is rubbish.


Yes it is....after seeing fig. 2 in your reference I can't believe companies can do that. It's clear that the LCP is reached within minutes after sunrise. 

Guess I can put them in the politician pile...you can tell they're lying when their lips start moving.


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## 141130 (Jan 25, 2014)

Hoppy said:


> The measure of light that counts with plants is PAR. If one bulb produces a PAR of 10, then two closely spaced bulbs will produce a PAR of 20. "Penetration" is a meaningless term. In water, all light loses intensity at a rate proportional to one over the square of the distance from the source, until you go to over 2 feet of water, then the absorption of light in the red part of the spectrum begins to become significant.
> 
> Completely independent of PAR is the length of the photoperiod. Some plants, if not all plants, will only continue photosynthesis for a limited time. After that the light isn't making the plant grow. If that time is 6 hours, doubling the photoperiod to 12 hours is no more effective than keeping it at 6 hours.


Is there a way to light an aquarium for 16 hours a day without negatively affecting the plants' biological processes; in other words, without stunting good and strong plant growth? I'd like to be able to see my fish all day. What about high-light (T5s, 2 WPG, 8 hours) but with an additional very-low intensity bulb (on for 16 hours)? Can I avoid algae? Would there be other problems?


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## 141130 (Jan 25, 2014)

plantbrain said:


> I have not suggested a w/gal range since the 1990's for this very reason and if so, 2w/gal and no more.


Really? 2 WPG? That'd be 2 WPG of high-intensity light, right? So why do people always insist on 3-5 WPG of high-intensity light?


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## roadmaster (Nov 5, 2009)

BlueJack said:


> That's right...It's not a linear relationship.
> 
> 
> 
> ...


 
I have a single 32 watt T8 bulb over 55 gal and anubia,crypt's,vals,grow just fine albeit slowly which suits me.


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## kman (Dec 11, 2013)

david765 said:


> Is there a way to light an aquarium for 16 hours a day without negatively affecting the plants' biological processes; in other words, without stunting good and strong plant growth? I'd like to be able to see my fish all day. What about high-light (T5s, 2 WPG, 8 hours) but with an additional very-low intensity bulb (on for 16 hours)? Can I avoid algae? Would there be other problems?


I _think_ if you can arrange for 0 PAR lighting that's enough for you to see your fish (colored LEDs and the like, perhaps), this may work.

But I'd love to hear an answer from someone who might actually know the answer.  (especially with regard to algae formation, since algae might be able to better use the 0 PAR LED spectrums?)


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## 141130 (Jan 25, 2014)

kman said:


> I _think_ if you can arrange for 0 PAR lighting that's enough for you to see your fish (colored LEDs and the like, perhaps), this may work.
> 
> But I'd love to hear an answer from someone who might actually know the answer.  (especially with regard to algae formation, since algae might be able to better use the 0 PAR LED spectrums?)


Me, too. I'm surprised that someone hasn't tried to light a tank for 16 hours a day using _additional _low PAR lighting (surely people want to look at their tank all day).


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