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Lighting Theory of a Planted Aquarium- RQE/PFY- PAR, PAS, & PUR


Updated/Revised: 9/19/15

Planted Aquarium Lighting- Relative Quantum Efficiency vs. Photosynthetic Action Spectrum

What light quality energy is "useable" for plant photosynthesis or what's the Photosynthesically Usable Radiation for planted and coral aquariums?

This topic has long been discussed in Aquatic Sciences for about the last 30 years. The idea of what light might be useful started with studies published in Plant and Aquatic Science journals, which described the Absorption Spectrum(1) of light used for Photosynthesis. The spectrum mostly in questions for Photosynthesis, was known the known Action Spectrum of both Chlorophyll a and b, as Chlorophyll is the most abundant plant biomolecule, which drive the process of converting light energy into chemical energy for a plant. Chlorophyll is critical for plants to be able to capture light energy for the energy conversion process.

Calculation of the absorbed Chlorophyll specifically has been known as PUR.

[Reference 1]- Chlorophyll a Fluorescence in Aquatic Sciences: Methods and Applications
[Reference 2]- Photosynthetically Active Radiation/Absorption Spectrum

(1). Absorption Spectrum- The spectrum of electromagnetic radiation that has passed through a medium that absorbed radiation of certain wavelengths. The spectrum used to measure absorption where various wavelengths of light represent different colours of light absorption spectrum.
[Reference 3]- Biology Dictionary

This can "differ" plant to plant as to what is specifically needed, but as of today's science, we have a pretty solid understanding of what makes Photosynthesis work for plants.

Photosynthesis is what makes a plant grow mainly through the chemical process of Chlorophyll. While for the longest time the light in the Action Spectrum of Chlorophyll a and b (blue and red) were thought as the only "useable" energy plants needed for photosynthesis, recent research has shown if green light is present, it will be used and converted into energy in the photosynthesis of Chlorophyll a/b.

For this reason, green light has also been considered as "useful" energy. This is how some graphs showing an Absorption Spectrum, does show some amounts of green being absorbed. (See above graph with the PAS graph of Chlorophyll A&B/Carotenoids, along with the Absorption Spectrum).

We can also actually see how much green is is being utilized in the RQE Mcree 1972 graph, which is about around 70%. Meaning if green is present, most will be used and only about 30% will be reflected. This when when we are only talking about a green plant. This however does not prove totally efficiency based on both recent and older real world studies, but it certainly is a good starting point.

[Reference 4]-Plant Physiology- Light

When considering green as a "useful" energy of green plant photosynthesis, the most common term describing this useful energy is different than the most "useful" energy captured for Chlorophyll. When maximum amount of photosynthesis per incident unit of energy is considered, the term used is Yield Photon Flux.

Here is how it's described in some current research describes this process:

This is when considering the Mcree graph and saying about 650nm of light equals one unit of co2 assimilation and 450nm will be about .7 of co2 assimilation. See reference for more of what thought of be active/inactive & useful/nonuseful energy.
[Reference 5]-Accuracy of Quantum Sensors Measuring Yield Photon Flux and Photosynthetic Photon Flux

[Reference 6]-Photosynthetically Active Radiation/Absorption Spectrum

Section Summary

The PAS graph shows the two spectrums absorbed for the plant growing process of Chlorophyll a & b. The RQE/YPF graph shows the total light used through the entire Photosynthesis process, as oppose to the light, which starts the action of Photosynthesis.

Blue and Red light is responsible for the action of Photosynthesis. Greens and yellows are not used in this process as seen in the PAS graph.

Green and yellows are shown in RQE/PFY Mcree graph to be used in the whole process of Photosynthesis.

People will think with RQE/PFY, if green or yellow light are the only color light available to the plant, Photosynthesis cannot take place as these colors do not provide any photosynthesis action. In fact, this light slow Photosynthesis if provided in higher amounts.
[As depicted in the first picture of this article]

We do know now, that even if a plant is grown under all green light, it will grow, it will just use have less of a grow rate. [Reference 7]- Influence of Green, Red and Blue Light Emitting Diodes

We also know different amounts of Blue, Green , and Red can affect different growth rates and mass of the plant (as in weight or more leaves).
[Reference 8]- Green light: Is it important for plant growth?

This is how blue and red light from the PAS have been considered "Photosynthesically Usable Radiation" in the past aquatic sciences and much talk about this subject involved increasing or decreasing the amount of blues and reds to see how plant growth is effected. Current research will now also consider how much green light is actually useful to improve growth or harm growth.

Affecting Usable Radiation

Energy waves excite photons and the amplitude of a wave is generally a measure of how energized the photons can be carries. In the same way, the displacement of a bow string, determines the amount of energy that propels the arrow. Energy is a positive quantity, so the amplitude is a positive number.

This matter has to be considered when thinking about what most useful for a water application such as an aquarium and also what is considered useful for the application such as a reef tank or planted tank. When speaking about light in air, the wavelength frequency does not matter as much as we can use a red spectrum having the highest usefulness of photosynthesis, with a less intense frequency. In applications where light needs to provide a more useful amount of energy to deeper water, blue light is considered "more useful", because of it's frequency intensity.

The blue light is required, because it has a higher frequency of photons (not higher quatity though), which will reach deeper in the aquarium for the plants or corals (zooxanthellae) to use for photosynthesis. Corals are known to require more energy to grow, so blues are "useful"/"Important", the right "quality spectrum", with the needed amount of energy for corals. Zooxanthellaes in corals have also shown to be adapted to blue light and all photosynthetic creatures are able to adapt the light being provided to them.
Affects of intensity can also have an affect on useful light, as in depth, as in light delivery such as a lens, spread, dissolved solids, salts, movement, or blockage (clouds or shade).

In most aquatic application, blue light has been best for useful light for both corals and plants due to the frequency of the photon. Better is the combination of blue with red, and now with current science, green is useful because since its frequency can also be used under water, albeit at a lower rate. These three colors together make up a "white" light.


Blue light has the most penetrating power of all the light, because it's more energized excited (dense wavelength) for more protons to be delivered, therefore it will provide more useful growing when water is considered. Red is the first color to filter out. Blue light does helps keep plants fuller with more leaves. Higher blue concentrated energy is needed for corals. To much blue can stunt growth.

Green does help green plants look brighter as it does have the highest value for how much is reflected back (30%) according to the RQE Mcree graph. It's also the color our eyes see best (CRI), so lamps with more greens in their spectrum will appear brighter. Amounts of green in white light has shown to help with thicker growth (more mass). Usually more is not required, unless it's for extra visual boost.

Red has shown to be very most for Photosynthesis in air. This color does have to be balanced for appearance reasons. Reds also help red plants look redder, but is not what causes the plant to actually be red. The boost in color is what is being reflected to the eye.

This information has not been considered for the photosynthesis of red plants and this information still needs to be researched.

Some green plants can turn red, which is almost like a Sunblock as they have to adapt to intense energy.

A light photon is a photon. There is no blue photons, green, or red photons. Blue deliveries more photos in their wave as it's more energetic.

Kelvin Spectrum

This is the light, which is more of a visual rating, based on the color a black radiator being heat up to a certain temperature. This is like the Sun as it heats up and is a certain color. This is not a measurement of how useful of light will be, but what color the light will be.

The Sun will be 6500K at high noon hour.

White lights, weather it's a cooler white or a warm white, will have more or less blue, green, and red. Warmer whites such as 3000K will be closer to the red side of the spectrum with much more reds, yellows and greens. While this may have more reds, there is less intense blues for aquarium application.

Spectrum of a 3000K Warm White Light

Cooler or natural Whites closer to 6500K + will be closer to the blue side of the lighting spectrum, with less greens and yellows based on design. 6500K has been described to be the best growing spectrum watt per watt of energy used to create the light, as it has the best balance of useful energy.
It is worthy of note that the licensed Cree XB 6500 emitter used in the AquaGro GroBeam 1500 & 600 has a spectrum that very closely mimics the RQE Mcree graph without the need for added emitters.

Product Resource: AquaGro GroBeam 1500 & 600

Spectrum of a true 6500K Natural White LED

A spectrum higher than 6500K (7000-10000) will be lacking in the required reds, so the addition of a red light may be used if considering photosynthesis, but not necessary.

Higher rated blue light such as 10,000K will be used for deeper applications above 20-24 inches, as it penetrates best.

How much greens and yellows are in a Natural White 6500K will vary based manufacture. These two colors in this lighting is more than enough to help feed the photosynthetic process in plants. Added greens and yellow/orange can be added energy for a visual appeal reason. They do add growing power as well.

How this conversion can gets so debatable and how it has been a popular topic in the aquarium hobby, is because of the effort of trying to define what is "useful" for an aquatic plant. Even the most current definitions found from people who are well known in this area are not concise as to what is "useful/needed". Statements such as this are made: "PAR defines the type of light which is needed to support photosynthesis in plant life".
And "PAR, ... solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis, make this topic not clear"..."plants do NOT however use the green light which is simply reflected - which is why we see plants as green."

Today's train of thought is simply to provide all the light in the PAR range, which is why people say a "full spectrum light is needed". What's important to try and do, is balance how much blue, red, orange, and how much green, yellow they want for appearance and just make sure there's enough growing power of blue or red in particular based on known studies and real world applications.

If lighting fixtures are using color light sources, one should ask for what reason and try to understand what color (blue, white, red) is being used for the actual action of photosynthesis. While appearance is important, what can happen is people overly using color heavy in everything but blue or red, then loose growing power for the plant. Plants will have great coloration, but lack in any growth rate.

No aquarium lighting fixture will strictly use highest "energy" lights, since what is being lighted is an aquarium and visual pleasure does come into play. There has to be a balance of growing energy and coloration, and energy used to create the light needs to be considered. This idea of "usefulness" has clearly been seen in our florescence technology, where we went from T-12 to T-5 to T-2 and used less watt energy to create more intense growing light.

It's often harder to apply this "usefulness", now that most aquarium lighting has shifted to LED. There are many variations in emitter bins, combinations of bins, and how the fixture is design, which detract "usefulness".
However while a photon is a photon, they do differ in frequency and we do know that different frequencies have different penetrating abilities in water, plant & algae tissue, even human tissue. This still translates into certain wave lengths being more "useful", there simply is not a huge gap.
This also does not change the input side of energy; the joules or watts as much can wasted as heat long before it becomes a photon of light due to poor drivers that daisy chain emitters, as well as the use of linear or analog reduction (aka current reduction) instead of the more efficient PWM.

So, how much blue, green, orange, and red energy is needed for a planted aquarium? This is very subjective and will need another article to go over the most popular combinations of color along with other factors, which can affect usefulness and what is "best".

If we can strictly apply useful energy, what is create is the best possible growing spectrum at the least amount of energy used. Newer research has proven that we can't just look at input energy, but also the quantity of photons and energy these photons can supply based on the frequency of these photons.

As for frequency of photons we do know based on studies already cited earlier that green light can and will grow plant life, just less efficiently. In fact as much as 80% less efficient than blue light based on one US government study.
The picture to the left demonstrates another study from about 2007 where the growth rate is quite different between two different 6500K light sources. Even after one subtract the the considerable amount of energy waster as heat by the Metal Halide light, we can see there is still a higher amount of lumens/photons from the MH light.
This leaves us to figure the difference in photon frequency/energy levels is what is making the difference in growth, possibly more blue energy, but since we do not have a spectrogram/spectrograph we can only speculate as to what spectrum is making the difference. But what we do not need to speculate on is there still is clear difference in "quality of light".

[Reference 9]- Real World Application of RQE, PAR. PUR, PAS, & Photons

Further Reading:

From this website
PUR/PAS vs PAR in Aquarium Reef/Planted Lighting; LED Wavelengths

Aquarium Lighting; Guide to Complete Facts & Information

St Mary’s Marine Biology LED Experiments

By Devon Trigg with editing by Carl Strohmeyer
Copyright 2015

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In Chronological order of writing with the newest at the top

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  2. Whirling Disease in Fish
  3. Reef Aquarium Chemistry Maintenance
  4. Use of RO, DI, Softwater in Aquariums
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  7. Tuberculosis in Fish
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  9. Head Pressure in Aquarium and Pond Water Pumps
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  11. Angelfish Virus/Aids
  12. Activated Carbon
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  25. Basic Fish Anatomy, Fin Identification
  26. Aquarium Moving/ Power Failures
  27. Octopus as Aquarium Pets
  28. Aquarium Nitrates
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