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dono where T2 fits
T5HO vs LED, where lumens rates, ...
T5HO beat LED arrays of the same size in watts per lumen. (what people see)
unfortunately this doesn't mean anything with PAR or PUR (what the plants use)
If youur looking at Data from 5 years ago this is correct but just in the last year LEd's have come a long way. The quality LED manifactrers are reaching 120 LUMs per watt on regular bases now for almost a year now. They are also projecting that the next generation which should be out mid this year could be exceeding 200 Lums per Watt.

On my reef tanks I run roughly 1/2 the wattage of LED light that I do with HOT-5's and my results are very simular to slightly better growth and much better color with the LED's.

for Flouresent lights and their color spikes, ... the visible light part of the electromagnetic spectrum includes red, orange, yellow, green, blue, indigo, violet (ROYGBIV), the human eye i think really makes all it's colors based off red, green, blue, being more sensitive towards the green/yellow.

we see orange just fine, based on how much red & how much green and the brain interprets the rest. (i'm going by really really old memory, so it might be beyond fuzzy)

as for "white" light, ... it appears white for the same reason, ... the balance of everything, a generally balanced whole, but it's a balance. if the reds and the blues are heightened or diminished equally things will still appear "white" to us. just dimmer.

it's like scales, if the right and the left aren't balanced it tends to shift one way or the other

spikes are somewhere between intentional by design and intentional for marketing.
plants don't make much use of green, but our eyes do, and without that spike it's going to look dark to us regardless of the benefits to the plants, and marketing is scared of loosing sales.
You are correct in this aspect however you are overly simplifing it. What the eye sees is reflective light off the object that gives it its color. If the surface of an object reflects light at say 500 nm and there is light hitting the object at 500nm it has no light to reflect back at us. You can create a white light with 3 LED's peaking at 470nm, 550nm and 640 nm that will look white to you reflected off a white peice of material but if there is an object that only reflects cyan light at 500nm it will look black to you since it has received no light it can reflect back at you.

for google searches on light spectrums of various bulbs, (5500K and higher) they can nail the blues, and the spikes the plants crave. the reds the plants want, ... it seems like they're all shifted, slightly towards yellow/orange.

a google search for a 6500K bulb

this has a balanced spectrum, but i notice the sides tend to be a little less than the middle. for everything i've read, this would give a good balance and be very bright, ... well bright to us, the reds & blues as they've taken a hit, and as these are the wavelenths the plants crave more, ... it's a good bang for your buck if you want to light a room, but for your plants, ... your money could be spent better elsewhere
Yes there is some truth to this. Very little light is utilized in the green spectrum for photosynthesis. Most of the green light is not absorbed by the plants but is reflected back at us which is why many plants look green to us. You could create a light source especially with LED's that has peaks just at the wave lengths that are utilized by photosynthesis and be able to get in thory a light that is extremely efficient. You could even individual tune this light for a specific plant only creating light that it absorbs for photosynthesis.

However if your not giving it any light that it is not absorbing then it has no light that it can reflect back at us. How do you think the human eye would see this plant? It would appear black or near black with only the excess light it could not absorb reflecting back at us.

I say this in theory as most plants also have a low level of florescence in them. You can see this florescence in corals where you might light a reef tank with light at 455nm and the result might be a bright red coral standing out as it absorbs the 455nm light and emits 660nm light. Many fresh water plants also do this but to an extremely less extent.

The bases of goo reef lighting is to provide enough blue light 400nm to 500nm to make the corals grow than add enough full spectrum light to make the observers eye happy. For fresh water plants it is simular only the red and the blue light are important.

When I was heavy into planted aquariums I found that the best combination of lighting was a combination of Chroma 50, 5,000K lights and Daylight 6,500K lights. I had truied lower K rated bulbs but found with them it was nearly impossible to keep algea under control because of abundance of red light.

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The most common mistake people make with planted tanks is to not understand photosynthesis and the visible spectrum of lighting that affects plant growth.
Wow! This was an awesome collection of science info clustered around solving problems of planted tanks and generally about designing light sources for plants. Thanks and really this is one of the best collections of info on the subject--I've read quite a few over the last year.

PS--ignore the bullies and their attacks disguised as "helpful comments". As a teacher of 27 years I recognize bullies and I recognize deep understanding. Please keep participating in forums, we need your knowledge.

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7,436 Posts
........ Very little light is utilized in the green spectrum for photosynthesis. Most of the green light is not absorbed by the plants but is reflected back at us which is why many plants look green to us. .....
I know this is a pretty old post, but I do think it should be corrected:

If you look at the illustration in the first post here, that shows the spectral range for lumens, which are intended to be close to normal human eye spectral response, you will see that our eyes are most sensitive, by far, to green light. And, much less sensitive to red and blue light. So, if we were to look at light with a perfectly flat spectrum from 400 to 700 nm, we would most likely see green light. So, the fact that we see plants as green is mostly because that's the color we see most intensely.

It is true that plants use more red and blue light than than green light, but they use more than a trivial amount of green light. Complicating this is the fact that we see a variety of spectral mixes of light as green, not just light of a wavelength associated with green.

To standardize our data on light, I think it is best to just use photosynthetically active radiation intensity as our measure of light (PAR - miicromoles of photons per second per square meter). No method within reach of hobbyists allows us to measure that with great accuracy, but even very cheap PAR meters are useful for judging how intense our aquarium lighting is, and the standard Apogee meters are even better, but still no where near perfect.

The light intensity in an aquarium varies widely, as much as from 20 PAR at the center, at the substrate to 100 PAR at the water surface to 10 PAR at each end of the tank at the substrate. So, to be useful I like to specify light intensity by the PAR at the center at the substrate level, and just accept the variation as the way it is. That lets us use a simple and useful set of PAR readings as a measure of low light, medium light, high light. And, we can define what low, medium and high mean several ways.
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