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while i know lumens aren't useful in deciding what fixtures are adequate for plant growth, the many different K ratings and amounts of red/blue light somewhat confuse me. how much is optimal for different types of plants? for example i'm not sure how much is required for low light plants as apposed to medium or high. (forgive me if this is a ludicrous question lol im a total newbie)
 

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Light is measured in a number of different ways.

K is a value that tells you what color the light will look to our eyes, and is a blend of several wavelengths. It does not tell you which wavelengths are present. It is reasonable to select house or office lighting based on a K value, but it does not say much about if this light is good for plants.

PAR is Photosynthetically Active Radiation and is the best way to select the correct lights for plants (in aquariums, green house or other).
This is an actual measurement of how much radiation is in the optimum range for the plants.
Plants use a certain narrow part of the red and a certain narrow part of the blue parts of the spectrum a lot more than they use the rest of the light. They do have color pigments that can help them use the other parts of the spectrum (yellow, other parts of red and blue), but they use these a lot less.
Here is a link that includes 2 charts (scroll down). The upper one, like a rainbow, shows all the colors plants can respond to. The lower one (colored lines) shows the most active plant pigments, the most important ones. Note how plants do have some reception of the whole spectrum (per the upper chart) but the most active, most important pigments really only use a bit of red and a bit of blue, the rest of the spectrum is close to zero (lower chart).
When you get a light that is really targeted toward the plants it will look sort of purple-pink, and may make the animals in the tank look funny. To make the tank look good to us, and have a high PAR rating for the plants I generally use 2 or more bulbs. One is a plant specific bulb. The other is a bulb that may be sold as "Daylight" or "Cool White". To me, this combination makes the fish and other things in the tank look their correct colors.

https://smartgrowtechnologies.com/l...s-are-revolutionizing-horticultural-lighting/
 
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