Garak, Light meters reading in lumens or foot-candles (since one foot-candle is one lumen per square foot) do show a reading with red, orange, or blue LEDs, but the question is how useful that is for our plant-growing purposes. The lumen is a measure that reflects - no pun intended - human eye sensitivity to various frequencies, which peaks in the yellow-green. A PAR meter, which claims to measure Photosynthetically Active Radiation, is a better - if considerably more expensive - tool. I've even got a couple which calculate the light integral over some time interval for me, which helps a lot when I'm comparing apples and oranges (light spectrum and intensity are both different, and I'm trying to adjust duration to compensate). On the other hand, as others have said, even a cheap light meter will help you quantify how the light is distributed in your particular setup and how it falls off with distance from your source (which is usually pretty different in practice from what the ideal equations would predict for point, line, or planar light sources) and that's very useful. Having said all that tech-speak, one of the best growers of orchids under lights that I know is completely low-tech - she takes no measurements, doesn't worry about spectra or color temperature, works purely from observation and amazingly acute intuition. Steve On 11/5/2015 3:54 PM, Garak wrote: > Do you happen to know if a basic light-meter will work with the > red/orange/blue-LEDs which are designed to only deal light at the > absorption maxima of chlorophyll or will it need light from the full > spectrum? > > > Am 05.11.2015 um 20:46 schrieb Nhu Nguyen: >> If growing under light is something you'd like to pursue, I would >> purchase a relatively cheap light meter for about $30. That will give >> you a more scientific and consistent way to work relative to natural >> sunlight. The edges of shelves away from the cone of light that Kipp >> mentioned have much reduced intensity so they're good spots for >> growing more shade plants. Nhu >