When using led lights we need a new thinking about how we measure the spectrum for the plant to use. The typical rating familiar to most growers is "lumen". Lumens are defined as the total light produced within the scope of human visual response. It does not tell us the distribution of this light energy in the spectrum, and most importantly, it does not tell us how much is useful for plants.
Lumens are particularly problematic when measuring light at the far end of the human visual response curve. Consider three lights -- red, green, and blue -- that emit the same amount of light energy per lamp. Compared to green lights, the red and blue lights have a much lower lumen rating, for the simple reason that red and blue people have a low visual response and green lenses have the highest ratings. This is why high lumen ratings do not necessarily make the lamp more suitable for planting. Similarly, the light meter measured in "Lux" rarely tells us about the plant's ability to grow. The light sensor in the Lux meter has its own spectral response curve, which may over or underestimate the light in various colors. This is why Lux meters usually have different settings for "sunlight", "fluorescent" and "incandescent". Again, because the lux table is used to measure the amount of light available to humans, they do not tell us how the plant will react.
PAR Plant biologists define light in the 400 nm to 700 nm spectral region as "photosynthetically available radiation" (par). The unit of measure, micromoles per second (μmoes), indicates how many photons per second fall on the plant in this spectral range. The cheap par meter uses a sensor that responds across the entire 400-700 nm spectrum and has its own sensitivity curve that requires different calibrations for sunlight, fluorescence, and hid illumination. All of these systems have a wide range of responses and cannot measure the narrow emission spectrum of led. They make the hid light look brighter by over-measuring the yellow-green light, and make the led light look darker by measuring the lack of red and blue light.
In order to properly measure the energy required for photosynthesis, we must use a spectrometer. The instrument measures the energy (in watts) for each specific wavelength over a range of wavelengths. Spectroradiometers can directly compare the ability of each lamp to produce light that plants can use for photosynthesis. Spectrometers are expensive instruments and are usually not found outside the laboratory. (A more common instrument called a spectrometer can display relative light output over the spectral range, but does not measure energy in watts.
PPFD PPFD is the photosynthetic photon flux density. PPFD measures the number of par actually reaching the plant, or, as scientists might say, "The number of photosynthetic active photons that fall on a given surface per second ". PPFD is a "point " Measurement of a specific location on the canopy of a plant, measured in microns per second per square meter (μmom/m s).