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What are lumens, and are they useful for evaluating grow lights?

Lumens are a measure of luminous flux, or the total amount of visible light radiating from a source, weighted by the human eye's sensitivity to the particular wavelength of the light. Lumens are the best measurement to use when evaluating how well a light will illuminate an area for human eyes. The human eye is most sensitive to light in the yellow range of the spectrum, so 100 photons of yellow light have a higher lumen rating than 100 photons of blue light or 100 photons of red light.

Plants preferentially absorb red and blue light. Lumens preferentially weight yellow light and de-weight red and blue light, making lumens just about the worst light intensity measurement possible for evaluating how well a light will grow plants.

Lumen Weighting (yellow) versus Photosynthetic Efficiency (green):
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Select Spectral Data
Black Dog LED Phyto-Genesis Spectrum®
Relative Photosynthetic Efficiency by Wavelength
Lumen Weighting Function (Relative Spectral Sensitivity of the Human Eye)
PAR range
YPF Weighting Function (Yield Photon Flux)

Lumens' measurement of human-visible luminous flux differs from PAR / PPFD, which measures radiant flux -- the total number of photons in the visible spectrum without weighting for human visibility. Yield Photon Flux (YPF) is like lumens in that photons are weighted based on their wavelength, but YPF weights them based on their usefulness to a plant rather than to the human eye, and YPF considers photons outside of the human visual range.