Plants & lighting

Measuring Light for Humans:
Lumens and Lux

First, how do we measure light quantity for humans? The obvious way is based on how bright the source appears and how "well" the eye sees under the light. Since the human eye is particularly sensitive to yellow light, more weight is given to the yellow region of the spectrum and the contributions from blue and red light are largely discounted. This is the basis for rating the total amount of light emitted by a source in lumens.

The light emitted from the source is then distributed over the area to be illuminated. The illumination is measured in "lux", a measurement of how many lumens falls on each square meter of surface. An illumination of 1000 lux implies that 1000 lumens are falling on each square meter of surface. Similarly, "foot-candles" is the term for the measure of how many lumens are falling on each square foot of surface.

Clearly, both lumens and lux (or foot-candles) refer specifically to human vision and not to the way plants see light.

How then should the rating for plant lighting be accomplished? There are two basic approaches to develop this rating: measuring energy or counting photons.


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