E-mail - What is a Lumen, and How Much Do I Need

Hey Doc,

    How many lumens per square foot should I have in my garden? What is a lumen anyway?

A lumen is a unit to measure light coming from an angle or a source (like your grow light). A lux is another unit of measure, it takes into consideration area. It is complicated, but basically the larger the area you consider, the lower the lux you get with the same amount of lumens. Lumens are the output, lux is a measure of that energy over an area. There are two types of lumens, luminous flux which measures light that is visible to the human eye, and power or radiant flux, which measures total output. If you are going to measure your grow lights in lumens, you want to use radiant flux. Plants and the human eye both have pigments so we can see color, and plants can absorb lights energy. We have different pigments though, so comparing what we ‘see’ as color, to what plants need to drive photosynthesis is, well comparing apples to oranges. It does not correlate to good growing!

I do not like the lumen (or lux) for measuring light for plant growth. One reason is that lumens would include green light in its measurement. However, plants do NOT absorb green light, they reflect it. So, you could have a grow light that emits a good percentage of its energy in the wavelength of green light, and it would register a high lumen output, but the electricity you are buying to run this high green output light is not available for plant growth.

Good growers may want to read my post comparing light intensity of MH and HPS