Light Unit Conversion Table

 *These are general conversion factors. Some conversions may differ based on lamp type and brand*

Unit of measure Type of measure Mainly used by Sunlight HPS Metal Halide
Footcandles Visible (human eye) Industry (US) 1 1 1
LUX Visible (human eye) Industry (Europe) 10.76 10.76 10.76
umol.m.2s1 of PAR Quanta of light in PAR range Horticulture research 0.02 0.013 0.015
moles/day (PAR) "daily light integral": accumulated PAR light during an entire day Horticulture research x hours of light footcandle x 0.00071 x hours of light footcandle x 0.00047 x hours of light footcandle x 0.00054
Watts/m.2 (total energy) Total energy Engineers, research 0.101 0.074 0.089
 
Compares units so that you can make conversions to units with which you are familiar. The horticultural industry measure light in footcandles. This measure of visible light (i.e., light visible to the human eye) does not exactly correspond to the spectrum of light energy used by plants in photosynthesis (the range of 400 to 700 nm wavelengths, termed "photosynthetically active radiation or PAR")

 

Definitions
Lumen: A measure of light given off by a source. One lumen equals the amount of light emitted by one candle that falls on one square foot of surface located one foot from the candle.
Footcandle: A measure of radiant light from a standard candle that falls on one square foot of surface area at a distance of one foot.
LUX: A measure of radiant light from a standard candle that falls on one square meter of surface area one meter from the source.
Micromol: One micromol per square meter per second (umol.m.2s1). A unit of measure of the amount of light hitting a surface that is in the range of 400-700 nanometers.
WM.2: Watts per square meter (WM.2). A unit of measure of the amount of light energy hitting a surface that is in the range of 400-800 nanometers.