Source: From Apogee Instruments
PPFD to Foot-candle Reference Table
Definitions
Photometric Units, Illuminance
Foot-candle: one lumen per square foot. The 16th General Conference on Weights an Measures (CGPM), Oct. 1979, decided that the candela is the luminous intensity of a source emitting monochromatic radiation of frequency 540 x 1012 Hz and radiant intensity 1/683 watt per steradian. This corresponds to 683 lumens per watt of radiation at approximately 555 nm wavelength, which is near the maximum of the standard photopic spectral luminous efficiency curve.
LUX: one lumen per square meter; differs from foot-candle by about a factor of 10.
Quantum Units, Photon Flux Density
Microeinstein: per second and square meter (µE m-2 s-1). The einstein has been used to represent the quantity of radiant energy in Avogadro’s number of photons and also Avogadro’s number of photons. The second definition has the einstein equal a mole of photons. While commonly used as a unit for photosynthetically active radiation (PAR), the einstein is not an SI unit.
Micromole: per second and square meter (µmol m-2 s-1). This term is based on the number of photons in a certain waveband incident per unit time (s) on a unit area (m2) divided by the Avogadro constant (6.022 x 1023 mol-1). It is used commonly to describe PAR in the 400-700 nm waveband.
Definition Source: Thimijan, Richard W., and Royal D. Heins. 1982. Photometric, Radiometric, and Quantum Light Units of Measure: A Review of Procedures for Interconversion. HortScience 18:818-822.
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