Subject: Light meter
From: [email-masked] (Gordon T. Uber)
Date: Sun, 26 Jun 1994 18:26:36 GMT
Newsgroups: bionet.plants

Per Allen Stimson "Photometry and Radiometry for Engineers",
John Wiley & Sons, 1974, p. 309 ff:

Ev = log2 (ES/C)
where:
Ev = Exposure Value
E = scene illumination in foot candles
S = film speed
C = constant = 22 +/- 7 for 35 mm cameras and larger

Reference: 
American National Standard, General-Purpose Photographic Exposure Meteres
(Photoelectric Type), Ph3.49,
American National Standards Institute, New York.

Caution: The human eye and light meters (by definition) respond most
strongly to yellow-green light (555 nm). Plants respond most to red and
blue light.  Hence the spectral distribution (color) of the light may
need to be taken into account.

Hope this helps.

Gordon