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Re: foot-candles, lux, lumens, sunlight, PAR
I forgot to include the following useful information which was
originally posted by Mark Fisher on 11 Nov 96 in the APD. This will be
extremely handy to anyone trying to estimate natural light intensities
TO MEASURE LIGHT LEVELS CHEAPLY:
Do you have a 35mm camera with a built-in light meter? One of the
"older" kind, in which you manually set the shutter speed, ASA or DIN
film speed, and hand-adjust the f-stops?
I found an approximation for converting readings from a camera's light
meter into foot-candles (Source: Greenhouses--planning, installing and
using greenhouses. Ortho Books, 1991).
Set your camera to ASA 25 and 1/60 shutter speed. Aim and focus the
camera so the object you want to measure fills the viewer. Adjust the
f-stop until the needle indicates the correct exposure.
If the object you are metering is WHITE, then:
If the object you are metering is GREEN (like a plant leaf), then you
must increase the f-stop by two settings to get the correct amount of
light (e.g., if it reads f5.6, add two f-stops, resulting in f11 and
A camera's light meter only reads reflected light, so if you measure
from something white (e.g., 90%+ reflectivity), you are getting a pretty
good measure of the amount of incident light. Green plants reflect much
less light (about 20%), so you must adjust your f-stop accordingly.
Steve (credit to Mark Fisher)