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Alain J Francois

Illumination calculation

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The answer to this question seems to elude me in a most vexing way.

Given a desired aperture (say T/5.6) and an ASA rating (say 125), I know how to calculate the required FootCandles - answer 200 Fc.

What I would like to know is how to calculate the amount of lamps, given the known wattage of each lamp required to produce that amount of 200 Fc. (I do realise that it is a mistake to consider wattage as output - it is a measure of lamp consumption), however there must be some calculation or simple relationship that would allow me to estimate the amount of lamps I am going to need for a particular scene. :unsure:

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The output of any given lamp can vary wildly depending on many variables, like par vs. fresnel; flood vs. spot; clean vs. diffusion; efficiency of the reflector; age of the bulb; and so on.


There's no practical way to calculate this. What you do instead is get familiar with the behavior and quality of the different lamps, and make an educated choice when to employ them.


You can look up the photometrics (light behavior) of many lamps in the book, Set Lighting Technician's Handbook, and sometimes on the websites of the lamp manufacturer.

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