Sort of a techy question

alwayslearning

alwayslearning

Well-known member
I'm thinking along the lines here of the inverse square law. Suppose I have a 500 W lamp. At 2 meters, I have x amount of light on the subject. Double that to 4 meters and I'll have 1/4x. If I want to double the distance as just mentioned, but keep the luminance on the subject the same as at 2 meters, do I need to double or quadruple the wattage of the lamp. I'm not sure how or where to find out about wattage of lamps as pertains to luminance.

Thanks for any assistance in this and please ask questions in helping to answer if I'm not clear in my question.

Larry
 
Quadruple the wattage. Or, change the beam pattern so that the light isn't spread over a uniform area, but if you tighten up the beam (by spotting in with a fresnel, for example) then you will maintain more of the footcandles hitting that same surface area.

Wattage per luminance is not easily researchable because it all depends on the type of fixture. A tungsten lamp has a different light output per watt than an HMI or a fluorescent or an LED. And then, even in the same lamp family, it'll vary based on the type of fixture you're using; an open-face can put out about 1/3 as many footcandles as an equal-rated fresnel lamp, and a PAR can do many many more, etc.

But assuming you're using the same fixture and the same beam pattern, yes, double the distance means you need 4x the watts to deliver equal footcandles to the subject.
 
Thanks Barry. Reading your reply made me wish I'd been more specific for your convenience. Let me explain ....

I'm involved in some aspects of a studio construction. This question applies only to overhead, tungsten light. The ceiling follows the roof and the roof is the roof of a barn that is being converted. On one side, along the length, there will be two rows of lights. The lower row will be about 10 feet above the floor. The next row in (hope that makes sense) will be about 16 feet above the floor, but we want the same illumination in the area. So the higher up fixtures will need higher wattage lamps. I didn't know if the wattage/lumens thing was linear (e.g. 100W lamps in the "low" row and 160W in the "high" row) or if it was something else.

Just typing this makes me feel like it's almost better to just experiment ......

Based on what you said though, I may be able to figure it out.

Larry
 
1.4x the distance means you'd need 2x the watts. The increase in wattage necessary is the square of the distance. So no, you wouldn't go with 160 watts, as that's only a 60% increase; you'd instead need 2.56x as much light, so if you went with 100w lamps in the low row, you'd need 250w lamps in the high row... assuming, of course, that all other things are equal (same type of lamps in the same types of housings, same reflectors, same lenses etc).
 
You're not only a genius Barry but you're stinkin' fast! :)

I just came back to tell you I "got it" and found your better articulated reply. Thanks. That is exactly what I needed.

Have a great evening.

Larry
 
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