# Obtaining White Light

2 replies to this topic

### #1 Malik Sajid

Malik Sajid
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• Lahore, Pakistan

Posted 13 December 2008 - 06:18 AM

As a student we usually work with tungston lights, mostly 500 watt and 1000 watt. I understand the light color they produce. I want to know how can i achieve the white color with these lights. Should i apply some kind of jell on them? What i normally do is put a butter paper on it, which makes it somehow whitish yellow, and reduces the light intensity as well.

What things should i keep in mind to produce white light?
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### #2 David Mullen ASC

David Mullen ASC
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• Los Angeles

Posted 13 December 2008 - 11:36 AM

"White" light is just when the color of the light matches the color balance of the film stock (or video camera). But normally it doesn't have to be a perfect match because you can color-correct the film stock or white balance the video camera, etc. So if your light is 3000K and your camera is balanced for 3200K, then that little difference is either acceptable or it can be fixed.

But in real world situations, your light isn't the only light in the scene, so the question is whether to match your light to the surrounding lights or vice-versa. But often real world tungsten practicals are warmer than 3200K, so you often add a little warming gel to a movie tungsten lamp to match the warm look more closely.

But yes, if for some reason your light gets too warm for you, you can compensate by using CTB (Color Temperature Blue) gels, usually something light like a #1/8 or #1/4 CTB.

If you want to be precise, you need to learn something about the MIRED system, since color temp numbers themselves are not linear in effect. Then you can figure out the MIRED shift number needed to correct, let's say, 2800K to 3200K, and then find a gel with the same MIRED shift number, or as close as possible. See:
http://en.wikipedia.org/wiki/Mired

You divide 1,000,000 by the color temp to get the MIRED value.

So 1,000,000 divided by 3200K gets you 312.5 and 1,000,000 divided by 2800K gets you 357 -- so the difference is 44.5 MIRED's. You would need a +44.5 MIRED shift to lower 3200K (312.5) down in color temp to 2800K (357), and you would need -44.5 MIRED shift to go the other way.

So you look through your handy Rosco or Lee gel swatch book and see that a Rosco 1/4 CTO has a MIRED shift of +42 (and so does 1/4 CT Straw) and to go the other way, to correct 2800K to 3200K, we see that a 1/4 CTB has a MIRED shift of -30 and the hard-to-find 1/3 CTB gel has a MIRED shift of -49.

Anyway, in the real world, it doesn't have to be perfect, you just have to get in the ballpark. And as gels age and fade, they lose some of their correction strength. Only color-correction gels have their MIRED values listed, otherwise most gels only have their transmission value listed.
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### #3 Malik Sajid

Malik Sajid
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Posted 15 December 2008 - 08:42 AM

hmmmmm......got it
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