Kelvin Shift Calculations
Published : 7th February 2005
I'm playing around with some 6 watt triphosphor fluorescent tube fittings that link together and would be very useful for 'in car' shots I have coming up on a 'no budget' shoot. A sort of micro flo without the expense idea. Scene is night, wide shot, sea front lights ...lots of them… and it would be nice to add a 'tickle' to the faces in the car, motivated from dashboard.
They are balanced to 3400k and I'd like to play with the colour temp. Is it a simple case of full CTO/CTB delivering shift between 3200k and 5600k equating to 2400k and therefore 1/2 = 1200k, 1/4 = 600k and 1/8 = 300k shift?
Subsequently how would you get 200k shift from 3400k to 3200k?
Also, I'm sure there is a very good reason Kino Flo's (and other fluoro systems) do good business but would someone with a mind for such matters mind explaining how it works and why we can't / don't / won't use domestic fittings?
Thanks in advance
It's better to work with MIRED shift values when trying to determine how to shift a light from one color temperature value to another. Correction gels often list their MIRED shift value in the swatchbook.
You divide 1,000,000 by the Kelvin degrees to determine the MIRED value, so 3400K is 294 and 3200K is 312. The difference is +18. A 1/8 CTO gel usually has around a +20 MIRED shift, so that's the closest gel.
Although you probably could have done this by eye, particularly for a night driving scene where "white" light is pretty rare anyway.
The other issue is whether your fluorescent needs any Minus Green gel...
David Mullen, ASC
Some links about Mired Values :
EI Customer Technical Services
Eastman Kodak Company