Originally posted by Photodana Hello Steve. The reason is a topic that has emerged in social media - how to choose which gel (color filter) to put on a flash, in order to color balance ambient light and flash light.
Most flashes are balanced to achieve light at appr. 5500. Knowing the Kelvin value / color temperature of the ambient light helps to use the right gel for the flash CTO or CTB:
Flash Photography Tutorial : How to use Color Correction Gels with Strobes - YouTube Oh! I guess that explains the sudden surge of interest in this. I am familiar with the video, but was unaware of any buzz associated with it. Most of what he says is very helpful, though he is a little fuzzy regarding color temperature and emittance spectra and also on what can be done with RAW capture. I don't know how many camera brands provide a Kelvin equivalent for manual/custom white balance. Sony (in the video) does. Nikon, as with Ricoh/Pentax, does not. If Canon does, it is not clear from their manuals, at least not for their higher-end product.
An option might be to iterate through available Kelvin degree WB settings until you get one that provides a nice balance to the ambient light. (Put a gray card in the frame for reference.) You can then use that value to assist in your gel selection and fine tune in Post. Where things get complex is when the ambient lighting does not fit conveniently into any of the discrete Kelvin degree temperatures. Stage and venue lighting are sort of famous for the ambient spectra being unlike any of the Kelvin degree emittance spectra. Manual/custom white balance can do a reasonable correction to neutral gray from a gray card, but a close gel match might not be possible.*
A more sophisticated approach is to make best effort for both ambient and gels and use a colorchecker (or similar) to make a color profile** for the session that might be applied later as part of RAW processing. This is done using a test shot that includes the colorchecker that is analyzed later to create a detailed color profile for that session and any others that might use the same lighting. Product shoots, particularly for clothing or fabrics, often use this approach.
Robert Hall mentions color meters and apparently is unaware that ambient light colorimetry is bread and butter for colorists in the motion picture industry. Prices for the meters start at over $700 and go up from there.
Sorry if this reads a little lame, but in practice, the gels are a much more rough approximation than manual/custom WB against a gray card, so I suspect that one's best effort is usually fine.
Steve
* How Sony does it and whether it always works for matching gels, I don't know.
** A good example might be session-specific DNG camera profiles (DCP) for Adobe and other products that support them.