There are a formidable variety of correction filters available to aid us in the battle of the color temperature. In their simplest applications, correction filters are intended to adjust the color temperature of full spectrum (black body) light sources to match shooting conditions and available light sources. Color Temperature Orange (CTO) filters shift the color temperature downwards and Color Temperature Blue (CTB) filters shift the color temperature upwards.
In theory, full CTB is intended to shift a standard 3200K incandescent studio lamp up to 5700K (the color of summer sunlight), while a full CTO is intended to shift 5700K daylight down to match 3200K incandescent lamps. In practice, no two brands of CTO and CTB filters are the same and are not always safely interchangeable.
Here is a test that you can perform in the privacy of your office. Take a piece of full CTO filter and a piece of full CTB filter, place one in front of the other (the order is unimportant) and look at a white surface through the combination. What you should see is the white surface, but at reduced brightness (the same effect as a neutral density filter).
What you probably also see is a color shift in the white surface due to the upwards and downwards color temperature corrections not being equal. The results of an in-depth survey (picking up the two swatch books laying within arm's reach on my desk) revealed that the filter book with the yellow cover produces a noticeably warm gray, while the one with the blue cover produces a slightly green gray. Doing this test between different brands of correction filter produces even more interesting evidence of color inconsistency.
THE VALUE OF MIRED
On closer examination, the specifications for the different brands of filter quote different correction factors, despite giving very similar descriptions of the filter's functions. Color correction factors are usually quoted in Mireds, a measurement system with long history in color photography. Mired is an acronym for Micro Reciprocal Degree, a measurement derived by dividing one million by the color temperature in Kelvin. For example 3200K is 313 mired (1,000,000 ÷ 3200) and 5700K is 175 mired (1,000,000 ÷ 5700). Therefore the filter needed to correct from studio incandescent to summer daylight is 137 mired and the reverse, from 5700K to 3200K, requires a -137 mired filter.
Mireds have proved really valuable for selecting correction filters. Although the color temperature shift produced by a filter is dependent on the color temperature of the light source it is correcting, the mired shift for that filter is always the same. The CTB correction from 3200K to 5700K produces a 2500K (-137 mired) shift, however the correction from 3600K (278 mired) to 6100K (164 mired) is also a 2500K shift, but its mired shift is -113, which equates to a Lee 3/4 CTB filter. One of the major benefits of using mireds is that you can add and subtract mired values to get the mired shift needed for a particular job. Surprisingly enough, a Cinegel 1/2 CTO correction (81 mireds) has the same effect as combining two Cinegel 1/4 CTO corrections (42 mired), and a Lee 3/4 CTB (-112 mired) is equal to combining a Lee 1/4 CTB (-35 mired) with a Lee 1/2 CTB (-78 mired).
If the notion of mireds is new or unclear, you can download an MS Excel spreadsheet that I prepared to go with this article. The spreadsheet will give you the chance to play with mired and color temperature values, and experiment with different filter and light source combinations.
In practice, there is rarely a circumstance where a single off-the-shelf filter will solve the mismatches between lightsources. Our 5700K daylight reference point of northern hemisphere summer sunlight is almost never the actual color temperature that we need in our pictures. If our shot is not in direct sunlight, or not in the middle of the day, or not in the summer time, then using a Readhead or a tungsten camera light for fill on a daytime shot, we usually require more correction than that offered by a single full CTB. A Double CTB (-260 mired or -274 mired, depending on where you shop) kicks your color temperature up into the 10,000K range.
NO STANDARD DAYLIGHT
On the other hand, if there is effectively no such thing as "standard daylight" outside a standards laboratory, how can we rely on an HMI or other Metal Halide "daylight" source to provide the appropriate fill in for our daylight shots? The answer of course is that we can't use "Metal Halide daylight" straight from the can: we have to spice it up a little to make it work.
Other than direct sunlight, which can range from 1500K at dawn/sunset to around 5000K in the middle of a summer's day, the majority of daytime light comes to us via reflections off clouds, the sky and everything else around us. The huge blue sky component of this reflected light can easily shift the color temperature up into the 10,000K to 16,000K region. This is what we are unconsciously adapted to seeing in a picture-a warmer key source of sunlight, filled in by much cooler sky light.
If we don't follow this pattern in our own lighting setups, we can produce a look that our audience will identify as artificial, even if they can't pinpoint the reason. The daylight fill that we add to a typical talking head shot should probably be at least a thousand or two cooler than the prevailing key source. It may seem a little odd at first to be correcting daylight sources with daylight filters, but this is the mark of an enhanced, naturalistic picture.
When dealing with conversions (in either direction) between tungsten light and daylight, be very wary of relying on the labels on the lamps or filters. Just because it's labeled "Daylight" doesn't mean that it will do the job for you in all circumstances. Your eye, or better still a color monitor, will give you a good indication of whether or not you are within the right color-temperature ballpark.
Applying a layer of CTO over a window so that you can use tungsten light for an interior shot often results in the window looking way too blue. The chances are that the light coming through the window is well above the 5700K for which CTO is intended to correct. Try out a full CTO combined with a 1/4 CTO for a healthy 220+ mired conversion that will deal with 12,000K skylight. If the shot shows that you are looking through a window, be careful to leave the exterior looking just a little bluer than the interior, because that's what the audience expects to see.
Despite the numbers and the formulae, color temperature correction is much more of an art than a science. The numbers are a big help in looking for the right solutions. However they rarely provide you with answers that will look great, rather than merely adequate, in your pictures. Becoming an artist with color temperature correction firstly involves very careful observation of the way light behaves in the world. Next, grab a camera, a few lights and a selection of conversion filters and try some looks out for yourself. It's only then that you discover tricks like using uncorrected tungsten for fill in the last fleeting moments of a sunset.