Understanding colour temperatures can be simple once you get your head around it. I'll break down the basics of what colour temperature is, how to white balance to your creative advantage and briefly cover different colour temperature filters and gels.
Let’s remind ourselves of the three most common temperatures you’re likely to encounter.
• 3200K - Tungsten (typically perceived as warm light)
• 4400K - Fluorescent (mix of tungsten and daylight)
• 5600K - Daylight (typically perceived as cool light)
Bear in mind that there will be occasions where the temperature exceeds either end of this range. Candlelight or fire for instance is very warm, with a white balance somwehere around 2000K. Different times of day can be much cooler and exceed 7000K.
Worth mentioning too, is that daylight is sometimes specified as 5500K or even 6000K. Different manufacturers have different preferences. 5600K was ‘defined’ as daylight when, once upon a time, the colour temperature of the sun was measured at precisely 12 noon on a cloudless summer solstice in the middle of the Nevada desert. Or something fairly non-scientific like that anyway. The point is, the actual colour temperature of daylight varies every day and all over the world - it is dependant on the weather, your location and probably numerous other unquantifiable, uncontrollable factors. 5600K was simply settled on as the accepted value so that sensors (film stocks, back then) and lights could be designed to a defined standard of what daylight is.
One of the things that took me a little while to get my head around is why is daylight is a cooler source than tungsten, yet we use a ‘hotter’ temperature (K is for Kelvin). The key to remember is that it's a white balance. To balance the temperature, in other words, to make the light 'white', for a cool source such as daylight, we need to use a warmer temperature such as 5600K to neutralise that colour and make it white. Equally, we only need a cooler temperature (3200K) to balance out the warmth of a tungsten source. This is why if you were to accidentally shoot at 10,000K, everything would appear very hot regardless of what lighting was used… yes, I've seen it done!
Whilst tungsten is usually perceived as warm and daylight is perceived as cool, if shot with a correctly balanced camera, the image will always appear white and neutral. The perception comes from either intentionally or unintentionally white balancing the camera away from the actual light source’s temperature.
You can use this to your creative advantage. If you're filming with all tungsten sources and your camera (or stock) is set to a tungsten WB, all of the light will be white, however if you added a daylight source, it would appear cooler and introduce a blueish tint to the shot. If you're shooting an interior for instance, be aware that any sunlight through windows will appear quite cool if you're using tungsten lighting and are balancing to 3200K. Conversely, using the same interior example, you could use a daylight WB/stock so that the sunlight is white and any interior, tungsten lights would then appear warmer and have a more orange tone.
Fluorescent lighting can be found the tubes you often see on the ceilings of offices. It's half way between tungsten and daylight. Real fluorescent lights often have a horrible colour cast (either magenta/pink or green) but that's another conversation. Let's use the interior scene example again. Often if I'm shooting such a scenario where there's a mix of source temperatures, I might decide to shoot with a fluorescent WB so that we get the best balance of everything. The daylight will appear a little cool and any tungsten sources will be a little warm, but neither will be more distinct than the other.
When shooting an cloudy, overcast day, it isn’t always necessary to white balance perfectly. Whilst the actual temperature of the scene is likely well over 7000K, by shooting at a regular daylight temperature of 5500 or 5600K, the image will look somewhat cold - exactly how a dull, overcast day might feel, right? The opposite is true, too. When shooting fires or candlelights, I would never try to balance the camera to 2000K. Shooting with a tungsten balance of 3200K, the flames will appear warmer - visually connecting the viewer with the literal heat that the flames produce, immersing the audience with the scene. If I were to balance a camera to 2000K, the flames would appear white and, to our eyes, peculiar. Whilst our eyes auto white balance themselves (you’ll notice this when wearing tinted sunglasses), I’m not certain our eyes can white balance down to such low temperatures. A fire will always look yellow or orange to our eyes.
Let’s imagine a shooting scenario and run though some white balancing options available, other than balancing the camera itself. You're shooting on daylight film and don’t have any tungsten stock, yet you have to shoot an interior scene - one with lots of tungsten sources. You could simply opt to shoot as is - and let the scene feel warm. This would depend on the context of the scene and whether a warm, homely feel is appropriate of course.
Another option would be to gel all of the light fixtures to daylight. You'd use CTB (colour temperature blue) to do this. It comes in full, half, quarter and eighth strengths. A full would render a tungsten source daylight (3200K>5600K), a half would make it around fluorescent (3200K>4400K) and so on. The quarter and eighth strength gels are often just used for creative purposes, to add a little tint to an otherwise white light (assuming shooting on a correctly balanced camera/stock). These gels can be folded to double up on their strengths, or to create fractions not possible with standard gels. A quarter folded over would be the same as a half, or a half and a quarter together would be a three-quarter.
In addition to CTB, there's the opposite - CTO (colour temperature orange). This would turn a daylight source into tungsten (using a full strength). Also remember that you don't have to use CTB on tungsten and CTO on daylight. I often use a little bit of CTB on daylight sources to inject some blue - maybe I want the scene to look like a nice fresh, crisp morning perhaps. Using CTO on tungsten sources would make the light even warmer - I use this to emulate firelight for instance. There are also plus green and minus green gels which compensate for any colour cast (such as fluorescent tubes I mentioned earlier).
Finally, going back to the example scenario, the third option would be to use filters on the lens. These work in the same way as the CTB and CTO gels and are usually in the same format as other glass filters like NDs or polarisers and fit in matte boxes. Some lenses have special slots near the back of the lens to insert a small cut of gel. These are often ultra wides where the field of view is too wide to use a matte box, or super telephoto lenses where the front of the lens is too large to fit in a matte box. Old Super-8mm cameras usually have a CTO filter built into them: 8mm film cartridges are all tungsten balanced, so when shooting outside, the user would flip a lever to engage the filter, warming up the light passing through the lens, rendering it balanced and white on the tungsten film stock.
Filters have what is called a Wratten number to identify what they are. The '80' series filters such as 80A, 80B, 80C and 80D are the cool filters. They do the same as the CTB gels. The '85' series filters (85, 85B, 85C) are your warming filters. They do the same as the CTO gels. There are also 81 (warming) and 82 (cooling) filters that offer more subtle adjustments, similar to eighth and quarter gels. More specific information about the exact temperature adjustments can be found on Lee's website, or alternatively, Wikipedia has an extensive list of filters and their effects here.
One important note to remember however, is that when using either gels or filters, they will reduce the light passing through them. They're 'subtractive', meaning they're filtering out certain wavelengths of light to provide the desired output. A gel on a light will mean the light isn't as bright, and a filter (or gel) on a lens will lower the exposure. The heavier the filter or gel, the more light loss.