Colour temperature is measured in ‘Kelvins,’ which is usually abbreviated to ‘K.’ It is measured on a numbered scale, where the higher the number, the ‘cooler,’ or bluer the light. The lower the number, the ‘warmer,’ or yellower the light. For example, a wax candle emits a very warm white light of roughly 1000K, while at the other end of the scale, a blue sky would measure around 9000K.
Colour temperature is a characteristic of visible light that has important applications in lighting. The temperature of a light source is determined by comparing its chromaticity with that of an ideal black-body radiator. A black body is a theoretical radiator and absorber of energy at all electromagnetic wavelengths. No electromagnetic radiation passes through it and none is reflected because no light is reflected or transmitted, the object appears black when it is cold.
The temperature (usually measured in kelvins K) at which the heated black-body radiator matches the colour of the light source is that source's colour temperature. Counter intuitively, higher colour temperatures (5000K or more) are cool (green–blue) colours and lower colour temperatures (2700–3000K) warm (yellow–red) colours.
Cool coloured light would be considered better for visual tasks. Warm coloured light is preferred for living spaces because it is considered more flattering to skin tones and clothing. Colour temperatures in the 2700–3600K range are recommended for most general indoor and task lighting.
Colour Temperature Applications
Film sometimes appears to exaggerate the colour of the light since it does not adapt to lighting colour as our visual perception does. An object that appears to the eye to be white may turn out to look very blue or orange in a photograph. The colour balance may need to be corrected while capture with filters or while printing to achieve a neutral colour print, or ultimately in post during the grading session.
Film is made for specific light sources using different sensitive emulsion layers (most commonly daylight film and tungsten film), and used properly, will create a neutral colour print. Matching the sensitivity of the film to the colour temperature of the light source is one way to balance colour. If tungsten film is used indoors with incandescent lamps, the yellowish-orange light of the tungsten incandescent bulbs will appear as white (3200K) in the photograph.
Filters on a camera lens, or colour gels over the light source(s) may also be used to correct colour balance. When shooting with a bluish light (high colour temperature) source such as on an overcast day, in the shade, in window light or if using tungsten film with white or blue light, a yellowish-orange filter will correct this. For shooting with daylight film (calibrated to noon daylight approx 5400K) under warmer (low colour temperature) light sources such as sunsets, candle light or tungsten lighting, a bluish filter may be used.
If there is more than one light source with varied colour temperatures, one way to balance the colour is to use daylight film and place colour-correcting gel filters over each light source.
Lighting technicians sometimes use colour temperature meters. Colour temperature meters are usually designed to read only two regions along the visible spectrum (red and blue); more expensive ones read three regions (red, green, and blue). However, they are ineffective with sources such as fluorescent or discharge lamps, whose light varies in colour and may be harder to correct for. Because it is often greenish, a magenta filter may correct it. More sophisticated colourimetry tools can be used where such meters are lacking.
Your eye/brain automatically compensates for differences in colour, film however does not. Therefore different film stocks need to be employed for different shooting conditions. Manufacturers try to help with this colour balance problem by making two different kinds of film: TUNGSTEN and DAYLIGHT films.
Tungsten films are calibrated to provide natural colours in many artificially lit situations, such as stage shows. However, even table lamps will often be too "orange" for tungsten film. Tungsten film is indicated by a large "T" on the film box and canister.
Daylight film, which accounts for nearly all film made today, works well outdoors. Fluorescent light provides a special challenge, images under fluorescent light can vary from green to magenta, which can give really atmospheric looks within their own right but often not the look that is intended.
You can compensate for colour balance problems by using corrective colour filters at source, which consist of coloured glass that usually mounts on the front of the camera lens. Some filters will cause you to lose an f-stop of light, so maybe it would be better to fix it in post during the grading session and capture as much dynamic range as possible.
Film is projected using yet another measure of luminance - the unit of measurement of brightness in the USA. A foot lambert equals a candela per square feet. The brightness of a reflecting surface in the foot lamberts is equal to the incident illumination in footcandles times the reflectance of the surface.
The accepted SMPTE projection standard is 16 ft lamberts (55 candelas per square metre) and is generally accepted as giving the brightest result. In Europe, the level is set at 14 Ftl, prints are graded with the assumption that the projection will be within tolerance. Any theatre used for answer prints and QC should be as close to the acceptable standard.
Obviously the colour temperature of the projector lamp is key to keeping these standards throughout the film industry - the normal standard is that of daylight film. Xenon arc lamps are approximately 5400K, we can see that all of these figures are beginning to continually appear.
TV & Video
NTSC and PAL TV norms call for a compliant TV screen to display an electrical/digital black and white signal (minimal colour saturation) at a colour temperature of 6500K. Most video and digital cameras can adjust for colour temperature by zooming into a white or neutral coloured object and setting the manual 'white balance' basically calibrating the scene to know what exactly is white; the camera then shows true white as white and adjusts all the other colours accordingly.
White balancing is necessary, especially when indoors under fluorescent lighting and when moving the camera from one lighting situation to another. Most cameras also have an automatic white balance function that attempts to determine the colour of the light and correct accordingly. While these settings were once unreliable, they are much improved in today's digital cameras, and will produce the white balance in a wide variety of lighting situations.
Hopefully this will help to understand the different correlation between film and video. Film ultimately cannot control it's colour temperature and capture, whereas a digital camera can measure the surrounding and compensate or self calibrate to capture a 'normal' image.
As you may have realised we really do underestimate the way we perceive colour. Mimicking the process through film stock and video cameras is a lot harder than it would at first seem. You should remember film is calibrated to approx 5400K and a television signal works at 6500K, which corresponds roughly to a mid-day sun in Western Europe / Northern Europe, hence it is also called a daylight illuminant.