FIGURE 1 While our eyes might disagree, as we view the golden light of sunset, we do generally consider the light supplied by our sun as being white.
Evolution has conditioned us to consider the light supplied by our sun as being white. This is not a distinct colour such red, green and blue, but is a blend of all the other colours combined. However, the value of white is dependent on the surface temperature of the sun of about 5,000°K. Any object heated to a higher temperature would appear to us as a bluer white, and if heated to lower than 5,000°K would appear as a redder white. This link to temperature has been taken as a useful way to measure all white values, which is why camera flashes are rated at 5,000°K. It does not mean that they are heated to this level, otherwise they would melt, it just means that they emit a white value equivalent to an object that had been heated to this temperature.
FIGURE 2 While we do not assume these are actually heated to the levels indicated, the table simply shows the white value equivalent to an object that had been heated to this temperature.
When any cold object is heated it radiates colour which changes as the temperature rises, first red, then yellow, white, and eventually blue. A paradox appears here as cooler objects emit redder white light, a candle for example, and hotter objects emit bluer white light, such as acetylene torches for welding. The paradox exists because humans have a psychological reaction to colour. We associate blue with objects that make us feel cold, such as ice and water, and we think of red as being hot because our everyday experience of fire is in the redder 3,000°K range. If you were to ever stand next to a fire of acetylene gas you would feel a lot hotter, a lot quicker, despite it being blue.
NOTE: Colour temperature is measured in the Kelvin scale (°K), where “0” is absolute zero, not in Centigrade (°C) or Fahrenheit (°F).
FIGURE 3 Colour temperature in film is usually set to one of two standards, 5,000°K for outdoor use in daylight, or 3,200°K for indoor use under tungsten light. The example image here was photographed using daylight film when the press conference was lit with television lights that are set to a far lower temperature. As a result the film recorded the image on the left, whereas the brain saw something closer to the image on the right. Usually this would be corrected for by adding filters to the camera lens so that the film and light temperatures match. The alternative often means extra work has to be done fixing the image in Photoshop.
The human brain adjusts very well to variations in the different values of white light. In most cases white still appears as white, despite the change in light source. This is very useful for us in everyday life, but when working as a photographer this can be a problem as the subject being viewed is often seen differently from what is being recorded on film.
FIGURE 4 In the example image above the night time street lighting was about 2,500°K. If the temperature is set too low then the result is a lot bluer, although this is closer to what the street looked like when seen in daylight. If the temperature is set too high then the image becomes orange.
Digital cameras have made life easier for photographers as the recorded colour temperature can be set to any value within a wide range, often 2,800°K to 10,000°K. However, it is common to just set the camera to Auto White Balance (“AWB”) and let the temperature adjust frame by frame.