| If an object is heated in a furnace, or by passing an electric current through it, it glows with a particular colour (red, orange, yellow, white, or bluish white) depending on the temperature. This type of radiation is called 'black body' radiation, not because it is black, but because it is free from spikes or gaps in its spectrum at particular wavelengths (ie., colours). Tungsten light-bulbs produce good black-body radiation. Electronic and gas-discharge light sources (e.g., flash tubes) do not produce perfect black-body radiation, but if there is enough red, green and blue light in the mixture, the source can often be correlated to a black body at a particular temperature. The correct term to express the colour rendering quality of a particular light source is therefore "correlated colour temperature", which is expressed in Kelvin, K, (degrees above absolute zero). Standard photographic daylight is assumed to have a colour temperature of about 5500K, and except when using special tungsten balanced films, the closer the light source is to this value, the better the result. Xenon flash has a correlated colour temperature of about 6000K, which is fine for most purposes, but sometimes a little too blue. Underwater flash makers sometimes use a yellow reflector or filter, to reduce the flash colour temperature to around 4500K, to compensate for the blueness of the water. Domestic light-bulbs have a colour temperature of about 2950K, which is very orange. Quartz-halogen lamps have a colour temperature of 3200 - 3400K, which is yellowish, but electronic and video cameras can compensate for this by adjusting the relative sensitivities of the red, green, and blue channels (white balance). Humans do not usually notice the effective temperature of a light source except by comparison, because our eyes adjust automatically (look at the colour of a tungsten light-bulb while there is sunlight streaming through the windows - the electric light seems orange or yellow in the daytime, but perfectly white at night). |
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