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What is Light Colour?

In simple terms the light colour is what a metal would look like if you heated it up to that temperature in Kelvin (Kelvin follows the Centigrade scale -273 In other words 1000 degrees Centigrade is 727 Kelvin.) See here the Wikipedia article for the kelvin scale

So with that in mind, you can understand why a "warm white" lamp that is said to have a colour temperature of 2500K would be quite yellow. Where as direct sunlight during the day is said to have a colour temperature of about 5500K. Anything over 5500K starts to go more into the blue end of the spectrum and are contradictingly called "cool" LED lamps that have colour temperature figures above 6000K for instance will be classed as cool white.

Here's what Wikipedia Has to say about colour temperature

Colour Spectrum

Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields. The color temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable hue to that of the light source. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K.

Color temperatures over 5,000K are called cool colors (blueish white), while lower color temperatures (2,7003,000 K) are called warm colors (yellowish white through red). This relation, however, is a psychological one in contrast to the physical relation implied by Wien's displacement law, according to which the spectral peak is shifted towards shorter wavelengths (resulting in a more blueish white) for higher temperatures.

Categorising Colour Temperature

The color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in kelvins, or alternatively in mired (micro-reciprocal kelvins). This permits the definition of a standard by which light sources are compared.

To the extent that a hot surface emits thermal radiation but is not an ideal black body radiator, the color temperature of the light is not the actual temperature of the surface. An incandescent lamp's light is thermal radiation and the bulb approximates an ideal black body radiator, so its color temperature is essentially the temperature of the filament.

Many other light sources, such as fluorescent lamps, emit light primarily by processes other than thermal radiation. This means the emitted radiation does not follow the form of a black body spectrum. These sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp. Because such an approximation is not required for incandescent light, the CCT for an incandescent light is simply its unadjusted temperature, derived from the comparison to a black body radiator.

Examples of Colour Temperature
Temperature Source
1,700 K Match Flame
1,850 K Candle flame, sunrise/sunset
2,700 - 3,300 K Incandescent Lamps
3,000 K Soft White compact fluorescent lamps
3,200 K Studio lamps, photofloods, etc
3,350 K Studio "CP" light
4,100 - 4,150 K Moonlight, xenon arc lamp
5000 K Horizon daylight
5000 K Tubular fluorescent lamps or Cool White / Daylight compact Fluorescent
5,500 - 6000 K Vertical Daylight, electronic flash
6,500 K Daylight, overcast
6,500 - 9300 K LCD or CRT screen

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GreenerU Ltd
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Morley
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