The colour temperature gradient

‘Colour temperature’ is the unit measure often used to describe the colour of light emitted by a light source. But what is meant by colour temperature? Well, if you put a black iron poker in a forge it will heat up until it glows and starts to emit light. It starts out a dull deep red and as it gets hotter it becomes orange, then yellow, and if it could resist melting it would eventually go blue. At each stage, the physical temperature of the poker tracks the change in its colour. This isn’t just true for pokers, any item heated to the same temperature will glow the same colour and that colour is given a temperature value as it’s measure. We describe this light as having come from a ‘black body radiator’. It is therefore possible to tell the temperature of a coal fire by looking at the colour of the coals and reading the temperature off the corresponding colour on a colour temperature chart.

As the filament in a tungsten bulb is given more energy it heats up and glows in just the same way as the poker. The hotter the filament the less red the colour becomes and the higher the colour temperature gets.

Flash light sources emit a bluer light than tungsten halogen bulbs and have a higher colour temperature as a result. Red light has a low colour temperature and blue light has a high colour temperature.

The colour plot showing the white locus curve

Light sources that have a colour that falls on the curve (often called the white locus of ideal light) from red to blue via orange are exactly what we want in photography. Candles, Tungsten bulbs, all forms of daylight and flash units are all ‘good’ sources of light. Our eyes like these light sources too. We can easily tell that a piece of white paper is indeed white whether it is lit by a candle or by moonlight. Incidentally moonlight is just reflected sunlight and is not ‘blue’ as shown in the movies. It is however bluer than the tungsten light in our homes and this difference causes us to think of moonlight as blue.

Our eyes and brain are constantly ‘white balancing’ adjusting the white point accordingly so that we can accurately asses the colour of items in the world around us. Cameras can sort of do the same thing using auto white balance. Auto white balance does not try to neutralise the colours in a photograph otherwise a picture of a green football pitch would be rendered grey. Instead cameras look at the red/ blue content at the brightest points in an image and try to determine the colour temperature from the data.

‘Bad’ light comes in many forms and the biggest culprits are; energy saving bulbs, mercury vapour lights, sodium street lights, fluorescent tubes, and carbon arc lights such as CSI or CID. ‘Bad’ lights don’t sit on the white curve and often contain spikes of energy at odd wavelengths that give green, orange or magenta colour casts. These non Linea lights make skin look ghastly and give people depression.

Why Kelvin and not Celsius? Well that’s scientists being pedantic. Kelvin = Celsius -273

The approximate colour of daylight:

Sunrise 2000K
Morning/ afternoon sunlight 4000K
Summer sunlight at noon 5600K
Sun through clouds at noon 6500K
Cloudy sky 7000K
Clear blue sky 10000 – 20000K

Midday sun has a lower colour temperature than deep blue sky. This is not because the sky is lit by a hotter light source but it is because of the make up of the atmosphere and the scattering of light. The shorter wavelength (blue) light is affected more than the longer (red) wavelengths. The sky appears blue until the angle of the sun is shallow and the blue light is further scattered by the increased atmosphere the sunlight travels through. This leaves just the red light and a lovely sunset.

The approximate colour of other common light sources:

Candle 2000K
Domestic light bulb 2750K
Tungsten halogen bulb 3200K
Photographic flash 5600K

Fluorescent tubes have many colour temperatures and spectral responses. Because fluorescent tubes use fluorescing gasses to make light and are not ‘black body radiators’ relying on heat energy to make light they can run cool yet emit a colour of light that correlates to a hot black body radiator. In the 60’s there were just warm white and cool white tubes but now there are many types. They range from specialist tubes with red spikes designed to make meat in a butchers counter look redder or ultra violet designed to highlight underwear in a disco. Generally though ‘florries’ produce spikes around the 550nm wavelength leaving a slight green colour cast.

Colour correction filters. Blue filters increase colour temperature and orange filters reduce colour temperature. With good light sources it is easy to match the colours by filtering one or the other. We can put a full orange filter on a Speedlight to give it the same colour of light output as a tungsten halogen light. Or we can put a full blue filter on the tungsten halogen light to match the colour of the Speedlight. With ‘Bad’ light however, some green or magenta shift with filters may help but it is often imposable to get a good match.

‘Bad’ light turns ‘good’. Over many years now researchers have found certain combinations of various fluorescing gasses can provide ‘good’ light in a tube. The most expensive of these fluorescent tubes are badged for photographic and technical use and are often overpriced because you can get the same ‘gas mix’ in fluorescent tubes made to combat ‘SAD’ the ‘I don’t like the dark winter light’ syndrome. The best of these anti SAD tubes are from the Sylvania professional range so if you want well balanced florescent tubes for your photographic production room you now know what to look for.

© Damien Lovegrove.