Know Light. Colour temperature. What it is and how to use it.

Why do my photos sometimes have an horrible blue or orange cast? What does that K stand for on my digital camera? If you are asking  these questions you probably should want to know about Colour Temperature. So let's get to it!

Light is an electro-magnetic radiation which carries energy; one of it characteristic is frequency which is translated by our brain in colour. Different frequencies produce different colours. The higher the frequency is, the higher the energy and bluer the colour. Colour temperature is the scale on which this energy is measured and unit measure is the Kelvin degree (K). The colour temperature depends on the heating temperature of a metallic incandescent light, from red, which needs the lowest temperature, through the orange, white and up to blue (highest temperature).

Normal daylight has a temperature of 5500K and we associate it with white light; a lower temperature energy measures, for example, 2000K so the light will be red; on the other hand if the temperature of the energy goes higher than 5500K, the light will turn blue. 

This is what you need to remember in spite of the confusion common terminology may cause by referring to red or orange light as warm and to blue light as cold (because it is not related to the temperature but to the psychological effect of the colours).

Why should you know this? to avoid a colour cast when taking picture in certain conditions. If you are using a daylight balance film or the sunny setting in a digital camera you will be correctly shooting  coloured images at 5500K, typical of a midday sunlight or a flash. If the colour temperature rises (or lowers) and you not balance accordingly, your images will have a blue cast (or a orange one): when taking pictures during an overcast day, for instance, your images will turn out with a bluish tone; when taking pictures inside a house or a restaurant they will be reddish. 

To avoid this you need to white balance your camera, if you are shooting digital, by changing the setting accordingly (from the sunny setting to the cloud one, for example), or manually choosing the colour temperature in kelvin degrees, or as well doing a personalized white balance photographing a white paper or using an expo-disc. If you are a film camera user using a daylight balanced film, you can remove the colour cast by using filters: 81 and 85 series will remove blue tones, 82 and 85 series will remove orange tones.

But you do not always want to remove a colour tone. One of the function of the light, and in my opinion the most important, is to create a mood to enhance the message of the image: would you correct the orange and red tones in a picture of a sunset ? Therefore you can create a warm and familiar feeling by adding orange tones, while blue cast is related to feelings of cold and distance. The use of psychological effects of the colours is another very interesting theme which we will discuss in next posts. stay tuned!