Thursday, 2 August 2012

White Light and Primary Colours

We are all familiar with the colour wheel that we have seen during our school days. And we know for a fact that when rotated it turns white. Reason being white light is composed of all other colours of the rainbow (VIBGYOR). If we pass white light through a prism we could create all colours of the rainbow.

Actually we do not need all these colours to make white light, we just need the three which are called the primary colours, RED, GREEN and BLUE (RGB). These are called the additive primary colours because when added together in the right proportion (equal quantities), they produce white light.

RGB Additive Colour Mixing
RGB Additive Colour Mixing

The opposite of these additive primary colours are the subtractive primary colours (CYAN, MAGENTA and YELLOW) which we get when we subtract any of the primary colours (RED, GREEN and BLUE) from white light.

Digital cameras, computer monitors, Television sets etc. all use the RGB colour values to create white. But the printing industry relies upon CMY colour values for chemical dyes and pigments. In order to produce black, equal quantities of CYAN, MAGENTA and YELLOW are added together.

CMY Subtractive Colour Mixing
CMY Subtractive Colour Mixing

Although this is theoretically correct, actually in CMYK printing, true black is added separately because ink pigments are not pure enough so if mixed together they produce a colour similar to purple – brown instead of pure black.

Importance of understanding light and its colours in photography

Professional photographers come across different situations where a proper understanding of light and colours could save the day. Especially when they have to shoot under mixed lighting conditions. Daylight, fluorescent light, tungsten light, candle light etc. all have their own colour casts.

Photographers manipulate light to suit their various needs, either by changing the colour of light by adding gels, or putting on colour filters in front of the lens or to correct colour casts, a combination of both or using the white balance settings in their camera etc. A thorough understanding of light and its colours is necessary in order to effectively tackle such tricky situations.

Colour Wheel
Colour Wheel

A colour wheel is most useful to easily understand the principle. In the colour wheel Red, Blue and Green are found (12o clock, 4o clock and 8o clock positions respectively) exactly one third of the distance apart (120 degrees) from each other. All other colours are made of different combinations of these primary colours. So in order to correct colour casts you compensate by adding the colour opposite of the colour cast in the colour wheel. For example say your image has a green tint to it, to correct it you need to increase the values of magenta. And if your image has a blue tint you compensate by adding yellow.

In the next article we will discuss about Shadows

Related Reading

  1. An Introduction to Light
  2. Wavelengths and Colours
  3. What Happens When Light Falls On a Surface
  4. Quality Of Light
  5. Shadows