If we take a white screen and shine light of peacock blue and red onto it, we will produce a white colour, both these colours being complementary.

Vẽ hình trang 6/8

When we talk about Primary colours we must be careful to define what we mean. If we mix different coloured lights, then Red light plus Green light plus Blue tight gives a White light providing the intensities of the colours are correctly balanced. These three colours, Red, Green and Blue are known 35 the Additive Primaries. This method of reproducing colours was used by Clerk Maxwell in 1861 in the first practical demonstration of colour photography based on three projectors.

Vẽ hình trang 6/9

There is another set of primaries known as the subtractive primaries-yellow, magenta and cyan. These are so called because each one can be produced from white light by taking away one of the additive primaries. Thus Yellow = White - Blue, Magenta = White - Green; Cyan=White – Fled.

lf Yellow light were to be mixed with Blue light, then providing that the intensities were correct, the result would be White light. But, if yellow pigment is mixed with blue pigment the result is green pigment. The reason is this: yellow pigment absorbs red and blue: the blue pigment absorbs red arid yellow, so green light is reflected.

The thing to remember is that light is an energy source and that the white screen reflects all that energy, thus two energy forms can combine to form white light or all the primary colours as seen on a colour television.

AFTER IMAGE

If we inadvertently glance at the sun or a light bulb, we can suffer ‘blobs before the eyes‘. This after image applies to colours as well. If you stare at a bright yellow object, your brain will try and neutralise it with a purple-blue to achieve a more neutral colour (less information being sent to the brain). If you then look away you may see a purple-blue after image of the same shape. The longer you stare, the more pronounced the effect.

The implications for the visual colour matcher are very important. A quick decision is essential, the longer you look the less you see.

The colour surrounding a colour also effects the colour we see. If we take two identical red objects, place one in front of a bright lemon background, the other in front of a magenta background, the red objects will appear different, depending upon their background.

It is not only bright colours that confuse the brain, grey will produce the same effects.

This means that when colour matching, try, when assessing a colour, not to have anything in your field of vision which contrasts too strongly.