how do we perceive colour? how do colours add?. what is colour? light comes in many “colours”....

How do we perceive colour?

How do colours add?

What is colour?

• Light comes in many “colours”.

• Light is an electromagnetic wave.

• Each “colour” is created by a wave with a different wavelength.

• When we see all the colours mixed together we perceive the light as white.

• But we can observe the entire visible spectrum of colours when the spectrum is dispersed by a prism.

• However, our perception of colour is a result of our biology.

• Other animals see the world very differently to us, because they have a different biology.

• Here is a little about how our biology affects the way we see colour…

So how do our eyes respond to colour?

Our eyes have two kinds of receptors – known as rods and cones.

• Rods respond to light intensity, and build up a picture in “bright and dark” (otherwise known as “black and white”).

• Rods are sensitive and can work in low light levels. They are important for vision at night – which explains why you cannot distinguish colours at night.

Here is a scene as it might appear to us at night.

• Cones are not as sensitive as rods, but respond to colour and send three types of signal to the brain – Red, Green or Blue.

• That is what makes Red, Green and Blue the primary colours. They are the additive primary colours.

• Pure red light sends a RED signal to the brain. Pure green light sends a GREEN signal to the brain and pure blue light sends a BLUE signal to the brain,

• but pure lights of other colours send a mixture of signals to the brain.

• For instance, pure yellow light sends a RED and a GREEN signal to the brain. The brain then combines these signals and creates the conclusion YELLOW, so…

• …so our biology cannot distinguish between a mixture of red and green light or pure yellow light.

• We all grow up mixing paint colours. But here, though we are not consciously doing it, we are using colour subtraction. Our intuition would tell us that mixing Red and Green should give a brown colour.

• When combining coloured lights, the three additive primary colours therefore create colours we do not expect. We say they add.

• But remember this is not physics… • …it is only a result of our biology!

So… how do our eyes add the Red, Green and Blue primary

colours?

Here is a white screen in a dark room.

Watch what happens as we shine coloured torches

at it? First we will shine a red torch at it.

What will happen as we shine a green torch at it too?

Look at the overlap. Is this what you expect?

Remember that this kind of colour addition is a result of our biology!

What will happen as we shine a blue torch at the screen too?

Notice how the three primary colours add to make white,

just as a jumble of all wavelengths is perceived as white.

Look at the overlap again. Is this what you expect?

All the colours have names:

BLUE

YELLOW

CYANMAGENTA

RED GREEN

WHITE

but they are also the subtractive primary colours.

YELLOW

CYANMAGENTA

Cyan, Magenta and Yellow are known as secondary colours,

but this is a mistake (or a simplification for those

learning colour mixing rather than physics!)

YELLOW

CYANMAGENTA

Some people call these colours Blue, Red and Yellow,

• Computer screens, televisions and colour photography (both film and digital technologies) build images that the brain sees in full colour, but using just the three primary colours – Red, Green and Blue. This is known as the RGB colour scheme.

• To be effective, colour films and digital cameras have to respond to the full colour spectrum in the same way as the rods and cones in the human eye do.

• You can play with defining your own RGB colours in any painting software using custom colours. By controlling how much of each channel goes into the colour mix you can create any discernible colour.

Now let us investigate colour subtraction.

Here is a white computer screen. Remember this is RGB white.

What happens as we place (subtractive) coloured filters on it?

We place a cyan filter on it…

What happens when we place a magenta filter on it too?

Look at the overlap. Is this what you expect?

Remember that these filters are each removing just one of the three RGB colours at a time.

The overlap therefore shows just the one additive primary (in this case blue) that is able to go through both filters.

What happens as we add a Yellow filter too?

Notice how the three primary colours collectively subtract to make black, removing all light.


Notice too how any two subtractive primary colours combine to make additive primary colours.


The additive primary colours are the subtractive secondary colours.


• Computer printers mix inks in such a way as to selectively filter light reflected from white paper. Though in daylight they are not just dealing with RGB colours, they still use Cyan, Magenta and Yellow inks – and usually Black too. This is known as the CMY colour scheme.

• Artists too mix colours to obtain others. By starting with the three CMY colours, white and black they can create a vast array of other colours. Adding more of one of the subtractive primary colours to the mix will selectively remove more of one of the additive primary colours, thus controlling the final colour as perceived by our biology.

Here is a scene filtered to show the three RGB channels separately.

Here is the scene filtered to show the three CMY colours separately. Each contains all the

information from two of the RGB channels.

And here is the complete picture with all the information from the three RGB channels.

Created by Christopher Goldsack

how do we perceive colour? how do colours add?. what is colour? light comes in many “colours”....

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