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Monday, April 28, 2008

Colour Perception


The human brain is a strange and wonderful thing. It's ability to perceive colour is by no means straight forward.

For instance, take the colour red; it has a tendency to come forward so that a red object looks comparatively closer to the viewer than it actually is. Some people have put forward the suggestion that this is because the earth's atmosphere is blue and that objects far away will have a blue cast to them as the light from them passes through more and more blue atmosphere and that this produces the psychological effect of bringing red objects closer.

Sounds reasonable doesn't it. But at night, green objects look closer and red looks less 'forward'. Try seeing if you can notice this effect with traffic lights, comparing daytime to nighttime.

Most people think of their eyes as the organ that sees the world around us. This is not strictly true as the real visual processing is done with our brains; if the central processing unit (CPU or microprocessor) is the computer equivalent of our brain then our eyes would be the equivalent of the keyboard; they are simply the input device. Our brains are far more complex and capable than any current computer's CPU and this is reflected in the way it works.

For example think of a photograph you have taken indoors, almost certainly with a sodium light bulb (normal household lamp). The photo had an orange colour cast didn't it. That is because the camera can only reproduce what it sees and what is saw was a room flooded with orange light; sodium lamps produce more of the red colours of the visible spectrum than the green/blue end (all the colours of a rainbow mixed together produce white light - literally). If you take a photograph of a room lit by fluorescent lamps the colour cast in the print will be green. But you didn't see an orange or green room when you took the photo did you.

That is because the brain is using its power to 'normalize' the view. It looks at the entire spectrum it sees before it and registers where the ends of the light spectrum are (red at one end and blue the other). It then moves the visible spectrum of the view along so that everything fits into what it thinks is correct. If there is a piece of pure blue paper in the room, the orange lamp would make it look less blue and more orange and a camera would reproduce this effect in the print. However, our brains see the blue paper as being the bluest object in the room otherwise others colours would not fit into its know spectrum. So it slides the spectrum along so that you see the paper as being pure blue.

Moreover, it can make these adjustments at an incredibly fast pace. If you walk out of a shop lit by fluorescent lamps (green cast) you do not momentarily see everything with a blue tint as you walk into the street, the scene in the street outside the shop is immediately corrected to the right colours and no disorientation takes place.

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