The Retinex Theory was devised by Edwin Land in 1977. It is sometimes also known as the Land Effect.
Land is also famous as the inventor of instant photography and the Polaroid camera.
What is the Retinex Theory all about? Well, colour constancy is the well known tendency for an object to always appear to have the same colour, no matter what the viewing conditions are. In other words a bright red post box appears red by daylight, by moonlight, and even under sodium street lights.
According to Land, we decide the colour of something by comparing its ability to reflect short, medium and long wavelengths with that of adjoining objects. Land considered that the eye and the brain (the retina and cortex) form a single optical system, which he called the retinex.
Have a look at the picture above. What colours do you see? Red, range, green, yellow, brown maybe? In fact this is a demonstration of the Land Effect, since you are looking at a monochrome red image, with a black and white overlay. The only 'real ' colour here is red. However our brain tends to 'see' other colours being present. The illusion works best under slightly yellow light, in other words direct sunlight is not the best way to view this illusion.
This is an A4 design, with a black design printed on glossy paper, and then a transparent overlay, with a red image printed on it.
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