The chessboard illusion is an optical illusion that was published by Professor Edward H. Adelson in 1995. The picture shows a chessboard pattern with light and dark squares. The optical illusion is that the area marked A appears darker than the part marked B, even though they are exactly the same colour. 

The following methods can be used to prove that the colours of the two areas are actually identical:

Open the image in Photoshop and use the eyedropper tool. Both zones (A and B) will have an RGB value of 120-120-120.

Print out the picture, cut it out and separate the two areas from their surroundings so that the illusion is cancelled out.

Use a light meter (photometer).