Checker shadow illusion
The image depicts a checkerboard with light and dark squares, partly shadowed by another object. The optical illusion is that the area labeled A appears to be a darker color than the area labeled B. However, within the context of the two-dimensional image, they are of identical brightness, i.e., they would be printed with identical mixtures of ink, or displayed on a screen with pixels of identical colour. However this theory has no conclusive evidence proving it true from a scientific point of view. 
That the two squares are the same color can be proven using the following methods:
- Open the illusion in an image editing program and using the eyedropper tool to verify that the colors are the same.
- Cut out a cardboard mask. By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two circles removed will work as a mask for a computer screen or for a printed piece of paper.
- Connect the squares with a rectangle of the same color, as seen in the figure to the right.
- Use a photometer.
- Print the image and cut out the squares. Cut out each square along the edges. Remove them. Hold them side by side.
- Isolate the squares. Without the surrounding context, the effect of the illusion is dispelled. This can be done by using the eyedropper tool in image editing programs, such as Gimp to sample the values of A & B, and to color in the newly adjacent rectangles using the paint bucket tool.