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Mach Bands

This low sun photo of the shadow of a wind turbine shows - beside the perspectivic effect - three more interesting phenomena. Firstly, the shadow is extremely blue - the snow scatters the light of the cloudless blue sky. The colour of the sky itself is due to the strong scattering of short wavelengths. Secondly, the edges of the shadow are diffuse - because the sun is no point source.

Thirdly, there is an effect based on the human visual system, it is not real: the shadow shows dark bands along the borders on the inside and bright bands on the outside. If the lightnesses of the respective bands on the unprocessed photo are measured, they prove to be inexistent. These bands are named after the physicist Ernst Mach (1865). The effect is caused by the neurons that of the human retina - the neurons influence the neurons next to them - and occurs if the eye detects regions with different lightnesses, separated by a narrow strip of a uniform lightness gradient. The image processing of the brain has a similar effect.

The effect also occurs if unsharp masking is applied during image processing. Probably already the camera's internal software with its sharpening algorithms does the same to a lesser degree.

Above: Panasonic Lumix GH1, unprocessed compressed image. Below: the same image after unsharp masking.

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