Uniform diffuse back light. 

The camera heads for the source of light, seeing a uniform white surface. Any object interposed between the camera and the source of light produces a shadow detected by the camera as a black shape on the white background produced by the enlightenig, obtaining the maximum contrast. 

Applied in measurements it specifies the profiles of the pieces. It is also used in the measurement of transmission and impurities in transparent and traslucid objects.