In luminance measurements of, for example, relatively dark tunnels from the approach road in daytime, considerable errors are sometimes involved. With systematic measurements it has been found that the light from the surrounding field, i.e., outside the measuring field of the luminance meter, causes a considerable amount of veiling luminance, due to scattering in the optical elements incorporated in the luminance meter, which is superimposed on the luminance of the object to be measured. It was found that the angular characteristics of scattering in the optical elements are similar to those in human eyes regarding the equivalent veiling luminance.
