Patent ID: 8345230

Claim:
An illumination apparatus for illuminating a sample surface to be measured, the illumination apparatus comprising: a plane light source having a spatial distribution of Lambertian characteristic positioned on a normal passing through a center of a measured area of the sample surface; and a mirror having an internal reflective surface being positioned between said plane light source and said measured area of the sample surface so as to reflect light from the plane light source to the measured area so that the light reflected on said internal reflective surface directly enters the measured area keeping the spatial distribution of Lambertian characteristic at an angle of 45 degrees with respect to the normal, said internal reflective surface having a circular or polygonal shape in a section perpendicular to said normal, said circular or polygonal shape substantially corresponding to an imaginary circle which is centered on said normal and has a radius equal to half a distance between said plane light source and said measured area of the sample surface.