Measurement apparatus for optical transmission factor

A measurement apparatus for measuring optical transmission factor comprising a pair of identical units located at opposing sides of an object to be measured, each unit comprising;

• means for generating a light beam having predetermined restricted wavelength band, the light beams of the respective units being generated alternately,
• a first beam splitter through which passes a P polarization component of said light beam to an object, and offsets a S polarization component,
• a second beam splitter which couples said offset component from said first beam splitter, and a P polarization component from the other unit through said object, to a single common path,
• a photo-detector to receive said two components through said common path to provide two electrical signals relating to intensity of said two components, characterized in that
• said first beam spliter (47, 53,) has at least a first pair of opposing planes (22, 23) for input and output of an optical beam, a second pair of opposing planes (24, 25), and a polarization film (26) located along a diagonal plane of the beam splitter,
• one (24) of said second pair of opposing planes (24, 25) having a mirror surface,
• an input beam (27) to one (22) of said first pair of opposing planes (22, 23) of the beam splitter being split at the polarization film (26) into a transmit beam (28) which is transmitted towards the other (23) of the first pair of opposing planes and a reflect beam (30), which is reflected toward said mirror surface and then toward said other (23) of the first pair of opposing planes,
• said reflect beam (30) sharing a substantially common point (200) on said other plane (23) of said first pair of opposing planes with said transmit beam (28).