Photometric apparatus including a rotatable optical density wedge

Claim 1

A photometric apparatus comprising: a light source; means for producing from the light emitted by said light source a measuring beam of light and a reference beam of light, said beams of light following different paths; a common light modulation system for both said beams positioned in the paths thereof; a photodetector and means for directing said beams of light on to said photodetector, said photodetector comprising means for determining and comparing the intensity of said measuring beam and said reference beam; said light modulating system comprising an optical density wedge having parallel isopaques which are contours of constant density; and said density wedge being rotatable about an axis perpendicular to the plane of said wedge and which is so located in the photometric apparatus that rotation of said wedge in a first direction increases the optical transmission of said reference beam and decreases the optical transmission of said measuring beam, and rotation of said wedge in a second direction decreases the optical transmission of said reference beam and increases the optical transmission in said measuring beam.