(under subclass 213) Subject matter including two optical paths, there being an optical element having known light absorbing properties in one of the paths or there being a light source in one of the optical paths having a known illumination characteristic (which may be dependent on the position of the light source), together with structure to facilitate comparison of the light in the two paths.
SEE OR SEARCH THIS CLASS, SUBCLASS:
222, for photocell type photometers with plural photodetectors involving comparison.
