Optical systems for defining the viewing and measuring fields in luminance/radiance meters

Luminance and radiance measuring instrumentations generally consist of an objective lens (or mirror system), a photodetector, and an optical system that defines both the viewing field and the measuring field of the instrument. In more versatile instruments, this optical system is generally required to define a multiplicity of different-size fields. This paper reviews existing methods-such as the beam splitter/reticle approach, the fiber light pipe approach, etc.-and explores several new systems. Comparative evaluation of the various systems is made using the criteria of efficiency, unambiguity of viewing field, alignment accuracy between viewing and measuring field, freedom from polarization, etc. The optimal system appears to be one that consists of a mirror disk fabricated with a multiplicity of elliptical apertures through it; the disk is oriented at an angle to the optical axis so that the photons being measured pass through the selected aperture, while the mirror surface reflects the balance of the inco ing radiation to the observer's eye for viewing. A new instrument that is based on this optimal system is briefly described.