Radiometric and Photometric Concepts Based on Measurement Techniques

The value of the fundamental quality in radiometry, the watt, is presently realized by electrical substitution in which the temperature produced in a blackened material due to absorption of radiant energy is balanced against that produced by electrical energy whose current and voltage can accurately be measured. A new method to measure optical radiation is being explored by the National Bureau of Standards in which photons are absorbed in a semiconductor and converted with efficiency closely approaching the theoretical maximum 100 percent. Other radiometric concepts, such as radiance, irradiance, and radiant intensity can easily be defined through simple geometric relationships. Photometry on the other hand, while sharing these identical relationships also introduces detector response modeled after human visual traits; new measurement unit names, and reliance on source intensity made in measurement techniques will be examined from the view point of detector-based radiometry and photometry.