Polychromatic source calibration by one or more spectrally filtered photodetector currents

摘要

A light source (12) emits radiation which is maintained at a desired intensity by use of a photodetector which views the radiation and has a fixed relationship between incident radiation and output photodetector (18) current. Intensity of the radiation is established by varying electric current applied to the source, via a feedback loop (42, 24, 22, 34), to attain a reference value of the photodetector current, the photodetector current being representative of the desired value of radiation intensity. This procedure may be performed first in an air environment with a specific optical assembly of source and photodetector to establish the reference value of photodetector current for a desired value of radiation intensity. Thereupon, the optical system may be operated in a vacuum environment, and a monitoring of the photodetector current establishes the desired value of radiation intensity. Alternatively, the source is operated at a fixed value of excitation current, and an iris (44) or other radiation attenuator is operated to produce the reference value of photodetector current.