CALIBRATION TECHNIQUE AND APPARATUS

• US 3,765,779 A
• Filed: 06/27/1972
• Issued: 10/16/1973
• Est. Priority Date: 06/27/1972
• Status: Expired due to Term

• Current Assignee: Graham R. Hunt, Lloyd M. Logan
• Original Assignee: Graham R. Hunt, Lloyd M. Logan
• Inventors: Hunt, Graham R., Logan, Lloyd M
• Primary Examiner(s): Schonberg, David
• Assistant Examiner(s): Clark, Conrad
• Application Number:
  US05/266,786
• Time in Patent Office: 476 Days
• Field of Search: 356/216,43,44,45,46,47,48,49,50 73/355,1F
• US Class Current: 356/216
• CPC Class Codes: G01J 5/52 using comparison with reference sources, e.g. disappearing-filament pyrometer

Abstract

A technique or method and apparatus utilizing energy available from metal mirrors having different emissivities as a calibration source for radiometers that determine absolute radiance of low energy, infrared targets near and below ambient temperatures. By the changing of a mirror in the optical train with one of different reflectivity, the relationship between reflectivity and emissivity enables the calculation of the number of photons in the field of view of a detector for each mirror such that the altering of the photon flux in a predictable manner by the change of mirrors enables calibration.

Claims

1. A method for determining the absolute radiance of a low energy source with a radiometer having a target optical system, a reference optical system, a modulator and a detector comprising directing the target and reference optical systems at a cold target, varying to predetermined values the reflectivity and emissivity of a reflector exclusively utilized in the target optical system to provide energy level outputs from the radiometer detector corresponding to the various values of emissivity at ambient temperature, directing the target optical system at an infrared target of interest while simultaneously directing the reference optical system at a cold target, comparing the output from the detector of said radiometer when said target optical system is directed at an infrared target of interest with the outputs corresponding to said various values of emissivity to determine the absolute radiance of the target of interest.

2. A radiometer comprising an optical system having a plurality of reflectors in optical alignment for collecting energy from an infrared target of interest, means for selectively varying the reflectivity and emissivity of one of the said reflectors, an optical system for collecting energy from a reference target, means for alternaTely passing energy from each of said optical systems, and a detector for alternately receiving the energy from said means for passing energy to provide an output.

3. A radiometer as defined in claim 2 wherein the said one of said reflectors has among its various reflectivities and emissivities the same reflectivity and emissivity as said means for alternately passing energy, said means for passing energy serving as a reflector to direct the output from said optical system for collecting energy from a reference target to said detector.

4. A radiometer as defined in claim 2 wherein said means for alternately passing energy comprises a chopper.

5. A radiometer as defined in claim 2 wherein said means for selectively varying the reflectivity and emissivity of one of the said reflectors comprises a series of reflectors, each of which has a different reflectivity and emissivity, and indexing means for separately placing each of said reflectors of said series of reflectors into optical alignment with the plurality of reflectors of said optical system for collecting energy from a target.

6. A radiometer as defined in claim 5 wherein said reflectors of said series of reflectors are mounted on a common mounting plate having a good thermal conductivity.

7. A radiometer as defined in claim 5 wherein one of the reflectors of said series of reflectors is gold.

8. A radiometer as defined in claim 5 wherein one of the reflectors of said series of reflectors is stainless steel.

9. A radiometer as defined in claim 5 wherein one of the reflectors of said series of reflectors is chromium.

10. A radiometer as defined in claim 5 wherein one of the reflectors of said series of reflectors is bismuth.