= Electrical substitution radiometer
= Electrical substitution radiometry
{synonym}
An <electrical substitution radiometer> measures <radiant flux> by replacing absorbed optical <power> with calibrated <Joule heating>. At fixed <temperature>, <feedback control> holds the receiver equally warm with the beam blocked or admitted: after corrections, absorbed optical <power> equals the reduction in supplied electrical heating. Incident <radiant flux> additionally requires <radiometric absorptance> and entrance-path corrections. This connects <radiometry> to electrical standards.<Spectroradiometric Detector Measurements: Ultraviolet, Visible, and Near-Infrared Detectors for Spectral Power by Thomas C. Larason and others>[§5.2, printed p. 30: electrical substitution and electrical standards]{ref}
Picture a small absorbing cavity connected to a colder reference block by a <radiometer thermal link>. A heater supplies <Joule heating>, a <thermometer> senses the receiver, and <feedback control> adjusts the heater. When <light> supplies some of the heating, the controller supplies less. The readout is the change in heater <power>, rather than the beam's brightness or its number of <photons>. NIST's institutional description gives a concrete receiver with a conical interior, absorbing coating and weak Kapton support, operated with a receiver heater and <thermometer>.<Pico-Watt ACR by National Institute of Standards and Technology>[Description, Design Parameters for Improved ACR Sensitivity; Key Design Features: conical receiver, coating, constant-temperature loop and thin-walled Kapton link]{ref}
\Image[/electrical-substitution-radiometer-cavity-control.png]
{title=Cavity, <radiometer thermal link> and heater readout in an <electrical substitution radiometer>}
{id=electrical-substitution-radiometer-cavity-control}
{description=Original functional schematic, not a drawing of a particular laboratory instrument. The shutter defines dark and illuminated states; incident <radiant flux> is specified at the receiver entrance. The cavity absorbs a fraction $a$, while the remainder escapes. The reference block and receiver have distinct controlled <temperatures>. The <thermometer> signal drives <feedback control>, and heater voltage and <electric current> provide the electrical <power> readout. Lead heating, stray radiation and spatial heating differences require separate checks.}
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