= Cryogenic laser-power substitution experiment
In NIST's 2001 comparison, the Laser-Optimized Cryogenic Radiometer held its cavity near $5.4\,\mathrm K$ and controlled the heat sink separately. Opening the beam reduced heater <power> at fixed cavity <temperature>; an AC bridge read the <temperature>. Tests used stabilized, spatially filtered <lasers> at $488$, $514$ and $633\,\mathrm{nm}$, with horizontal input and a movable <photodiode> transfer detector in the cavity's optical plane. The paper contrasts this with the High Accuracy Cryogenic Radiometer's optical heating followed by two electrical heating cycles and interpolation.<Comparison of Two Cryogenic Radiometers at NIST by Jeanne M. Houston and others>[§§2–3, printed pp. 642–643: apparatus, wavelengths, readout and the two substitution protocols]{ref}
The two laboratories compared calibrations of the same transfer detectors, and checked their spatial response. Alignment differences and transported dust limited that comparison; its agreement did not prove universal equivalence. Its correction tables retain window transmission, cavity <radiometric absorptance>, alignment, electrical readout and <optical-electrical heating non-equivalence>. The heat sink was controlled, but §3 does not specify its numerical setpoint: the $5.0\,\mathrm K$ reference in the preceding example is stipulated.<Comparison of Two Cryogenic Radiometers at NIST by Jeanne M. Houston and others>[§§3–5, printed pp. 643–646, especially Tables 2–3 and spatial-uniformity checks]{ref}
For a reproducible substitution measurement, record the beam's <wavelength> and entrance boundary, receiver and reference <temperatures>, heater terminal voltage and <electric current>, dark readings before and after illumination, and the settling rule. Check beam collection at the <aperture>, window transmission, <radiometric absorptance>, deposited lead heating, and sensitivity to beam position. Repeatability alone misses a stable correction error. <Photometric calibration> can subsequently use this <radiant flux> scale, but <photometry> still needs the appropriate spectral visual weighting; the cold receiver does not itself measure <luminous flux>.
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