= Thermopile absorber
A <thermopile absorber> converts incident <electromagnetic radiation> into <heat> near the <thermocouple measuring junctions>. Electrically separate junctions can share its <temperature> through thermal contact. The reference support provides a path for <heat transfer> and a comparison <temperature>; “cold” denotes this reference role, even if it is warmer than the absorber.
Distinguish incident <power> from absorbed <power>. A window, the illuminated <area>, and the absorption of the coating determine their relation. A detector's measured voltage sensitivity can depend on where a small illuminated spot lies.<Thermopile detectors by Hamamatsu Photonics K.K.>[p. 3, §3-1 Sensitivity, Eq. (3-1) and following paragraph: uniform illumination versus a localized spot]{ref} The model in <thermopile thermal response> takes absorbed <power> as its input, so an optical calibration is a separate step.
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