= Thermopile radiative measurement
PASCO's documented lamp experiment connects a TD-8553 <thermopile> sensor to a millivoltmeter and places it level with a lamp filament along a meter stick. Lamp and detector axes are aligned. With the lamp off, readings at several positions establish an average ambient <thermal radiation> background. With the lamp on at a fixed supply setting, detector readings at different distances are compared after background subtraction. Between readings, a reflective shield prevents the detector warming; its shutter also protects the reference junction when idle. Moving the shutter can move the sensor, so fixed geometry matters. The manual's lamp limits must be followed.<Thermal Radiation System: Instruction Manual and Experiment Guide, TD-8553, TD-8554A and TD-8555 by PASCO scientific>[p. 1, Radiation Sensor; pp. 9–11, Experiment 2, Fig. 2.1, procedure and calculations: alignment, lamp-off ambient readings, heat shield and distance comparison]{ref}
This protocol tests whether the background-corrected <electric potential difference> approximately follows the <inverse-square law> over a suitable distance range. It is a relative <electromagnetic radiation> measurement; those readings alone do not calibrate absorbed <power> in <watts>. The manual treats the filament as an approximate <point source>.<Thermal Radiation System: Instruction Manual and Experiment Guide, TD-8553, TD-8554A and TD-8555 by PASCO scientific>[p. 3, Stefan-Boltzmann Lamp; p. 11, Experiment 2 calculations: point-source approximation and comparison against inverse squared distance]{ref}
For an absolute calibration, independently known absorbed <power> and a monitored reference <temperature> would be needed. A useful additional control is to repeat a fixed detector position before and after a distance sweep: disagreement reveals possible drift. These additions test the measurement chain rather than assuming that background subtraction fixes every error. A finite source, changing illuminated <area>, reflections, changing reference <temperature>, and insufficient settling can alter the measured dependence. In particular, the <time constant> in <thermopile thermal response> explains why a quick reading and a settled reading may differ. The detector infers its own absorber <temperature> difference; inferring the source's <temperature> additionally requires its <emissivity>, optical geometry and an appropriate <thermal radiation> model.
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