= Comparing surface emission at equal temperature
Oberlin College's documented demonstration heats a cube internally with a bulb and compares polished <aluminium>, dull <aluminium>, black-painted and white-painted faces at the same <temperature>. The apparatus includes a <thermopile> and a digital meter. Its preparation specifies 20 minutes of preheating and heater setting 6; these are instructions, not measured outcomes. The <thermopile> supplies the electrical readout for the relative <thermal radiation> comparison. Cover it between readings: otherwise detector warming changes its reference <temperature>. The account reports little difference between painted black and white faces and a larger contrast between shiny and painted faces. The comparison motivates a surface-dependent <emissivity> and shows why visible color alone is insufficient.<Leslie Cube by Oberlin College>[Oberlin College, Leslie Cube, apparatus list, Description and Notes: four finishes, common temperature, preheat and detector-cover control]{ref}
Equal source <temperature> is the principal control: it holds the <Stefan–Boltzmann law> factor $\sigma T^4$ fixed. For a sharper comparison, also hold detector distance, viewing <angle>, viewed <area> and surrounding <temperatures> fixed, and record a covered-detector baseline. These are additional diagnostic controls. Under those assumptions, a changed readout can reveal changed <directional emissivity> averaged over the detector's wavelength response. The demonstration supplies neither an absolute calibration nor a scan over all directions and <wavelengths>; it therefore does not directly measure <total hemispherical emissivity> or establish the fourth-power law. Comparing cooling rates would additionally require separating <convection>, <thermal conduction> and <heat capacity> effects, as in <net radiative heat exchange>.
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