An engineer is testing the design of solar-thermal absorber plates to determine how different surface treatments affect the rate of thermal radiation emission.
Four identical metal plates, each treated with a different surface finish, are tested:
- Anodized selective black
- Polished bare alloy
- Painted matt dark-grey
- Painted gloss white
The experimental setup is shown in the diagram below:
Each plate is heated to a uniform, steady temperature of 120∘C120^\circ\text{C}120∘C using an electric heating element. An infrared thermal imaging camera is positioned directly above the plate to record the intensity of the infrared radiation emitted.
State one variable that the engineer must keep constant when taking measurements for each plate to ensure a fair comparison.