by Bernie Lyon
Referring to Figure 1, the “ideal” block has an emissivity of 0.2 and a reflectivity of 0.8 We are assuming that these values are constant and do not change with angle. This is not the case. If thermal radiation strikes a single surface, 20% of the radiation is absorbed and 80% is reflected. (Keep in mind 20% of the radiation – Not the temperature!) If it strikes another surface before exiting, 20% of what remains is absorbed and 80% of that is reflected, and so on.
Therefore, with multiple reflections caused by cavities, the effective emissivity, or emittance, increases, while the effective reflectivity decreases. This is the reason why holes and cavities appear dark. They reflect less visible light.
In Figure 2, there is a thermal image of two heated aluminum blocks, with holes drilled in them. Additionally, a small washer was placed between the two blocks, creating another cavity. The cavities certainly appear warmer. If you look closely, you can see that the deeper holes on the right appear warmest.
A thermocouple was placed in the cavity between the blocks, making good contact with the surfaces. The thermocouple read 125°F. The table shows the temperature measurements obtained with a longwave (8-12mm) infrared camera, with the TReflect properly set, for various emissivity settings.
Notice the close correlation between the thermocouple and the infrared camera when fairly high emissivity values were used. The majority of values found in emissivity tables, 0.2 or less, would have produced temperatures much too high because tables do not take into account the cavity effect.
I like to think of cavities as “truth indicators.” They are not always warm. If they appear cool, the object is genuinely cool.