Blackbody Radiation and Masking Tape
You knew your high school physics lesson on black body radiation was sure be of practical use, and recall that all matter above absolute zero continuously emits electromagnetic radiation.
Max Planck, in 1901, was the first to define the relationship between the intensity of radiation for a black body as a function of temperature and wavelength.
From his work we understand as the temperature of an object increases, the peak intensity shifts to shorter wavelengths. By slowly heating a piece of metal we can observe this relationship. At room temperature, the metal does not emit any visible light. As the temperature reaches approximately 500°C, it begins to glow red as the metal emits energy in the red portion of the visible spectrum. As the temperature of the metal increases to approximately 1500°C, it begins to glow white emitting energy at all visible wavelengths.
Emissivity describes the efficiency, compared to a black body, with which a material radiates energy. The temperature of an object and its emissivity define the amount of infrared energy an object will emit. At room temperature most of the emission is in the infrared region of the electromagnetic spectrum.
Measuring metals can be challenging. Non-contact IR sensors cannot separate incoming IR radiation into real object and reflected parts. Materials with a low emissivity is a very specialized IR measurement field. Gold has an emissivity of 0.05! A typical value for aluminum (rough finish) is 0.18 and less if polished. Copper can vary widely depending on the degree of oxidation, ranging from 0.2 to 0.8.
To assist with more accurate measurements, masking tape provides a simple paper layer that will quickly reach the same temperature as the object in question. Paper has a high emissivity of 0.85 and higher. An alternative technique for flat surfaces is a thin layer of baby powder.
Non-Contact IR Sensor
Another example of emissivity: human skin 0.99. This is the key to the success of non-contact medical temperature devices.