Understanding Infrared Thermography Reading 7 Part 2 of 2.pdf

Thin films of many plastics are virtually transparent to most infrared
wavelengths but do emit at certain wavelengths. Polyethylene, polypropylene,
and other related materials, for example, have a very strong, though narrow,
absorption band at 3.45 µm. Polyethylene film is formed at about 200°C in the
presence of heaters that are at about 700°C.
Figure A-21 shows the transmission spectra of 1.5- mil thick polyethylene film
and the narrow absorption band at 3.45 µm. The instrument selected for
measuring the surface of the film has a broadband thermal detector and a
3.45 µm spike band pass filter. The filter makes the instrument blind to all
energy outside of 3.45 µm, and enables it to measure the temperature of the
surface of the plastic film without seeing through the film to the heaters.
Charlie Chong/ Fion Zhang