Understanding Infrared Thermography Reading 7 Part 2 of 2.pdf

A.3.3 The Target Surface
The chart of the electromagnetic spectrum (Figure A-3) indicates that the
infrared portion of the spectrum lies adjacent to the visible. Every target
surface above absolute zero (0 Kelvins or -273° Centigrade) radiates energy
in the infrared. The hotter the target, the more radiant energy is emitted.
When targets are hot enough, they radiate or glow in the visible part of the
spectrum as well ( and beyond that, again becoming invisible again, example
UV & ɣ ray) . As they cool, the eye becomes no longer able to see the emitted
radiation and the targets appear to not glow at all. Infrared sensors are
employed here to measure the radiation in the infrared, which can be related
to target surface temperature. The visible spectrum extends from energy
wavelengths of 0.4 µm for violet light to about 0.75 µm for red light. (µ or µm
stands for micrometers or microns. A micron is one-millionth of a meter and is
the measurement unit for radiant energy wavelength.) For practical purposes
of temperature measurement, the infrared spectrum extends from 0.75 µm to
about 20 µm.
Charlie Chong/ Fion Zhang