Understanding Infrared Thermography Reading 7 Part 2 of 2.pdf

A.3.1 Some Historical Background
The color of a glowing metal is a fair indication of its temperature (the higher
the temperature, the whiter the color). The ancient sword-maker and
blacksmith knew from the color of a heated part when it was time to quench
and temper. This technique is still in use today; precision optical matching
pyrometers are used to match the brightness in color of a product with that of
a glowing filament. The brightness of the filament is controlled by adjusting a
knob that is calibrated in temperature. The next logical step is to substitute a
photomultiplier for the operator’s eye and, thus, calibrate the measurement.
Finally, a differential measurement is made between what the brightness of
the product is and what it should be (the set point), and the differential signal
is injected into the process and used to drive the product temperature to the
set point. With the advent of modern infrared detectors, the precision
measurement of thermal energy radiating from surfaces that do not glow
became possible. Measurements of cool surfaces, well below 0°C, are
accomplished routinely with even the least expensive of infrared sensors.
Charlie Chong/ Fion Zhang