Understanding Infrared Thermography Reading 3

When there is a solid material, such as a glass or quartz viewing port,
between the target and the instrument, the spectral characteristics of the solid
media must be known and considered. Figure 1.11 shows transmission
curves for various samples of glass. Most significant is the fact that glass
does not transmit infrared energy at 10μm where ambient (30 °C, 86 °F)
surfaces radiate their peak energy. In practice, infrared thermal
measurements of ambient targets can never be made through glass. One
practical approach to this problem is to eliminate the glass, or at least a
portion through which the instrument can be aimed at the target. If a window
must be present for personal safety, vacuum, or product safety, a material
might be substituted that transmits in the longer wavelengths. Figure 1.12
shows the spectral transmission characteristics of several infrared
transmitting materials, many of which transmit energy past 10μm. In addition
to being used as transmitting windows, these materials are often used as
lenses and optical elements in infrared sensors and imagers. Of course, as
targets become hotter, and the emitted energy shifts to the shorter
wavelengths, glass and quartz windows pose less of a problem and are even
used as elements and lenses in high temperature sensing instruments.
Characteristics of the measuring instrument are addressed in Chapter 4.
Charlie Chong/ Fion Zhang