Figure 2-2: blackbody radiation curves – RayTek 3i User Manual
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Raynger 3i Series Operator’s Manual
2-3
2.2
PRINCIPLES OF OPERATION
An infrared thermometer and the human eye are very similar. An infrared ther-
mometer has a lens that focuses infrared radiation from an object onto a detector. The
eye focuses light onto the retina. The detector is stimulated by the incoming infrared
energy and produces a signal that is transmitted to the circuitry. The retina is stimu-
lated by incoming light and sends a signal to the brain. The circuitry processes this
signal and computes the temperature of the object.
The intensity of an object’s emitted infrared energy increases or decreases in propor-
tion to its temperature. The higher the temperature of the target, the greater the inten-
sity of infrared radiation.
To calibrate a noncontact temperature measurement instrument, the manufacturer
uses a blackbody. A blackbody is a perfect emitter because it absorbs and emits all
radiant energy but reflects or transmits none. The emissivity value of a blackbody is
1.00. Figure 2-2 shows the radiant emittance values of a blackbody at various tem-
peratures and wavelengths.
Most objects have emissivities that are less than 1.00 but are reasonably uniform at all
wavelengths of the infrared spectrum. These are called graybodies. The non-ideal
(less then 1.00) emissivity values of different materials can be compensated for, by the
emissivity controls, so that accurate temperature readings can be obtained. Emissivity
values for many common materials (both metals and non-metals) are listed in
Appendix B.
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14
10
10
10
10
1
10
10
2
1
-1
-2
-3
-4
Wavelength (Microns)
Blackbody Radiant Emittance (Watt/cm )
2
1500°C (2730°F)
1000°C (1830°F)
542°C (1000°F)
260°C (490°F)
20°C (70°F)
Figure 2-2: Blackbody Radiation Curves