An infrared thermometer measures radiant energy beyond the sensitive range of the human eye.  All objects radiate this energy with an intensity relative to the temperature of the object.  Should an object become sufficiently heated, the infrared energy will become visible as the object becomes red hot.  As the temperature increases, it emits wavelengths further across the visible range.  Ultimately, the object glows white hot. 

Infrared radiation (IR) is electromagnetic radiation lying in the wavelength interval from about 0.75 microns to an indefinite upper boundary.  This boundary is sometimes arbitrarily set at 1,000 (1 mm).  This radiation is simply “light” that is slightly outside the human eye’s sensitive range.  It upon how hot the object is.  The spectrum band consists of the near infrared (0.75 microns to 1.5 microns), intermediate infrared (1.5 microns to 7 microns) and far infrared (7 microns to 1,000 microns), as shown in Figure 1 below.  The infrared radiation obeys all the laws of light.  These include shadowing, reflection, refraction and other optical behavior.

Figure 1

In other words, the net radiation flow is proportional to the difference between the fourth powers of the absolute temperatures of the two objects and to the area of the cooler object that is exposed to sight from the hotter object. 

Instrumentation theory and practice teach that all instruments, in order to measure a quantity, must either consume or divert a small portion of that quantity in the process of measurement.  This act disturbs the original quantity, altering its magnitude, and since it is the altered magnitude that is actually measured, an erroneous reading results (Figures 2 and 3). 

Figure 2

Figure 3

This error-producing process applies to all know temperature measuring instruments except infrared thermometers.  All conventional thermometers require a probe to touch the object being measured, which is usually at a different temperature.  Consequently, heat transfer occurs.  After equilibrium occurs, the temperature measurement is said to have “come up,” but the object is now either colder or warmer than it was before the probe “intruded upon it”.  The amount by which the object is cooler or warmer than the probe is a part of the error of measurement.

Infrared thermometers produce no “intrusion error.”  A hot object “target” is radiating its infrared radiation in all directions whether or not the infrared thermometer is there taking its temperature.  The object’s radiation characteristics, and hence its temperature, are not disturbed by the presence of the infrared thermometer.

Assume that the instrument is the cooler of the two objects and that its front end optical “telescope” is the area that is exposed to sight from the hotter object being measured.

The optics then collect this sample of infrared radiation from the hot object being measured and focus it on the tiny infrared detector.  The detector, in turn, converts it to a proportional electrical signal, which is the exact electrical analog of the incoming infrared radiation, and hence the hot object’s temperature.

This minute electrical signal is then amplified in the preamplifier as shown in Figure 4, converted to a digital signal, and digitally linearized (to change the T⁴ radiation characteristics to a perfectly linear voltage-temperature relationship).  After linearization and further conditioning, the resultant temperature number is shown on the display of the instrument.

Figure 4