Omega Speaker Systems OS523E User Manual download pdf (Page 71)

Appendix: How Infrared Thermometry Works

A-2

Blackbody

When thermal radiation falls on an object, part of the energy is

transmitted through the object, part is reflected and part is

absorbed. A blackbody is defined as an ideal object that absorbs all

the radiation incident upon it. The best example of a real object that

acts like a blackbody is a small hole drilled deep into a large opaque

cavity. Thermal radiation entering the cavity is internally reflected

and has little chance of escaping the cavity before it is fully

absorbed.

Emissivity is defined as the ratio of energy radiated by an object to

that of the energy radiated by a blackbody. By definition, the

emissivity of a blackbody is 1. Most objects are considered gray

objects with an emissivity between 0 and 1. Various emissivities for

common materials are shown in Appendix B.

Spectral Distribution

Objects radiate energy at

different wavelengths, but

not with constant intensity

at each wavelength. Figure

A-2 shows the energy

radiated by a blackbody at

various temperatures as a

function of wavelength. As

a body is heated, the

intensity of the radiated

energy increases and the

peak of the curve shifts

towards the shorter

wavelength end of the

spectrum. The total area

under a spectral

distribution curve is

proportional to the total

energy radiated by the

blackbody at a given

temperature.

Figure A-2. Blackbody Spectral Distribution

Relative emission from a blackbody versus wavelength.

The area under the curve corresponds to the total ener-

gy, and is proportional to the absolute temperature to

the 4th power. The peak of the spectral distribution

curve shifts to shorter wavelengths as the temperature

increases.

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