Omega Engineering OS533E manual Blackbody

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.

AAppendix: How Infrared Thermometry Works

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.

A-2