ASNT Level III- Visual & Optical Testing

1.3 Blackbody Radiation
The light from common sources, particularly light from incandescent lamps, is
often compared with light from a theoretical source known as a blackbody.
For equal area, a blackbody radiates more total power and more power at any
wavelength than any other source operating at the same temperature.
For experimental purposes, laboratory radiation sources have been built to
approximate closely a blackbody. Designs of these sources are based on the
fact that a hole in the wall of a closed chamber, small in size compared with
the size of the enclosure, exhibits blackbody characteristics. This can be
understood with the help of Fig. 1. At each reflection, some energy is
absorbed. In time, all incoming energy is absorbed by the walls. Conversely,
a blackbody can be a perfect radiator. If the interior walls of the blackbody are
uniformly heated, the radiation which leaves the small opening will he that of
a perfect radiator for a specific temperature and its emission energy and
wavelength spectrum will be independent of the nature of the enclosure. From
1948 to 1979, the international reference standard for the unit of luminous
intensity was taken to be the luminance of a blackbody operating at the
temperature of freezing platinum.
Charlie Chong/ Fion Zhang