Blackbody-type source

Figure 2.12. Typical interferogram of a blackbody-type source in the region of the centerburst measured by a rapid-scanning interferometer. The slight asymmetry indicates a very small amount of chirping.

Most of what is known about atomic (and molecular) structure and mechanics has been deduced from spectroscopy. Fig. 1.7 shows two different types of spectra. A continuous spectrum can be produced by an incandescent solid or gas at high pressure. Blackbody radiation, for example, gives a coiilinuum. An emission spectrum can be produced by a gas at low pressure excited In heat or by collisions with electrons. An absorption spectrum results when light Irom a continuous source passes through a cooler gas. consisting of a scries ol daik lines characteristic of... 

Column 14 The spectral type of the source. Spectrum type C is a cool dust spectrum (typical Td 30 K), peaking beyond 100 m colour correction factors are of order 1.00, 0.95, 0.99, 1.00. Type W is a warm dust spectrum (typical Td 70 K), peaking between 12 and 100 m colour correction factors are of order 1.03,1.00, 1.00, 1.04. Type S is a stellar spectrum (typical blackbody of about 5000 K) colour correction factors are of order 1.43,1.40,1.32,1.09. A semi-colon indicates that the infrared spectrum is uncertain. Actual, colour-corrected fiux densities can be calculated from the quoted ones by dividing the latter by these colour correction factors (see IRAS, 1989a). 

Photoacoustic spectra can be observed with a simple microphone and optical apparatus. A diagram of a modern piece of equipment is given in Figure 42, in which there is a blackbody sample at the second input of a look-in amplifier. Corrected photoacoustic spectra can be recorded with this type of monochromator equipment. Frequency-modulated xenon lamps are used as white light sources, so that a chopper is not needed for modulation. 

The most convenient calibration sources for the middle and far infrared are black-bodies at constant and well-known temperatures. The range of blackbody temperatures should match approximately the range of expected brightness temperatures of the planetary atmospheres and surfaces under investigation. Of course, calibration sources are also subject to systematic errors the temperature sensor of a blackbody may be systematically off, or the actual emissivity of the device may not be as close to unity as assumed. However, these types of error generally can be kept small in number and in magnitude in comparison with systematic changes of the instrument response. 

In the article humorously titled Blackbody, blackbody simulator, blackbody simulator cavity, blackbody simulator cavity aperture, and blackbody simulator aperture are each different from one another, Bartell (1989b) points out important conceptual differences that should be recognized but are generally overlooked when we speak of BBS. A (true) BB is an idealization. The devices in our laboratories that we call BBS are actually blackbody simulators. One type uses an approximately isothermal cavity and a separate aperture. For a well-designed simulator, accurate radiometric calculations can be done by fleating that separate aperture (not the cavity itself, nor the cavity opening) as the IR source. The IR irradiance depends on the distance from the separate aperture. 

The most common source for detector testing in the 3-20 pm region is a cavity type blackbody simulator - a heated cavity with diffuse walls and a large wall area-to-opening ratio. To the extent that such a cavity simulates a true blackbody, its exitance can be predicted using Planck s law this is discussed in Chapter 2. Hereafter, we will follow convention and refer to such a cavity simply as a blackbody. Blackbodies rely on multiple internal reflections to achieve effective emissivities of 0.99 and more. 

Moore et al. (2009) Building Scientific Apparatus (2nd edition) by J. H. Moore, C. C. Davis, and M. A. Coplan, Addison-Wesley, Reading, MA. Describes the construction of an NBS-type blackbody source. ... 

Indirect Refinement of Radiometric Values One significant source of radiometric error is the optical distance between the source and detector. It is seldom possible to measure that overall distance directly or with great precision. Another source is the diameter of the apertures if you use a cavity-type blackbody. 

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