Apertures used with blackbody

Evidently, we cannot get all of the required background reduction with a cold shield alone, so we will need a spectral filter in conjunction with the cold shield. We pay a penalty for the use of the spectral filter the signal from the blackbody will be reduced. Assume that (like many labs) we have 500 and 800 K blackbodies with apertures that range from 0.050 to 0.500 in diameter, and that total distances from the blackbody aperture to the detector can be as small as 8 . We calculate the irradiance we can expect with three filters, each with a bandpass of 0.2pm, centered at 3, 4, and 5 pm, using two blackbody temperatures (500,800 K) and the two blackbody aperture extremes. Table 9.1 shows the irradiances that we can expect with the 12 different combinations. It also lists the signal-to-noise ratios that we will see, based on the expected NEI. From this table we can pick out the acceptable combinations. 

Blackbody radiation is achieved in an isothermal enclosure or cavity under thermodynamic equilibrium, as shown in Figure 7.4a. A uniform and isotropic radiation field is formed inside the enclosure. The total or spectral irradiation on any surface inside the enclosure is diffuse and identical to that of the blackbody emissive power. The spectral intensity is the same in all directions and is a function of X and T given by Planck s law. If there is an aperture with an area much smaller compared with that of the cavity (see Figure 7.4b), X the radiation field may be assumed unchanged and the outgoing radiation approximates that of blackbody emission. All radiation incident on the aperture is completely absorbed as a consequence of reflection within the enclosure. Blackbody cavities are used for measurements of radiant power and radiative properties, and for calibration of radiation thermometers (RTs) traceable to the International Temperature Scale of 1990 (ITS-90) [5]. 

A well-defined molecular beam strictly defines the source area and angular range of molecules and restricts the amount of background vapor that reaches the ionizer. Furthermore, by using a small field aperture one can make the source area of the molecular beam smaller than the cross-sectional area of the cell orifice. This definition of the beam effectively removes the effect of the shape of the orifice on the fiux distribution of the molecular beam and makes KEMS measurements independent of orifice shape. This effect is analogous to the requirements of sampling the radiation from fuUy within the blackbody when temperature is measured with a pyrometer. 

Example of Spot Size Calculation Consider an//1.5 lens with a 100 mm (4-in.) focal length, used to focus a 0.050-in. (1.25 mm) diameter blackbody aperture on a detector using a 15 pm spectral filter. Assume that we put the blackbody 60-in. from the lens the resulting image will be close to the lens focal point ( 4.12-in. from the lens using the image equations as described in Section 14.6.1). 

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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