Blackbody irradiation source

Background is, by definition, the irradiance level that remains when the source (usually blackbody) irradiance is set to zero. In low-background testing, we may use a blackbody to cause an irradiance level that simulates the background at which the detector will operate in another application. In this case, we would use the apparently contradictory expression the background due to the blackbody. ... 

Figures 7.4(a), (b) and (c) give the spectral characteristics of three sources (a) the sun at the earth s surface, (b) a JP-4 pool fire and (c) blackbody sources at typical room fire conditions (800-1100 K). The solar irradiance is mostly contained in about 0.3-2.4 fim while fire conditions span about 1-10 pm.

The blackbody radiative mechanism proposed here is in fact very reminiscent of the model advanced by Dimbar in which CW infrared laser irradiation can be regarded as a blackbody source raising the effective temperature of the system. In this model, the population of ions achieves a truncated Boltzmann distribution, which resembles that of a normal Boltzmann distribution characteristic of the effective temperature but which abruptly ends near the dissociation energy of the ion of interest because once this energy is exceeded, the species rapidly imdergo... 

To see how ASa depends on the intensity of the radiation, suppose an ensemble of absorbers is exposed to continuous light with an irradiance of I(y) = Ib(v) + Ir v), where Ib v) is the diffuse blackbody radiation fi om the surroundings at the ambient temperature (T) and /r(v) is the radiation from any additional source. Molecules that absorb at frequency v will be excited with a... 

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 IR work is a blackbody, and a blackbody is the obvious choice whenever it will generate acceptable signal levels at obtainable temperatures. A well-made cavity blackbody (technically a blackbody simulator) has an emissiv-ity so close to unity that we can safely assume that value. It requires no calibration except that required for the temperature sensors. Given the cavity temperature and the area of the separated aperture, Planck s radiation law yields the exitance and irradiance - both as a function of wavelength, and integrated over any desired spectral region. These were discussed in Chapter 2. Mounting and calibration of blackbodies is discussed in Section 9.3.1. 

Sources other than blackbodies are used when testing detectors that will be used with some other specific source, or when fhe needed IR irradiance cannot be achieved with a blackbody, or for visible applications. Alternate sources include lamps (incandescent bulbs), LEDs, Globars, and carbon arcs. These are available commercially with a little Web search a few references are included at the end of this chapter under the heading Sources of Additional Information - Sources other than Blackbodies. ... 

Temperature and Emissivity of the Shutter and Chopper The temperature and emissivity of the shutter and chopper enter our irradiance calculations. Assuming that the shutter or chopper is at room temperature is often adequate, but the validity of this assumption and its effect on the irradiance should be reviewed periodically. The blackbody will heat the chopper and shutter, and the increased temperatures can cause significant errors in some cases. For high blackbody temperatures, it may be essential to include a thermal shield between the source and the shutter. 

Data Required To measure linearity, we need the DC output at a number of different IR inputs to the detector. The specification usually requires that we vary the irradiance, but we may also change the integration time. The irradiance can be varied by changing the temperature of an extended-area blackbody, the distance to a cavity source, the current to an LED, or the stimulus for some other source. 

On the other hand, if we vary irradiance by changing the distance between a source (a cavity blackbody for example) and our detector, and the distances are in error (whether random or systematic), that error will contaminate our linearity plots. 

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