Blackbody calibration

In addition to a blackbody calibration, the software is usually provided with correction functions for ambient effects such as atmospheric attenuation as a func-... 

The DMR experiment utilized techniques more common to radio astronomy than infrared. The main part of the instrument consisted of sets of opposed antennas looking at different parts of the sky. These antennas looked for differential signals indicating different temperatures at different locations. FIRAS was a version of the FTS discussed earUer. It compared the spectral shape of the cosmic background radiation against the spectral shape of internal blackbody calibrators. The DIRBE instrument was more similar to standard infrared instruments than the other two COBE experiments. DIRBE used a suite of four different detector types to map out the sky at several infrared wavelengths. This instrument has contributed very important information on the contribution of various types of astronomical objects to the total background radiation in the universe. 

Blackbody Calibration Radiometric calibration is different from conventional calibration of micrometers and scales. We discuss the radiometric calibration problem in Section 9.6. For now, we discuss the emissivity and the more mechanical parts of blackbody calibration, determination (in a traceable way) of the temperature, and (for cavity type blackbodies) the aperture diameter. 

Sample cells were fabricated from tungsten. Additional crucibles composed of a Pt-40 w/o Rh-8 w/o W alloy were also used in experiments on the PuPt phase. Each tungsten cell was vacuum outgassed at 1800 for 1 h before an experiment. The cell temperature was determined during the measurements by sighting with a pyrometer (Pyrometer Instrument Co.) onto a blackbody hole in each cell base. The pyrometer and sight glasses were calibrated with an NBS standard lamp. 

For the calibration of most infrared ear thermometers the sensitivities S0 and R0 and the temperature coefficients Sj and a for both sensors have to be determined. Typically a two-step calibration is performed. In the first step the ambient sensor is calibrated by immersing it into two different temperature controlled baths. In the second step the thermopile sensor is calibrated by measuring the output signal while placing it before two different blackbody radiation sources. 

For the IRT 3000 a simultaneous calibration concept [6], that is shown schematically in Fig. 3.48, has been developed. At two different ambient temperatures two measurements at different blackbody temperatures are performed with completely assembled thermometers connected to external computers. This results in four independent sets of output values of both ambient and thermopile sensors (see Eq. 6 and 7) ... 

Insertion of the ambient temperatures in Eq. 1 results R0 in and a. Since the ambient temperature is calculated from a polynomial funtion (eq. 2) it is necessary to determine the polynomial coefficients by a least square fit regression over the ambient temperature range 0-50°C. Together with S0,. Si, b, and c, these values are written by the calibration computer to the non volatile memory of the microcontroller. As a calibration check two additional blackbody readings are performed at a third ambient temperature (see Fig. 3.48). 

Nd YAG and the lifetime measurement is made by the use of the phase-locked detection of fluorescence lifetime (PLD) scheme. By reference to the use of the fluorescence lifetime measurement, the problems, in pyrometry, of emissivity, and sight path factor in the blackbody radiation measurement could be corrected in such a scheme having an internal self-calibration. ... 

Figure 11.34 Calibration graph of the fluorescence lifetime (solid line) and blackbody radiation (dashed line) versus temperature.

A strip lamp is a convenient means for the calibration of secondary pyrometers (Fig. 9). The notched portion of the tungsten strip is the target. A pyrometer which has been calibrated against a radiating blackbody is sighted on the target, and the strip lamp current is adjusted to radiate at the intensity of the blackbody, as transferred by the primary pyrometer. The secondary pyrometer is then substituted for the primary, and the current required to raise the lamp filament to the brilliance of the target of the strip lamp is noted. 

No object can radiate more energy than can a blackbody at the same temperature, because a blackbody in equilibrium with a radiation field at temperature T radiates exacdy as much energy as it absorbs. Any object exhibiting surface reflection must have emissivity of less than 1. Pyrometers are usually calibrated with respect to blackbodies. This can cause a serious problem in use. The emissivities of some common materials are listed in Table 4. 

Blackbody radiation sources are accurate radiant energy standards of known flux and spectral distribulion. They are used for calibrating other infrared sources, detectors, and optical systems. The radiating properties of a blackbody source are described by Planck s law. Energy distribution... 

The most accurate measurements of the CMB spectrum to date have come from the Far InfraRed Absolute Spectrophotometer (FIRAS) on the COsmic Background Explorer (COBE) (Boggess et al., 1992). In contradiction to its name, FIRAS was a fully differential spectrograph that only measured the difference between the sky and an internal reference source that was very nearly a blackbody. Figure 9.2 shows the interferograms observed by FIRAS for the sky and for the external calibrator (XC) at three different temperatures, all taken with the internal calibrator (IC) at 2.759 K. Data from the entire FIRAS dataset show that the rms deviation from a blackbody is only 50 parts per million of the peak Iv of the blackbody (Fixsen et al., 1996) and a recalibration of the thermometers on the external calibrator yield a blackbody temperature of... 

On a more absolute level, a blackbody cavity surrounded by gold at its melting point (1064.43°C) can be focused upon for one calibration datum. Such a cavity is depicted in Figure 8.8. By placing a rotating sectored disk, or varying thicknesses of... 

Figure 8.8 Schematic of a blackbody source for temperature calibration. The graphite surface has a high emittance. The molten liquid (e.g. gold) surroundings guarantees temperature uniformity, and as it solidifies or fuses, its temperature is single-valued.

By using absorbing filters, the radiation from blackbody sources at higher temperatures can be down-rated to temperatures within the calibration range of the pyrometer. As a result, the range of the pyrometer can be extended well above the melting temperature of gold. 

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

Relative state populations (NvJ or Nv) are derived from the observed spectrum in two stages. First, the spectrometer-detector unit must be calibrated with a standard blackbody source to allow for changes in sensitivity with wavelength. Then the corrected relative intensities are converted to the NvJ (or Nv) using values of the spontaneous emission coefficients. This procedure is quite simple when individual rotational lines can be resolved [101, 102]. Karl et al. have described a computational technique for analyzing the overlapped first overtone (Av = 1) spectra of CO [261] and NO [262] when the rotational distribution is known to be equilibrated, and Hancock and Smith [256] have extended this method. 

I. Reda, J. Hickey, C. Long, D. Myers, T. Stoffel, S. Wilcox, J. J. Michalsky, E. G. Dutton, and D. Nelson, Using a blackbody to calculate net-longwave responsivity of shortwave solar pyranometers to correct for their thermal offset error during outdoor calibration using the component sum method, Journal of Atmospheric and Oceanic Technology 22 1531 (2005). 

Keyvan S, Rossow R, Romero C (2006) Blackbody-based calibration for temperature calculations in the visible and near-IR spertral ranges using a spectrometer. Fuel 85(5-6) 796-802... 

In order to achieve a reliable temperature resolution of 26 mK, the infrared camera had to be carefully calibrated with special designed blackbody. The geometrical arrangement of three different temperature standards and the reference bodies made the calibration box behave like an ideal black-... 

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]. 

FIGURE 7.17 Radiation thermometry (a) calibration against a blackbody cavity (b) measurement of a real surface. 

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