Abstract
The emissivity of most material surfaces that can be used as radiation sources is a function of wavelength. On the other hand, blackbody cavities with emissivities higher than 0.995 in a wide wavelength range are readily available in many laboratories. If it were possible to attenuate by a constant factor the radiation emitted by those blackbodies, then they could be used as gray-body radiators. A neutral density filter is not an option to attenuate the radiation from a blackbody source because its transmittance is wavelength dependent. Optical choppers, usually rotating disk shutters, are widely used to modulate the intensity of a light beam. The apparent transmittance of an optical chopper is defined in terms of the mark-to-space ratio. Most optical choppers have a 1:1 ratio which would be equivalent to 50 % transmittance. To attenuate the radiation coming from a blackbody, the optical chopper should have a stable rotating speed and a high chopping frequency so its mark-to-space cycle time is very short compared to a radiation thermometer response time. If this condition is fulfilled, the radiation thermometer would display a temperature reading as if it were aiming to a gray-body at the temperature of the blackbody and with an emissivity equal to the optical chopper transmittance. This method to obtain a gray-body radiator using a blackbody source and an optical chopper is discussed, and some measurements including its uncertainty analysis are reported.
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Abbreviations
- \(\varepsilon _{\mathrm{GB}}\) :
-
Spectral emissivity of a gray-body, having a constant value, independent of wavelength
- \(\lambda \) :
-
Wavelength (m)
- \(L\, (\lambda ,T)\) :
-
Spectral radiance of a surface at given wavelength \(\lambda \) and temperature \(T\) (\(\hbox {W}{\cdot }\hbox {m}^{-2}{\cdot }\upmu \hbox {m}^{-1}{\cdot }\hbox {sr}^{-1}\))
- \(L_{\mathrm{b}}(\lambda ,T)\) :
-
Spectral radiance of a blackbody at given wavelength \(\lambda \) and temperature \(T\), as defined by Planck’s radiation law (\(\hbox {W}{\cdot }\hbox {m}^{-2}{\cdot }\upmu \hbox {m}^{-1}{\cdot }\hbox {sr}^{-1}\))
- \(S(T)\) :
-
Signal produced in the detector of a radiation thermometer, produced by that part of the radiation energy, emitted and/or reflected from the objects under measurement, that builds an image in the thermometer optical system, arbitrary units
- \(T\) :
-
Temperature of an object that could be a blackbody, a gray-body, the surface of a chopper disk, or the surroundings walls (K)
- \(\tau _{\mathrm{CD}}\) :
-
Transmittance of a chopper disk, defined by its mark-to-space ratio
References
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Rodríguez-Arteaga, H., Cárdenas-García, D. Gray-Body Radiation Using a Blackbody Source and an Optical Chopper. Int J Thermophys 36, 1757–1765 (2015). https://doi.org/10.1007/s10765-015-1904-5
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DOI: https://doi.org/10.1007/s10765-015-1904-5