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Proceedings Paper

Beating the brightness theorem: thermodynamics of light recycling (experimental)
Author(s): Ling Fu; Ralf Leutz; Harald Ries
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Paper Abstract

The brightness theorem states that it is impossible to increase the spectral radiance of light by passive optical devices, which seems intuitive because spectral radiance is connected to temperature; increasing it seems to violate the second law of thermodynamics. However, consider a gray thermal source, that is a source that emits thermal radiation but with less than unit emissivity. Kirchhoff's law states that absorptivity is equal to emissivity. Thus if we redirect part of the emitted radiation back onto the source, some of it would not be absorbed but either transmitted or reflected instead. Consequently, this radiation would be superposed to the thermal radiation from the source in the same phase space and thereby the spectral radiance would be increased. In fact, thermal sources such as high pressure discharge lamps feature absorptivity-emissivity values far below unity. In this contribution with spectroscopic measurement we show that for such sources the spectral brightness can be increased. This does not contradict the second law because light recycling reduces the irreversible entropy production inherent to the radiation process. It is equivalent to increasing the optical thickness of sources for the price of reducing the total phase space of emitted radiation by the same factor.

Paper Details

Date Published: 11 January 2006
PDF: 6 pages
Proc. SPIE 6033, ICO20: Illumination, Radiation, and Color Technologies, 603304 (11 January 2006); doi: 10.1117/12.668058
Show Author Affiliations
Ling Fu, Philipps Univ., Marburg (Germany)
Harbin Institute of Technology (China)
Ralf Leutz, Philipps Univ., Marburg (Germany)
Harald Ries, Philipps Univ., Marburg (Germany)

Published in SPIE Proceedings Vol. 6033:
ICO20: Illumination, Radiation, and Color Technologies
Dazun Zhao; M. R. Luo; Hirohisa Yaguchi, Editor(s)

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