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The search for the CIBR is impeded by two fundamental challenges: there is no unique spectral signature of such a background, and there are many local contributors to the infrared sky brightness at all wavelengths, often quite bright. The lack of distinct spectral signature arises in part because so many different sources of primordial luminosity are possible (see, e.g., Bond, Carr, & Hogan 1986), and in part because the primary emissions are then shifted into the infrared by the cosmic red-shift and dust absorption and re-emission. Hence, the present spectrum depends in a complex way on the characteristics of the luminosity sources, on their cosmic history, and on the dust formation history of the Universe.

Setting aside the difficult possibility of recognizing the CIBR by its angular fluctuation spectrum (Bond, Carr, & Hogan 1991), the only identifying CIBR characteristic for which one can search is an isotropic glow. One must solve the formidable observational problem of making absolute brightness measurements in the infrared. One must then discriminate and remove the strong signals from foregrounds arising from one's instrument or observing environment, the terrestrial atmosphere, the solar system, and the Galaxy. Particular attention must be given, of course, to possible isotropic contributions from any of these foreground sources. Finally, if an extragalactic isotropic residual remains, one must evaluate the contribution from galaxies over their luminous lifetime to distinguish their light from that of pregalactic or protogalactic sources.