We present an experimental study of coherent backscattering (CBS) of photon noise from multiple scattering media. We use a pseudothermal light source with a microsecond coherence time to produce a noise spectrum covering a continuous transition, from wave fluctuations to shot noise over several MHz. The angle-dependent Fano factor of backscattered light shows an enhancement due to CBS in the wave fluctuation regime. The CBS line shape and enhancement factor of the noise power is consistent with theory in the weak-scattering limit and for a large number of open reflection channels. These initial experiments on weakly scattering media demonstrate that sensitive noise measurements can be combined with the separation of path lengths present in CBS, opening up new experiments on noise transport in the localization regime. Contents

The origin of spectral features, speckle effects, is explained, followed by a discussion on many aspects of spectral features generation. The next part gives an overview of means to limit the amplitude of the spectral features. This paper gives a discussion of all means to reduce the spectral features amplitude (SFA), i.e. inner pixel averaging, coherence effects, and angular averaging, and to a lesser extend coherence effects and polarization. A new approach to model spectral features is presented, that is based on the averaging mechanisms, in a similar way as is used for speckle patterns. Formulae for all averaging methods are given and the use is discussed. The paper starts off with a short description on Spectral Features as observed in earth observation spectrometers, followed by speckle effects and averaging mechanisms. Next the origin of the averaging mechanisms in a spectrometer are given and a method to calculate them.

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