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Digital holography under weak illumination
Digital holography at light levels below noise using a photon-counting approach
Opt. Lett. 39 (17), pp. 510-513 (2014)
Nazif Demoli, Hrvoje Skenderović and Mario Stipčević
The capability of acquiring information in low light or weak illumination (WI) conditions and is important for a variety of scientific and industrial applications, ranging from microscopy imaging to medical imaging (for example, objects sensitive to light or DNA sequencing). Particularly, the WI conditions are met in single photon detection (extra long photo-sensor exposures) and high speed imaging (ultra short photo-sensor exposures). In such conditions, it is known that both, Poisson noise and quantization noise (temporal fluctuations in intensity, limited dynamic range), become dominant sources of noise.
Digital holograms recorded with standard CCDs in the WI conditions are, depending on the beam ratio, either obscured by noise (1:1) or, otherwise, flattened by the bit depth conversion (usually 8 or 10 bit). Possible approach is to use a photon counting detector.
Optimum conditions for recording digital holograms of a very weak signal (0.44 cps) hidden below the detector’s noise (21 cps) were investigated in terms of the most important holographic measures, namely the fringe visibility (or contrast) and the signal-to-noise ratio (SNR), and in relation to the main holographic parameters (the beam ratio and the exposure). To preserve the originally recorded holographic information, high dynamic range of a photon-counting detector was employed and the possibility to vary the recording parameters in a wide range of their values. It is shown that the optimum recording parameters are possible to estimate theoretically and to implement experimentally. It is also found that increasing the contrast does not necessarily improve the SNR and that, in cases of nearly constant contrast, the SNR clearly reveals differences in the quality of holographic recordings.
Comparison of the holographic reconstruction images for the following parameters: τ (exposition time) and K (reference-to-signal beam ratio).