2023-12-20 23:00:00
The field of low-light vision and imaging is defined by its applications. Historically, it has been dominated by military applications. The needs of armies, to ensure their missions in a nocturnal environment, have driven these industrial developments since the end of the 1950s. The fundamental goal pursued is to provide man with a night vision capacity, the capabilities of which we are always seeking to increase. performance. This motivation can, of course, be expressed in several civilian applications such as pleasure boating or observing nature and nocturnal wildlife. Outside of the field of night vision, several fields of scientific applications exploit these technologies. To cite only two, it is undoubtedly illusory to want to be exhaustive, we can retain two extreme cases: astronomy and microscopy.
Low-light night vision is defined by a spectral range in which photon fluxes are weak, or even very weak, at night on the Earth’s surface. Initially, due to human capabilities, this domain was limited to the spectral band of the eye. Then, naturally, the limitation of the photon flow in the Visible domain and technological capabilities led to the expansion of this spectral band in order to explore and exploit the limits provided by the terrestrial environment. We will not address here the field of thermal infrared which, day and night, involves processing significant photonic fluxes while being only partially sensitive to the illumination of the scene. The thermal infrared field makes it possible to produce images dominated by temperature and emissivity contrasts. Thermal images of scenes are, therefore, very different from the images perceived by human vision which is sensitive to the different levels of reflectivity (or albedos) of objects illuminated by natural or artificial sources illuminating the scene. Also, we will focus here on this area where the signatures of the objects composing the images are dominated by the contrasts in albedos day and night. This limits the spectral domain in the short wavelengths to the near ultraviolet, around 0.3 µm, defined by the start of atmospheric transmission and in the long wavelengths in the infrared in band 1 to 2, 5 µm by the beginning of the domination of photon flows from the Black Body at room temperature.
This article focuses on the analysis of the different contributors to nighttime illumination, scene contrasts and the fundamental elements responsible for the signal-to-noise ratio in low light level imaging.
The reader will find at the end of the article a table of acronyms and a table of symbols used.
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