Low light level imaging - Fundamentals

The field of low-light vision and imaging is defined by its applications. Historically, it has been dominated by military applications. Since the late 1950s, industrial developments have been driven by the needs of the armed forces to carry out their missions at night. The fundamental aim is to provide mankind with a night vision capability whose performance is constantly being improved. This motivation can, of course, be applied to a number of civil applications, such as pleasure boating or the observation of nature and nocturnal fauna. Outside the field of night vision, there are many other scientific applications that exploit these technologies. To mention just two - and it would be unrealistic to attempt to be exhaustive - there are two extreme cases: astronomy and microscopy.

Low-light night vision is defined by a spectral range in which photon fluxes are low, or even very low, at night on the Earth's surface. Initially, because of human capabilities, this range was limited to the spectral band of the eye. Then, naturally, the limitation of photon flux in the Visible range and technological capabilities led to a broadening of this spectral band, enabling us to explore and exploit the limits brought about by the terrestrial environment. We won't deal here with the thermal infrared domain, which, day and night, requires the processing of large photon flows, while being only partially sensitive to scene illumination. Thermal infrared images are dominated by contrasts in temperature and emissivity. Thermal images of scenes are therefore very different from images perceived by human vision, which is sensitive to the different levels of reflectivity (or albedos) of objects illuminated by natural or artificial sources. We're going to focus here on this domain, where the signatures of the objects making up the images are dominated by albedo contrasts, both day and night. This limits the spectral range in the short wavelengths to the near ultraviolet, around 0.3 µm, defined by the onset of atmospheric transmission, and in the long wavelengths in infrared band 1, around 2.5 µm, by the onset of the domination of photonic fluxes from the Black Body at room temperature.

This article focuses on the analysis of the various contributors to night illumination, scene contrast and the fundamental elements responsible for the signal-to-noise ratio in low-light imaging.

At the end of the article, readers will find a table of acronyms and a table of symbols used.