Day-night vision

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Single images of a multispectral image (C) of a scene, whereby one image (A) is the scene in the visible spectrum and another image (B) is the scene in the near infrared spectrum and is only artificially illuminated in the near infrared spectrum. This lighting is invisible to the human eye. In the multispectral image shown, an object can be recognized well on both a dark and a light background.

Day-Night Vision (in German day-night-seeing or 24h-seeing ) describes the ability to see (machine) independent of ambient light. Mostly optical and imaging techniques are understood here.

Day-night vision systems differ from simple night vision systems in that they can "see" in sunshine as well as at night and especially in the transition phases between day and night. In addition to these long-lasting brightness transitions, there is also the challenge of making image sequences with rapid brightness transitions (for example when driving out of a tunnel at high speed) evaluable. A particular difficulty here is that part of the object of interest is photographed against a light background, while another part is photographed against a dark background.

The following light sources can be used for this:

• Artificial lighting through the system (active lighting)
• Ambient lighting (e.g. solar radiation, moonlight, street lamps, ...)
• Natural radiation of an object (e.g. thermal emission)

Depending on the characteristics on the basis of which the objects of interest differ from the background, one or more techniques for day-night vision are selected. The table below shows which features of the objects of interest affect the intensity on the imaging sensor.

feature	active lighting in the NIR (near infrared; infrared, the wavelength of which is close to that of visible light)	Natural radiation in the FIR (far infrared; thermal radiation)
horizontal / vertical position	yes, because of Light cone (only illuminated objects can also reflect the light)	no influence
Distance to the system	Quadratic decrease in intensity with distance (actually even with the fourth power - but the apparent size of the object becomes quadratically smaller with distance)	no influence
orientation	yes, there is usually only a retroreflection for objects that are perpendicular to the viewing direction . Longitudinally oriented objects usually act like a mirror.	no influence
size	no influence	no influence
Surface / material	Yes	Yes
temperature	no influence	yes, the warmer, the brighter / shorter-wave

From the table follow u. a. the following facts:

• People are warmer than the environment and can therefore be easily recognized by their own radiation (here: thermal radiation).
• The writing on traffic signs cannot be recognized by their own radiation, as the surface usually consists of exactly one material. However, this is possible through active lighting.
• Road markings cannot be reliably detected using natural radiation.
• When the road surface is wet, no road markings can be seen even with active lighting, since the emitted light is reflected in and not against the direction of travel and line of sight.