For some time now, we have been investigating to what extent sensors that are sensitive in SWIR (shortwave infrared, 0.9 - 2.5 µm) can be used at night. Of great interest is whether they can replace or supplement the camera and image intensifier systems in use in the visual spectral range (VIS).
It is therefore necessary to clarify how the illuminances measured in the visual system can be compared with the irradiances measured in the SWIR. Until now, for comparisons of cameras sensitive in different spectral ranges, measurements have to be performed during the night. This is costly and not always practicable. In the long term, therefore, an "illumination standard" for the short-wave infrared spectral range is to be established - just as it already exists in the visual range.
In order to be able to make statements on this, data must be collected in a first step. The main light source for the night measurements is, besides the moon, planets and stars, the so-called airglow, a luminous phenomenon of the upper earth's atmosphere. Its nocturnal appearance is called Night Sky Brightness and its intensity is highly variable. The Night Sky Brightness can be observed in all spectral ranges from the UV to the long-wave infrared. It shines particularly strongly in the SWIR between 1.4 µm and 1.8 µm.
Since the astronomical and especially the atmospheric parameters change significantly from measurement night to measurement night, it is difficult to generalize the results. As already suspected at the beginning of the measurements, the cloudiness has a great influence on the measurement results. Furthermore, the influences of the moon and of indirect artificial lighting have to be considered.
In order to do justice to this high variability of the measurement parameters, a sufficiently large number of measurement data is necessary. This can only be obtained by continuous measurement over a period of at least one year.