For some time, we at Fraunhofer IOSB are investigating to which extent sensors that are sensitive in SWIR (shortwave infrared, 0.9 - 2.5 µm) can be used at night. It is of great interest whether they could replace or supplement the camera and image intensifier systems used in the visual spectral range (VIS).
Therefore, for night vision applications it has to be clarified how illuminances measured in visual terms can be translated into irradiances measured in SWIR. Until now, outdoor measurements during the night are required for comparisons of camera systems operating in the VIS and SWIR. This is complex and not always feasible. Hence, in the long term a »lighting standard« for the short-wave infrared spectral range is to be established - just as it already exists in the visual spectral range.
In order to be able to make statements on this, data has to be collected in a first step. Besides the moon, planets and stars, the main light source for the night measurements is the so-called airglow, a luminescence of the upper atmosphere of the earth. Its nocturnal appearance is called nightglow and its intensity is highly variable. The nightglow is observable in all spectral ranges from UV to long-wave infrared. It shines particularly bright in SWIR between 1.4 µm and 1.8 µm.
Since the astronomical and especially the atmospheric parameters change significantly from night to night, it is difficult to generalize the results. As already suspected at the beginning of the measurements, cloudiness has a great influence on the measurement results. Furthermore, influences of the moon and indirect artificial lighting have also to be taken into account.
In order to cope with this high variability of the measuring parameters, a sufficiently large number of measuring data is necessary. This can only be obtained by means of a continuous measurement over a period of at least one year.
|Examples to illustrate the differences VIS - SWIR|