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Laser protection in military binoculars

Almost all custom-designed and modern optical instruments for military use are provided with an integrated protection device to secure human eyes from laser based distance measuring and targeting systems. The laser being used for these units is mostly a Nd:YAG laser with a wave length of 1064 nm (as well as variants).
The laser protection consists of interference filters rigidly mounted at the optical path of the binoculars. The filters, one in each tube at the newer Zeiss-Hensoldt binoculars for example, are made up of a dielectrical metaloxid multi layer coated face plate. Despite of modern coating technologies the reduction of light transmission, which means the difference between the incident light at the obectives and the leaving light at the oculars, because of the filters is mostly caused by reflections and can not be insignificantly avoided. The total loss of transmission with the filters inside is about 10 percent.

Brightness and color rendering of the binoculars will be affected by the laser filters. Owing to lower light transmission in consequence of the laser filters, leads to an overall lower brightness of the image and is thus a drawback at poor or low light conditions. Also color rendering is still not neutral and there is a blue hue in evidence.

Removing the laser filters needs some effort, particularly there is a need to readjust the binoculars and to refill them with nitrogen after that. You can incidentally see with one view through the objectives inside the Zeiss-Hensoldt Fero-D 16 if the laser filters are present or not. This won´t work for the Fero-D 17, Fero-D 18 and Fero-D 19, because the laser filters here are on a different place.

Picture: Laserfilter Hensoldt binocular upon a sheet of white paper

Here is a test of a Hensoldt Fero-D 16 without the laser protection filters...

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Some ex factory and new in box Hensoldt/Zeiss 8x30 M Fero-D 16 - still originally packed and not opened yet. Color is "Bronzegrün" (RAL 6031).
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