## How to use the reticle

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- Published on 18 June 2010
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The reticle is a thin metal coated glas plate with a scaled mapping on his surface, located at the image plane of an afocal optical system. The basic unit for the use of a reticle is the military angular measure called ´point´, which 1 point means about 1 meter at 1000 meters. The basic for the deduction of this military angular measure is the so called ´full artillery circle´, which is subdivided into 6400 points (NATO). This division is the result of the circle perimeter calculation of a circle with the radius of 1000 meters, the result is 2πr = 2 x 3.14159 x 1000 = 6283.18. From a mathematical point of view the circle perimeter is the sum of its chords, in which the precision of the calculation is depending from the number of this chords. For sufficiently approximation and for easier calculation in right-angeled coordinate systems, the nearest integer divisible by 4 will be used - 6400 (NATO). The result for this simplification is 6283.18 / 6400 = 0.98174, rounded 1 point.

With this reference value and the reticle the approximate distance of an object or other equation values can be determined. The equation is as follows:

Known dimension of an object multiplied by 1000 and divided by the readed number of points results in the approximate distance of the object.

For example, truck with the standard width of 2.50 meters:

If the truck fits 2 of the bigger points on the reticle, this is called ´0-10´ and it can be calculated 2.50 x 1000 / 10 = 250, resulting in a distance of 250 meters of the truck.

In former warsaw pact states the ´full artillery circle´ was suddevided into 6000 points, which means 6283.18 / 6000 = 1.04719, rounded also 1 point. The calculated difference per point is at 1000 meter about 6.5 cm, by what all these optical instrumantation have been useless!

Source: Guido Thürnagel

## Laser protection in military binoculars

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- Published on 19 June 2010
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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...

Thank you again for the deal, smooth and plain like silk, Stefan !