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Borescope
A borescope is an optical instrument that is used for visual inspection of the inside of a closed object through a small hole – for example to check the cogwheels inside an engine – or for covert inspection of a room or area that is currently in use, or to which one does not have direct access.

The device typically consists of a flexible or rigid tube with an eye piece at one end, an objective lens at the other, and a relay optical system in between [2]. In the espionage trade, it was often used for surveillance of an adjacent room, or for inspecting the empty space inside an object before planting a covert listening device (bug).

A borescope is also used as a countermeasures instrument, for example when searching for hidden bugs or explosives. The image on the right shows a rigid borescope that was used by the State Security (Stasi) of the former DDR [1].
  
Rigid borescope

It is about 30 cm long and has a diameter of 8 mm. At the side is a removable lens that allows a common portable torch light to be used for illuminating the inspected object, using an optic fibre around the relay system to guide the light to the tip of the device. Similar tools are used to look inside the human body, in which case they are referred to as endoscopes 1 . Generally speaking, there are three types of borescope: flexible, rigid (like the on shown here) and video (camera) [2].

  1. In German, Borescopes and Endoscopes are both known as Endoskop.

Rigid borescope Eye-piece and light inlet Light inlet with pocket torch Borescope with pocket light for illumination
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Rigid borescope
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Eye-piece and light inlet
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Light inlet with pocket torch
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Borescope with pocket light for illumination

Features
The diagram below shows how the borescope works. At the left is the eye-piece which is similar to the eye-piece of a microscope. It has a focussing ring to adjust the image of the object to the eye. At the bottom left is a removable lens that is used to inject light into the optical system.

Borescope seen from the side

Inside the tube are two optical relay systems: an outer one to guide the light from the removable inlet to the tip at the right, and an inner one to guide the image from the objective lens at the tip to the eye-piece at the left. It is suitable for inspecting small areas as well as large rooms. When used in combination with an inspection mirror (mirror probe kit), it is possible to look around a corner inside a closed object or behind ridges of a piece of furniture when searching for bugs.


References
  1. Detlev Vreisleben, DDR Borescope
    Personal correspondence, September - October 2018.

  2. Wikipedia, Borescope
    Retrieved October 2018.
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Crypto Museum. Created: Sunday 07 October 2018. Last changed: Monday, 08 October 2018 - 09:22 CET.
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