The periscope (from ancient Greek περί peri , German , in order, around ' and σκοπεῖν skopein , look') or periscope , also simplifies Wall watchers , is an optical instrument for the parallel displacement of the beam path . The invention goes back to Johannes Hevelius , who published it in his printed work Selenographia sive Lunae Descriptio in 1647 under the name Polemoskop (see drawing).
The periscope is mainly used in war to safely look out of bunkers , trenches (also scissor telescopes here ) and submarines . Also hovercraft make use of this technique, as no more troubled waters is at a higher altitude, which restricts the view. The Spirit of St. Louis aircraft also had a periscope for the pilot. Angle mirrors are often used in armored vehicles .
Structure and functionality
The basic structure of a periscope consists of a tube with mirrors or prisms arranged at its two openings . These direct light rays running perpendicular to the pipe into the pipe and, offset parallel to the original direction of incidence, out again. An observer who looks into the lower opening of a periscope gets the impression of looking at the surroundings from further above. The mirrors are arranged at a 45 ° angle to the incoming and outgoing light beam, which causes a right-angled deflection. The reflectance of metallized mirror surfaces is considered imperfect and continues to deteriorate over the course of months. The totally reflective diagonal surfaces of prisms, on the other hand, have a better and lasting degree of reflection and are preferred.
This simple structure leads to a field of view that is too small for longer periscopes . Periscopes equipped with lens systems can expand the field of view many times over with a suitable field lens .
Two such periscopes are shown schematically below. The arrangement of the lenses corresponds in principle to that of a telescope with low magnification. A magnification factor of 1.5 is often chosen, which for psychological reasons gives the impression of a natural, 1-fold magnification. However, since the image always appears rotated by 180 ° in simple telescope systems, inverting the image - as in binoculars or telescopes for terrestrial observation - uses either an inverting prism (g) or an inverting lens (c).
In the case of periscopes of the second type, a pair of erecting lenses (c 1 and c 2 ) can be arranged so that the beam path between them runs parallel. This allows the length between c 1 and c 2 to be changed over a wide range without affecting the image (e.g. for different heights of a submarine periscope).
If the second field lens (c 3 / b 2 ) is positioned a little above the intermediate image (it thereby takes on part of the task of the erecting lens), there is space for a crosshair at the location of the intermediate image, which appears sharply in the image as a sight. It can be illuminated from the side for night observation.
A panoramic telescope is called a periscope, the upper deflecting prism of which can be rotated 360 ° horizontally relative to the lower one. So that the image does not turn upside down when it is rotated, an additional prism system between the erecting lenses (c 1 and c 2 ) ensures a suitable counter-rotation of the image.
With the ring vision tube , the horizon is completely visible with the aid of a fisheye lens . The image appears greatly reduced and distorted in a ring. This periscope can complement other periscopes so that the horizon ring around the actual image is visible.
- Development and construction of the submarine periscope. In: Polytechnisches Journal . 329, 1914, pp. 417-420.
- Erecting lenses: lenses of a mostly 1: 1 image of an upside down intermediate image onto an additional upright intermediate image.