DOCTER DOCTER®nightvision 1x24 - Manual (Page 23)

Glossary of Technical Terms and Additional Information
Exit pupil
Unlike other long-range optical equipment, night vision devices have no physical exit
pupil. Nevertheless, the size of the exit pupil and its location can be defined. This is
called a reference pupil (Ø6±0.1mm) .
The exit pupil distance is the longest distance between the last lens of the eyepiece and
a reference aperture (that corresponds to the opening of the eye). In the observation
distance, the whole screen without any shading must be visible.
The exit pupil diameter is the diameter of the lens aperture in the exit pupil distance
within which the user is able to observe the whole screen without any detectable
shading.
Functioning
By means of this night vision device, residual light, which can only be perceived by the
human eye hardly or not any more, will be intensified and, thus, made visible again. For
that purpose, the object of observation at first needs a high-aperture lens that images it
onto a light-sensitive surface area. On that surface area, the light (photons) converts
into electrons. Due to the presence of a high tension at the micro-channel plate (MCP),
the electrons released by the photo cathode are accelerated into its direction. The MCP
consists of many small channels each of which acts like a little single amplifier. The
electrons impinge on the channel walls where they release new electrons again
(cascade effect). This process is repeated within the channels several times until they
are, after exit from the channel, attracted by the phosphor screen. When impinging it,
light is produced that is intensified 10,000 times. Here, the wavelength of the emitted
light depends on the phosphorous material and, as a result, gets the specific yellow-
green colouration. A fibre glass bundle consisting of approximately 50 millions of fibres
rotates the image by 180°. At last, the eyepiece magnifies, with a big eye distance, the
image of the phosphor screen for the eye.
Consequently, for attaining maximum performance of a night vision device, it is not only
the image intensifier tube that is of a vital importance, but also the optics used in the
objective and eyepiece that has to deliver an adequate performance and be tuned to the
specific requirements of high-grade tubes very purposefully.
Photo cathode
Photon-electron
conversion
Micro-channel plate
Electron intensification
Phosphor screen
Electron – photon
conversion
Entrance window
Vacuum Electrons
Exit window (fibre
glass bundle)