The viewing device consists of a diode vacuum tube in
which an objective optical element (lens or mirror) images the spatial pattern of
luminance from the scene on to the end window of an image converter tube which is coated
internally with a photo-emissive cathode material. The number of electrons emitted from
this material depends on the quantum efficiency.
These electrons correspond to an electron version of the scene and are accelerated to a
phosphor material coated anode on the other end of the tube by a high positive voltage.
The phosphor screen emits visible photons and in the process converts the original spatial
infra-red photon luminance of the scene into a visual image. The visual image is viewed
with the aid of of a magnifying eyepiece of either the single or the binocular type.
A beam of infra-red was supplied by fitting a suitable filter to
standard spotlights and searchlights to exclude the visible part of the lamp's
The number of infra-red photons incident on the
photo-cathode to produce one emitted electron.
This device couples together the
diode image converter developed for the active viewing system by fibre-optic
faceplates. These effectively divide the light image into a large number of elemental
areas or dots, each of which is efficiently transfered through its own lightpipe or glass
fibre. The advantage of this method of image coupling is its ability to transform the
images from one curved plane to another. Three stages are used to provide as much
brightness gain and to provide an erect image at the output. One variation of the cascade
geometry is to provide a diode with its external fibre-optic surfaces flat so that
separate diodes are coupled simply by butting together and another is a single tube
envelope comprising three internal diodes.
Rare-earth doped crystals of calcium and strontium fluoride fluoresced
in the visible region of the spectrum when irradiated by infra-red radiation. The rare
earth ions absorb radiation at one wavelength and when they are irradiated by a second
infra-red wavelength they emit visible light. By flooding the crystal with a local source
of infra-red, visible radiation was emitted when the crystal was irradiated by infra-red
radiation from the scene. With this mechanism image conversion from infra-red to visible
was obtained without intermediate electronics.
When light is propagated through an atmosphere containing water droplets
light incident on the surface of the droplets is successively reflected and refracted.
These effects contribute to a net backscattering of the light.
An optical component assembly containing a lens and a mirror.
By altering the chemical composition of the photocathode its response to
photons incident on its surface is extended to longer infra-red wavelengths.
If a bar pattern consisting of equally spaced black and white bars is
imaged by an optical system, due to its aberrations, a loss of contrast will be observed.
An objective method of measuring this loss of performance is by an optical transfer
method. If the contrast of the bar pattern (measured by the difference between the darkest
and lightest portions of the bar pattern) is plotted as a function of the number of bars
per unit length then a spatial frequency will be observed for square waves. Unfortunately
the square wave pattern is not very convenient mathematically and the modulation transfer
function is measured using a sine wave grating instead of a bar pattern. The black-to-white intensity of
a sine wave presented at the object plane of the optical component to
be tested undergoes transfer losses as it is relayed through the system. If the maximum
and minimum modulation levels of the sinusoidal image are B 1 and B 2 at any given spatial
frequency, then the contrast transfer function CTF is given by :
(B 1 - B 2 )/ (B 1 + B 2 ) or one half the amplitude of the output modulation divided by
its mean value.
The CTF values are plotted against the spatial frequency of the object pattern and thus
the perfomances of different components can be compared.The overall CTF of a complete
system can then be obtained by multiplying the CTF ordinates of each component part.