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8.
Eyepiece Lens
The
task of an eyepiece lens is to magnify the photocathode's image of the scenery
produced by the IIT of a night vision device. The usual magnification,
which fits to the human eye, is by a factor of 8 or 10 times.
Just
like it is the case with a camera the human eye has an adjustable diaphragm.
An human eye that is adapted to the darkness has a maximum pupil diameter
of approx. 7 mm. The image projected at a given distance to the eye (eye relief)
by the eyepiece lens should be larger than 7 millimeters, so that a movement
of the eye is possible within the projection. If it would be smaller
than the pupil diameter the generated image would be considered as being too
dark. In case of being equal in size one would be forced to keep the eye straight.
Only by moving the head the scenery would be registered. By extended viewing
time not only a fatigue of the neck musculature can occur.
The
right dimensioning and adjustment of the eyepiece lens to fit the night vision
device are also important, if regarding the different output window sizes
of image intensifier tubes. There are curved or
flat (glass fiber twist) output windows, which display the image upside-down
or upright. For example a strong blurring of
the image's edges can occur if the eyepiece is
not adapted to the respective screen (however, with most devices of the 1st
Generation still the IIT is mainly responsible for the notorious blurry edges).
Apart from the task of turning the image around by 180° an eyepiece lens
should also be capable of adjusting to users with a corrected vision by a
diopter adjustment. Using night vision equipment
without wearing eyeglasses is a matter of course even with occasional employment.
With so-called 'panoramic eyepiece lenses' this is unfortunately not possible.
Nevertheless they offer a comfortable night vision in a usually good quality.
But they still represent a compromise between the requirement of an observation
with two eyes and the need for keeping the original costs relatively low.
For NVDs of Gen0 and Gen1
it is a matter of precaution having embedded special x-ray
blocking additives in the eyepiece material (e.g. lead glass).
If
there is a choice to make in terms of material of an eyepiece lens (glass
or plastic) generally glass (despite it's higher weight) should be preferred.
Unter the same conditions plastic lenses become also foggy but glass lenses
are mostly of better optical quality. Besides that, often plastic is more susceptible
to temperature-dependent material expansions and scratches. To protect from
mechanical damage modern NVDs are equipped with so-called 'sacrificational
windows' at objective and eyepiece lens. At the eyepiece lens the also
called 'demist shields' serve a further purpose: In damp and cold weather they
prevent to a large extent fogging of the (cold)
optics due to the isolating air layer between the
window and eyepiece. Fogging of the optics from the inside is often prevented
by a purging the complete night vision device with
dry nitrogen.
9.
Infrared illuminator
Contrary
to civilian applications of night vision devices military users and law enforcement
units mostly refuse the use of additional infrared illumination (IR illuminator)
- of course with exception of IR designators. Their experiences with systems
of the Generation 0 (active night vision devices) accelerated the development
of image intensifier tubes, which did not need traitorous IR illumination
any more. If at all, a source of infrared light can only be seen with the
naked eye on short distances in complete darkness by directly looking into.
But to the opponent equipped with night vision devices it represents almost
a brightly 'position light'. In addition for
long observation distances efficient illuminators
were needed, which were mostly very large in size
and because of their power supply very
heavy.
Therefore many military night vision devices have only a small built-in
infrared LED, which can be used for map reading or for orientation
within short distances in areas completely without any low light (whereby
within buildings night vision devices are not used mostly at all because of
the reduced field of view and the blinding effect of strong white flashlights
on the opponent).
However,
the civilian user can quite improve the performance of a night vision device
by using infrared illuminators e.g. within security surveillance or wildlife
observation. Since different generations of night vision devices have also
different working ranges within the electromagnetic spectrum, additional IR-light
should always correspond to the respective wavelength
of the used generation of night vision device.
An IR illuminator designed for Gen2 and Gen3 would be less effective together
with 1st Generation image intensifier tubes, which have a main working range
next to the visible range (around 780 nm). Because of this, a suitable IR
light source for Gen 1 tubes is still visible with the
naked eye by a deeply red glowing. With
large ranges (e.g. over 100 meters) there must also be considered that power
demand and consumption behave to a linear increase of illuminating exponentially.
Under certain circumstances reflections by obstacles at short ranges can overload
the image intensifier tube and can decrease the image quality by over-radiating
the background. Lucky, if the IR light source has focussable
optics. Unfortunately only a few infrared illuminators are equipped
with a regulated power switch, which not only
saves battery power, but also provide more flexibility.
Basically
there are three categories of IR illuminators: infrared
LED illuminators, infrared laser illuminators and any other light
sources with infrared filter.
- IR
LEDs
have the smallest range, but a very equal illumination. They are mostly
of fixed-focused-design and have a very small power consumption. A variable
illumination performance can easily be achieved by simply connecting the
LEDs. They are also perfectly suitable as marking beacons because of their
small and longeval design.
- Infrared
lasers illuminators
emit bundled monochromatic light (i.e. infrared
light with only a defined cutout of the electromagnetical spectrum). In
the high IR range they are invisible also for night-active animals and can
illuminate very distant objects by being focussed
sharply. The power consumption and the size of the devices are mostly
not substantially.
Unfortunately IR lasers have a big disadvantage in possibly causing
irreparable damage to the equipment and to the human retina (device is eye
safe?!) due to their high power density (reflections by illuminated
objects). This represents a special problem to the naked eye because of
the invisibility: damage to the retina is not noticeable while being exposed
to, but only as an already occurred injury!
- A
simple flashlight with an infrared filter is
in many cases a serious alternative, if additional IR illumination is needed.
Compared to an IR LED illuminator generally the advantages are a higher
range and focusing options. Nevertheless this combination is suitable for
devices of each generation (depending on of the filter specifications),
since a flashlight emits a broad spectrum of
EM-wavelengths. In addition it is very practical if the filter is to be
opened easily in the darkness (so-called flip-open,
tip-off filters). In order not to over-heat the flashlight or the
filter, it should be checked that there cannot get enough heat enclosed
destroying the equipment.
Exception: in the AN/PVS-7B a so-called 'Non Inverting' tube is utilized.
This IIT does not turn the image coming upside-down from the objective lens,
because the following optics (collimator) flip & split the image upright
again. Both eyepiece lenses have a diopter adjustment of their own and are
adjustable in interpupilary distance for individual use.
From
left to the right: LED illuminator for Gen1, IR-laser illuminator (also focussable
for IR designating purposes), flip-open IR filter for flashlights, single
IR LEDs for building up own IR illuminators
Beside IR Beacons Western armed forces also use chemical infrared snaplights
for marking purposes (their brightness is also enough for close orientation
in the surrounding environment).
