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Buying a night vision unit
(some recommendations to the customer)
The task of choosing a high value Night Vision unit that satisfies
your requirements and budget is never easy. Stores and catalogs
often carry several types of NV by different manufacturers. Simply
reading the advertising materials may not give sufficient understanding
of the available selection and may ultimately mislead the customer.
Many marketing companies exaggerate the characteristics of their
units (sometimes 5-100 times) and aggressively advertise their product
without a clear explanation of all technical data to the customer.
Every Star Light NV works on the principle of multiplying the brightness
of the image in the wavelength of visible and near-visible infrared
light. The unit contains an objective lens, light amplification
tube with the high voltage power supply, and the eyepiece. Light
(starlight for example) reflects from the object to be viewed, passes
though the objective lens and creates an image on the photo cathode
of the light amplification tube. Inside of the light amplification
tube, the light is electronically amplified and projected as a yellowish
green glow on a screen that is then seen by the viewer..
The two most important characteristics for the NV are the characteristics
of the NV tube and optics of the unit. Internationally accepted
terminology classifies the NV tubes in I, II, or III generation
(with certain intermediate levels of I+ and II+ gen.)
NV Tube Gen. I
The Light Amplification tube for Gen. I employs a vacuum tube with
the photo cathode sensitivity 120-250 µA/lm. The light amplification
of these tubes provides for the output of 120-900 times the ambient
light. Resolution in the center: 25-35 lines per millimeter.
The NV products manufactured utilizing these tubes are popular because
of their low price (under $300). One of the characteristics of these
units is that the image is most clear in the middle of the field
of view while becoming somewhat distorted and less clear at the
edges. Bright light sources (street lights, lit windows etc.) sometimes
make viewing less satisfactory because they may fill the entire
field of view with light.
Even within a single class, Gen. I NV tubes may be quite different
in their performance. As a part of quality control during manufacture
the tubes are evaluated by a selection process and are sold for
significantly different prices depending on their amplification
and image quality.
Sometimes this difference in cost is reflected on prices of the
NV units from different manufacturers. Dedal-Handy and Dedal-140,
for example, utilize tubes that are 1.5-2 times more expensive than
those found in competitive products. Tube selection is based on
the sensitivity of the photo cathode, light amplification, resolution
and the clearness of the field of view. Small specks seen on the
screen, as a rule, do not distort the viewing in the dark and would
not be a reason to reject the NV. However, if the specs are bright
(star like) and the tube has light spots in its field of view -
that unit must be rejected.
A unit, which lacks contrast in its image, is also an indication
of lesser quality. When buying an NV, the purchaser can check the
clarity and resolution, whereas the sensitivity can only be tested
by professional using special testing equipment or by comparison
in the dark. Due to low sensitivity, single-stage Gen. I tubes are
very demanding with regard to light transmission of the optics.
Only scopes with special light gathering optics (magnification power
not more than 1.5) may provide satisfactory results when viewing
at a light value equal to ? of the moon. When light is less bright
an I.R. illuminator may be necessary to light the subject. Unlike
many other products in this class, all Dedal products using Gen.
I tubes are equipped with electronic overload flash protection system
to prevent damage in case of temporary exposure to bright light.
Multistage NV tubes Gen I
To achieve higher light amplification, some manufacturers connect
light amplification units in a tandem arrangement of two or three
stages. This allows light amplification to increase to 20000-50000
times, but at the cost of increasingly poorer resolution, especially
at the edges. Units made with multistage Gen. I tubes characteristically
have greater weight and size. Modern units using Gen I+ or II+ technology
have similar performance characteristics with few of the drawbacks
of the multistage amplifiers and have proven to be more popular.
Gen. I+ (Super I+)
This is a further development of the Gen. I NV tube with a fiber
optics plate being installed at the front or an exit of a tube.
This modification allows for a dramatic improvement of the image
resolution and prevents distortion from other light sources in a
field of view. Tubes of Gen. I+ have light amplification of approx.
1000 times, while the photo cathode sensitivity increases to min.
280 µA/lm. Resolution in the center min 45 lp/mm. NV units built
with the Gen I+ tubes differ from the Gen. I units by increased
image quality, lower distortions and better viewing distance in
passive or active mode (with or without an I.R. illuminator). These
scopes work well in an urban environment. In the open, the units
are effective with the ambient light levels down to ? of a moon.
When light conditions are lower, an I.R. illuminator is needed.
The cost of the Gen. I+ tubes is 4-9 times of the cost of regular
Gen. I. Our company manufactures the Dedal 220 and Dedal 200 utilizing
the Gen. I+ tubes.
Gen. II
The design of the Gen. II tubes differs from the Gen. I+ by utilizing
a special electron amplifier - the micro channel plate. The performance
characteristics of such NV tubes are a higher photo cathode sensitivity
of 240 µA/lm, with the resolution of 32-35 lp/mm. The lifetime of
these tubes is designed to be not less than 1000-3000 hours of operation.
Two types of NV Tubes utilizing micro channel plates (MCP) are currently
available: the 25mm and 18mm From the users prospective the larger
diameter is more efficient but also requires larger NV unit a whole.
In this category, Dedal manufactures a multifunctional NV Scope
- the Dedal-41 and series of rifles copes: D-40, 40m, and 300. All
of the above units are equipped with an additional manual brightness
control that allows the viewer to adjust the scope for the optimal
gain/noise ratio.
Gen. II+
With a light gain of 25000-35000 the Gen II+ tubes have photo cathode
sensitivity of 600 µA/lm and are sensitive to light in the infrared
spectrum. Resolution 39-45 lp/mm. Lifetime of the tube is estimated
to be 1000-3000 hours.
Since the Gen. II+ tubes are smaller (they do net have a speed up
chamber) they have a slightly lower light amplification than the
Gen. II. Yet due to the especially high sensitivity in the IR spectrum
the Gen. II+ units have better viewing distance in the open field.
If the main task for the unit is photo or video recording, the NV
of choice should be the Gen II unit due to its higher light gain.
NV units with Gen. II+ tubes are equipped with automatic gain control,
flash protection, protection from the side light distortions and
feature a high quality image throughout the entire field of view.
Units equipped with Gen. II+ tubes are used by the armed forces
of many countries and considered to be of professional quality by
the experts.
Gen III
Gen. III differs from the Gen. II+ by use of a photo cathode based
on Gallium Arsenate with even deeper sensitivity to the IR spectrum.
The photo cathode sensitivity is 900-1600 µA/lm, resolution 32-64
lp/mm and an expected life of 10,000 hours. This is 3 times longer
than the Gen. II tube.
The NV units utilizing the Gen. III tubes work very well in the
low light environment. The image comes out clean and with excellent
contrast. The only drawback of Gen. III units is their vulnerability
to the side light distortions due to lack of the fiber optical plate
on the front of the tube. Until recently Gen. II+ and III units
where used mostly by the military, recently these units became more
widely available.
Infra Red Illuminator
For civilian NV systems an Infra Red (IR) illuminator can be a
welcome enhancement that provides additional light on the object
to be observed. This is particularly valuable when the ambient illumination
is not bright enough for passive (starlight) mode or the object
is in shadows.
IR illuminators are designed with lasers, IR diodes and special
light bulbs. One should keep in mind that laser illuminators might
be harmful to eyes. Because of this, they are outlawed for civil
usage in many technologically developed countries. In Russia some
of the dangerous lasers are still being sold.
Illuminators that utilize IR diodes are harmless with the added
benefit that they provide more even illumination than their laser
counterparts. Presently most of the manufacturers are producing
the IR illuminators with diode IR components that deliver an output
of 3-12 mWt. Our company equips all it's NV units with the most
powerful IR source in Russia - 35 mWt. The viewing distance with
our IR illuminator is 1.5-2 times of the competition.
If your NV unit does not have a built-in IR illuminator it can be
purchased separately as an accessory.
Construction
The physical characteristics of NV units are usually designed with
three factors in mind, compactness, weight and observation distance.
Unfortunately sometimes these characteristics are in conflict with
one another. For example, greater observation distances can be provided
by a unit with a larger lens with the inherent disadvantage that
the device will be bigger in size and weight. The customer has to
decide which characteristic is a priority and buy accordingly.
One factor, which is especially important, is the construction of
NV rifle scope - it must be extremely rugged to withstand the shock
of up to 500Gs along the optical axis due to recoil. In this harsh
environment the cross hairs, sighting posts and targeting adjustments
must remain unmoved and be visible in all types of viewing conditions.
Many NV rifles copes currently offered on the market are not designed
to compensate for these stresses and may have weak mounting and
zeroing-in systems. This will require repeated readjustments to
maintain accuracy. This is a complicated subject that cannot be
discussed properly in a short article. We encourage you to contact
our company for a special (free) consultation on the subject.
To summarize, we suggest you use the following simple guidelines
when selecting an NV unit:
If image quality and distance to target are not vital for your
applications we recommend you choose your NV by price and its
size/comfort. When your image requirements are higher - select the NV (particularly
Gen I.) unit with the optical system that has a light transmission
co-efficient not less than 1.5 When selecting your NV choose a manufacturer that has been established
in the NV business and has a proven track record. Make sure you
are purchasing from a company that sold in the competitive international
markets and has developed the reliable customer service/product
support system. Do not buy without warranty - select a manufacturer
who has an established record for honoring their warranty. Try to compare several units, as the technical specs in advertisements
and manuals of many companies are often inaccurate and inflated.
Make sure that the unit works and everything is functioning
properly.
Congratulations on your wise purchase !
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