Night Vision products
collect and amplify existing light through the objective lens which
is then focused on the image intensifier. Inside the intensifier,
a photocathode is "excited" by the light and converts the photon
energy in to electrons. These electrons accelerate across an electrostatic
field inside the intensifier and strike a phosphor screen (like
a green monochrome TV screen), which emits an image that you can
see. It is the acceleration of electrons, which provides gain and
enhances the image.
What's inside a night vision
scope?
A night vision scope is made
up of a set of optics (glass), a simple power supply (batteries,
wire, transformer, regulators and capacitors) and an image intensifier
tube.
Generations of Night Vision
tubes.
The key difference between
the night vision generations is the intensifier technology. Gen.
I devices use an intensifier tube that amplifies ambient light by
accelerating electrons and striking a phosphor surface just like
a Television. Generation II devices add a micro-channel plate that
multiplies the number of electrons before they impinge on the phosphor
screen, thus increasing gain; Generation III devices further add
a Gallium Arsenide photocathode which creates significantly more
photoelectrons than Gen. II devices. Generation II and III devices
offer greater light amplification. Generation I devices are high quality and provide light
amplification adequate for most recreational activities and for
many professional uses.
Aren't all Night Vision
products the same?
No. Line Pairs, Magnification, Tube Grade and several other factors determine the difference between Night Vision items.
What is the effective range
of night vision?
A. It varies anywhere from
10-400 feet. The maximum viewing distance depends on the conditions
of use. Overcast conditions, fog, rain; etc. may reduce the effective
distance of a night vision. An Infrared Illuminator will increase
viewing range, especially in enclosed areas like a warehouse, cave,
or thick brush.
Why do you focus both the
front (Objective lens) and Eyepiece (Ocular)?
The short answer is that
you don't see through the night vision device, rather you see the
phosphor screen on the back of the tube. The objective lens focuses
the image on to the front of the intensifier tube. That image is
then converted in to electron energy. It paints that picture on
the back of the tube. You need to have a sharp image to start with.
Then your eye needs to be focused onto the back of the tube where
the image resides. Every eye is different and therefore the ocular
focus is very important. Focus the ocular first and then move to
the objective lens. You may have to adjust each lens several times.
What is Magnification (Power)?
A. Magnification is the degree
to which the object being viewed is enlarged. For example, with
a 7x42, the number 7 represents the "power".
A power 7 magnifies an image to seven times the
size it would be when viewed by the normal, unaided human eye. The
level of power affects the brightness of an image, so the lower
the power, the brighter the image it delivers will
be. In general, increasing power will reduce both field of view
and eye relief, which are also discussed here.
Why isn't there more magnification
in a night vision device?
A. The more you magnify the
more light you lose.
What is Field of View?
The size of the area that can
be seen while looking through a device is referred to
as the field of view. The angular field of view is indicated on
the outside of the device, in degrees. The linear field of view
refers to the area that can be observed at 1,000 yards, and is expressed
in feet. A larger field of view translates to a larger area seen
through the device.
Field of view is related to
magnification, with greater magnification creating a smaller field
of view, in general. A large field of view is especially desirable
in situations where the object viewed is likely to move, or when
the user is moving.
You can use angular field to
calculate the linear field by multiplying the angular field by 52.5.
For example, if the angular field of a particular is 8
then the linear field will be 420 feet, i.e. the product of 8 x
52.5.
What is Resolution?
A measurement of the
ability to distinguish fine detail and produce a sharp image. Better
resolution also delivers more intense color. Resolution varies in
relation to the size of the objective lenses. Generally,
a larger objective lens will deliver more detail to the eye than
a smaller objective lens, regardless of the magnification. Actual resolution is determined by the quality of the
optical components, the type and quality of the optical coatings,
atmospheric conditions, collimation (i.e. proper optical alignment),
and the visual acuity of the user.
Lens Coatings
The optical elements of the
device are coated to reduce internal light loss and glare, which
in turn ensures even light transmission, resulting in greater image
sharpness and contrast. Lens coatings range in quality as follows: coated -- fully
coated -- multicoated -- fully multicoated. Coated lenses are the
lowest quality and basically will not result in a product that will
satisfy you. Fully coated lenses are quite economical and can work
well for you, depending on your needs. Multicoated or fully multicoated
lenses are both very good choices. Fully multicoated lenses give
the best light transmission and brightest images, and are therefore
the most desirable.
Eye Relief
This refers to the distance,
in millimeters, that a device can be held from the eye and the
full field of view can still be comfortably observed. Eyeglass wearers
in particular benefit from longer eye relief.
What is I.R. Illuminator?
All night vision devices need
some available light to work. It is possible to see a bright image
in low light or no light conditions with the use of an Infrared
Illuminator.
What type of light source can
be harmful to a night vision device?
Night Vision devices are designed
for use in the dark. It may be harmful and can damage the device
if you use it during the daytime or in other very bright situations.
Keep in mind that strong direct light, such as projectors, car headlamps,
strong flashlights, and so on, may be harmful to your night vision
unit if you direct your device at the source of these intense lights.
Most devices have electronic protection
against such damage. Most of these are simple turn off overload
circuits, but some have more sophisticated and separate light sensing
mechanisms.
What characteristics might
I expect from my Night Vision device?
Each generation
of Night Vision has its specifications and output ratio.
The image of a night vision
device will not be as sharp as that of daylight binocular. This
is due largely to the image being generated on a phosphor screen.
Black spots may be visible in the image. This comes as a natural
process of manufacturing the intensifier tubes.
NIGHT VISION
