Starlight Technology
Night Vision Types
Night vision devices come in
multiple basic technologies. Digital night vision, Starlight technology,
and thermal-imaging are the most common. Both Starlight technology and
digital night vision amplify whatever available light there is, while
thermal-imaging (or infrared) devices look at heat. These three technologies
give very different results, are different in design, cost, and image
appearance. We will look at all of these technologies but the majority
of the night vision devices
available on the market are Starlight technology
and we will focus most of our discussion there. The “Night Vision”
terminology we use in this article refers to Starlight (image-enhancement)
technology.
The night vision industry has
grown dramatically in the last ten years. Originally developed and funded
by the military, the technology is becoming much more popular and affordable
for consumers. This is still an emerging technology. The more advanced
designs can only be sold within the US and are restricted for export.
Definition (Starlight technology)
Starlight technology is what
most people think of when they talk about night vision. Since this technology
uses available light, it does not work in complete darkness where there
is no light to amplify. However, most of these devices also come with
a built-in infrared (IR) illuminator. The IR illuminator emits a beam
of near-infrared light, invisible to our eyes, that the night vision device
can use to illuminate the scene.
Night vision devices come in
three accepted generations of design (Generation 1, 2, and 3). These can
be thought of as levels of design sophistication, although the basic components
and technology (defined below) remain mostly constant. The different design
generations are also significantly different in image quality, cost and
capability. These three and the proposed “Generation 4” devices will
be explained in greater detail in the Generations section of this article.
One of the major misconceptions
about night vision devices is that they are like a binocular that collects
light and focuses the image onto the eyes. Night vision devices are closer
to a camcorder in that available light is converted to electrons that
are then amplified and projected onto a screen that converts them back
into a visible light image that you look at through an eyepiece. Examining
this a bit closer, there are several major components that all Starlight
night vision devices use.
Starlight Technology Design
| Figure #1 |
1) Objective Lens
2) Photons
3) Photocathode
4) Microchannel Plate
5) Electrons
6) Power Supply
7) Phosphorus Screen
8) Eyepiece |
|
Objective lens
In front is a conventional
objective lens, which captures ambient light and some near-infrared light.
This lens focuses the incoming light onto a photocathode at the front
end of an image intensifier tube. The objective lens is adjustable and
is used to focus at different distances similar to the focus knob on a
pair of binoculars.
Image-intensifier tube
The photocathode at the front
end of the image-intensifier tube converts the photons of light to electrons
(electrical energy). These electrons pass down through the tube and are
accelerated using 5,000 volt bursts between oppositely charged electrodes.
The electrons are now effectively “amplified”. The problem with this
approach is that the acceleration of the electrons distorts a linear path
and the subsequent image. Generation 1 devices have significant distortions
around the edge because of this.
Starting with Generation 2,
a microchannel plate (MCP) was added into the tube to solve the distortion
problem. The MCP is a glass disk with millions of microscopic holes (microchannels),
made using fiber-optic technology. The electrons in the microchannels
are both accelerated and multiplied in number. As electrons pass through
the microchannels, they cause thousands of other similar electrons to
be released as they collide with the microchannel walls. Therefore, each
incoming electron sends thousands of similar electrons out the other end.
This was a major improvement in night vision.
In summary, the image-intensifier
tube has performed two of the essential steps toward light amplification:
first it converts photons of light into electrons of energy and second,
it amplifies the flow of electrons. The addition of a microchannel plate
in Generation 2 image-intensifiers improved the orderly output of electrons
and significantly multiplied their number.
Power supply
The power supply powers the
unit and display. It also sends pulses of electricity to the electrodes
that accelerate the electrons in the image intensifier tube. The power
supply may have a quiet buzz in Generation 1 units, while the device is
on. This is normal and unless it affects the usage of the device should
not cause concern.
Phosphorus screen
The electrons coming out of
the image-intensifier tube are hurled against a green phosphorus screen.
This converts the electrons back to photons of light and the final light-intensified
image is created. Green phosphorus is used because the human eye can distinguish
more shades of green than any other color. This is why night vision is
associated with the eerie green display color.
Eyepiece
Last of the basic standard
components is the eyepiece through which one looks to see the image that
has been projected onto the phosphorus screen. These are adjustable so
you can focus in sharp on the screen. Once focused for your eye this should
not have to be readjusted (unless switching between users). This adjustment
might be thought of like a diopter adjustment on a pair of binoculars
that you adjust for your eyes.
Infrared (IR) Illuminator
In front is a conventional
objective lens, which captures ambient light and some near-infrared light.
This lens focuses the incoming light onto a photocathode at the front
end of an image intensifier tube. The objective lens is adjustable and
is used to focus at different distances similar to the focus knob on a
pair of binoculars.
Conclusions
- Night vision devices come in three designs: starlight
technology, digital night vision and thermal-imaging. Starlight technology
designs are the most common and popular type of night vision for consumers.
- Starlight technology night vision works by amplifying
available light and can not function in total darkness without the aid
of an IR-illuminator.
- The basic components of all starlight technology night
vision devices consist of an objective lens, image-intensifier tube,
phosphorus screen, eyepiece and power supply.
Previous Article - Day
Optics Miscellaneous | Next Article - NV
Generations