Technology FAQ

An Image Intensifier is a vacuum tube that amplifies a low light-level scene to observable levels. The object lens collects light and focuses it onto the Image Intensifier. At the photocathode of the Image Intensifier the incoming light is converted into photo-electrons.

These photo-electrons are accelerated in an electric field and multiplied by a Micro Channel Plate (MCP). An MCP is a very thin plate of conductive glass containing millions of small holes.

An electron entering a channel strikes the wall and creates additional electrons, which in turn create more electrons (secondary electrons), again and again. Subsequently the highly intensified photo-electrons strike the phosphor screen and a bright image is emitted that you can see.

PHOTONIS produces a variety of different Image Intensifiers suited for applications running from X-rays to the Near-Infrared wavelength band. The application determines which type of input window and photocathode should be used
Image Intensifier Visualisation

Visualisation of image intensifier with Micro Channel Plate

More about the principles can be found at How It Works


Generation I
It started with electrostatically focused Generation I tubes featuring high image resolution, a wide dynamic range and low noise

Generation II
night visionIntroduced the Micro Channel Plate for much higher gain in the 1980’s. The original image resolution was less than that of the first generation intensifiers but the gain was much higher up to 30000 fL/fc

Generation III
night visionIn the late 1980’s an Image Intensifier with a GaAs photocathode was developed showing an enhanced sensitivity in the Near-Infrared. In the late 1990’s Gen III tubes with improved performance appeared on the market. These types are called Gen III Omni III and Gen III Omni IV.

A Gen III tube is virtually identical to a second generation device on the technology point-of-view, the only difference is that it includes a photocathode based on Gallium Arsenide (GaAs). However, Gen III tubes come with real drawbacks linked to the “fragility” of this GaAs layer.
As early as 2001 and confirmed by field experience, end-users and Governments concluded that an I² tube’s “Generation” was not a determinant factor of a tube’s global performance and therefore eliminated the term as a basis of requirements and export regulations. Unfortunately “Generation” of the I² tube is not an assurance of performance nor quality.
There are other important parameters to determine the performance and quality of the Image Intensifiers.
 


The important performance parameters are:

  • Signal-to-Noise Ratio (SNR)
  • Resolution and MTF
  • Lifetime

Signal-to-noise ratio:
This is by far the most important parameter for an Image Intensifier. It is a measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. For Night Vision devices it is measured at a light-level of 108 ulx.

The value of the SNR determines the resolution at very low light-levels. Therefore, the higher the SNR the better the ability to resolve image details under low light-level conditions. The SNR is related to the specific design of the tubes.

Low signal-to-noise ratio
 Low signal-to-noise ratio 

Resolution and Modulation Transfer Function (MTF):
Resolution is the maximum line density on a USAF target that can be resolved by a human eye and is expressed in line pairs per mm (lp/mm). A more objective performance indicator is given by the Modulation Transfer Function (MTF). High MTF values at low spatial frequencies provide - like for the XD-4™ tubes - sharp images with a good contrast.

Lifetime:
The life time of an Image Intensifier is an extremely important parameter for Night Vision applications. A number of different definitions are used depending on the manufacturer. For the XD-4™ Image Intensifiers of PHOTONIS the life time of 15000 hours is the expected life time, which is defined as the time after which still 50 % of the original sensitivity is left.
 


The distance between the photocathode and the MCP determines how big the halo will be. As you look to an active light source, you will see a halo around the light source. The closer the MCP is located towards the photocathode, the smaller the size of the halo. Typically 0.8 mm for the XR5™ and 2.0 mm for the GEN III.
night vision


There are many different variables that can effect the distance that you can see with a Night Vision device. The larger the object the easier it is too see. If you're trying to see details we call this recognition range and if you're just trying to see whether something there is or maybe you will just see movement but won't be able to 100% determine who or what it is. This is called detection range. Another variable is lighting conditions.

The more ambient light you have (starlight, moonlight, infrared light) the better and further you will be able to see. You can always see further on a night where the moon and stars are out then if it is cloudy and overcast. We typically state that you can tell the difference between a male and a female or a dog and a deer at about 75 to 100 meters However, if you were looking across an open field and there was a half moon out you could see a barn or a house 500 meters away.
 


At the photocathode the incoming light is converted into photoelectrons. As the photocathode is exposed to a high light source, it works at a very high level. Because the photoelectrons are limited, the photocathode will run out of photoelectrons.

The exhausted parts appear to become dark. When this event occurs over a longer time in the form of an image, it appears to be burnt into the photocathode. PHOTONIS Image Intensifiers are less vulnerable for a high light level than any other generations.

That is why PHOTONIS IITs will not be damaged immediately, when others do.