Frequently Asked Questions
A Brief History of Night Vision Devices
Night Vision Devices (NVD's) were developed over 40 years ago by the U.S. Military. The first NVD's were "Active" IR (infrared) devices, meaning the viewing device could "see" infrared light focused on an object, similar to the way the eye sees a conventional torch beam. The difference is that the IR light beam is invisible to the naked eye, so the viewer could observe the subject undetected. The user would focus an IR beam, then "see" what the beam illuminated with their scope. The problem with this method was that the enemy also developed IR viewers, and once the spotting IR beam was turned on, an enemy could see it too. This first NVD technology was called Generation 0.
In the 1960's, the first "Passive" Starlight Systems were developed, which used available light from moon and stars. These devices would amplify the available light by electronic means, resulting in a viewable image without the use of IR beams. This technology is called Generation 1. A true Gen 1 unit will provide a bright image in moonlight, or if used with a good IR (infrared) light. Some of the really cheap ($800.00 range) and poorly made NVD's on the market today use Gen 0 tubes with an IR light and call the product a Gen 1 unit. Let the Buyer Beware!
The 1970's brought us Generation 2 intensifier tubes, which offered much improved performance, weight gains, and reliability over the Gen1 units. These improved tubes made possible a great variety of helpful devices for the military as well as commercial users. These currently require a US State Department licence for use out of the US or Canada.
In the late 1980's and early 1990's, the state-of-the-art Generation 3 tubes were developed. These offered even greater enhancements and improved performance. These are the best of the best, and are considered current U.S. Military technology. You can see Gen 3 devices at work on CNN, and a host of other television programs, when night time pictures are shown. All Gen 3 devices require a U.S. Dept. of State Export License to be shipped from the U.S.
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.
An Image Intensifier is used to amplify light of which the intensity is so low that you cannot see it with your naked eye or any other detector. The exit pupil of your objective lens provides the intensifier with a small amount of light in the form of an image, regardless of whether you are looking at a target an inch or a mile away. This small amount of light has to be amplified by the Image Intensifier.
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.
1st GenerationNight Vision Devices (NVD's) generally use tubes made in Russia or Israel, and a few other countries. 1st Generation tubes are no longer made in the U.S. as this technology is considered outdated. This is one of the reasons why the pricing is usually less for Gen1 devices. Limitations of Gen 1 tubes are that they will "bloom" or "glare" when lights come into view, such as street or security lights, automobiles, or torches, causing loss of image. Tube life is also around 1000 hours (minimum spec).
Note: a 1000 hour tube life equates to 1-hour of use per night, every night, for 2.74 years. If used 10-hours per month, this equates to 8.3 years, or if used 5-hours per month, 16.3 years of tube life at minimum specifications. Some tubes will last longer, some less. Take these numbers x 5 for Gen 2 tubes, and x 10 for Gen 3 products. For the occasional user, tube life is not really a factor.
2nd GenerationNVD's incorporate a microchannel plate that improves spectral sensitivity and tube life, thus giving the 2nd Generation tubes increased performance, plus a smaller size and weight. These Gen 2 tubes offer outstanding performance in very low light conditions. Gen 2 tubes have a minimum tube life of 3000-5000 hours. Gen 2 devices perceive light in the upper ranges of the visible light spectrum, and work well in and around metropolitan areas because of the abundance of visible light. They also work well in rural settings when there is some moon light, or with supplemental IR light.
3rd Generationtubes feature an improved microchannel plate and photocathode for even higher sensitivity and clarity in extremely low light conditions. Gen 3 units can see in nearly pitch black surroundings. Gen 3 units see more in the near infrared spectrum than the visible light spectrum, which makes the Gen 3 units more specially adapted to the extremely dark, rural settings where the near infrared light from the night sky (starlight) can illuminate the landscape for viewing. These are the current "State-of-the Art." The typical Gen 3 intensifier tube life is rated between 10,000 and 12,500 hours.
Passive Night Vision Viewers have only a few components: ·
- An Objective Lens collects minute amounts of ambient light that are undetectable to the naked eye, and then directs that light into the Image Intensifier Tube ("tube" for short.)
- The Intensifier Tube has a photo cathode which absorbs the light directed into it by the objective lens, and converts the light energy (photons) into electric energy (electrons). Then the electrons are multiplied many thousands of times as they pass down the tube, where the highly electrified electron image strikes a phosphor screen, emitting light that is seen as a "image."
- An Ocular Lens then allows magnification, focusing, and viewing of the phosphor screen image with the eye.
- A fourth component would be an IR (infrared) Illuminator, used to illuminate objects in total absence of ambient light such as inner rooms, caves or deep forests.
Image Intensifiers - How They Work - Part 2
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.
Visualisation of image intensifier with Micro Channel Plate:
It started with electrostatically focused Generation I tubes featuring high image resolution, a wide dynamic range and low noise.
introduced 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
In 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 greatly improved performance appeared on the market. These types are called Gen III Omni III and Gen III Omni IV or Gen IV.
Infrared is an often confusing term. The reference applies to electromagnetic waves of certain frequencies that are above the visible spectrum (ie. what we can see naturally). Our eyes see only a small part of the overall radiation in our environment, which ranges from Gamma Rays on the lower end of the spectrum to Radio Waves on the upper end.
The human eye spectral sensitivity extends only from about 0.4 um ( .4 micron) to approximately 0.7 um. The colour blue falls within the approximate range of 0.4 to 0.5 um, the colour green falls between 0.5 to .6 um, and red falls within 0.6 to 0.7 um.
Adjoining the red end of the visible spectral region is the Infrared, which can be broken down into three overlapping categories:
- Near IR (from .7 to 1.3 um) which is reflective.
- Mid IR (from 1.3 to 3.0 um) which is reflective.
- Thermal IR (from 3 um to microwave) which emits rather than reflects.
Of these three categories, only Thermal IR is associated with the sensing (emitting) of heat, which occurs at frequencies of 3.0 um and up. Near IR and Mid IR are sensed as reflective, rather than as emitting, and these frequencies are found below 3.0 um down to the upper red regions of the visible spectrum (0.7 um) Near IR energy (reflective IR) can be seen with certain CCD and CCTV cameras, and Camcorders, as the chips used in these cameras have some sensitivity to IR light in the Near IR spectrums. Image Intensifier devices, commonly known as night vision devices (NVD's), also are sensitive to the Near IR regions below 1,000nm (1 micron). NVD's, CCD/CCTV cameras, and camcorders all supplement their night-time capability by adding IR lights. These are diodes of various configurations, that emit IR light in frequencies that the NVD or camera can see. An IR light supplements ambient light in really dark conditions, and enables photography or viewing in total absence of light.
Thermal IR (emitted IR) is heat energy emitted from objects at a frequency of 3.0 um and up. This emitted energy is seen with a thermal imaging camera, which will be tuned to sense thermal IR (heat) frequencies in the 3.0 to 5.0 um (short wave) or 7.0 to 14.0 um range (long wave), which are the two ranges with offer nearly 100% emission values. All objects emit energy in these frequencies.
EXAMPLE: A common hot plate, when turned on, will emit energy in the Thermal IR region as it heats, but no Near or Mid IR light, nor any visible light, will be seen. As the plate heats, and the heating element gets hotter, the plate will begin to emit Mid IR energy because as the plate heats, the frequency of the energy released decreases. So the hotter the plate gets, it will start to glow red (a lower frequency), which we can see with our eyes. In this instance, there will be Thermal IR energy emitted along with frequencies down to the upper red spectrum.. A thermal imaging camera would see the actual hot plate, plus the heat generated above the plate in the air. Your eyes would see the red glow of the plate, and if you viewed this scene in total darkness, your camera or NVD would also see some IR light. The electromagnetic spectrum from the upper red, to Near IR, to Mid IR, to Thermal IR, would all be present in this one scene.
Ever wondered why the image emanating from a Night Vision Device is the correct way up - similar to a pair of binoculars?
The binoculars achieve this result with the use of optical prisms, however this technology is not applicable in electro-optical devices. Electro-optical devices utilise an image converter tube which is an image inverter. That is, images that are erect on the input photochathode are up-side down on the output imager.
For imaging applications such as attachment to a recording instrument such as a SLR camera or camcorder, this may pose a problem when the image is upside down ( to centre objects that move to the left, you need to move to the camcorder to the right - objects that move up, you need to move down ), however to achieve a useable correctly aligned image when a Night Vision Device is used as a hand held spotting scope, this is achieved via a "optical twister" to obtain an image the correct way up.
The "optical twister" is situated between the Phosphor Screen and the output imager and through this optical fibre, the image is "twisted" as it's name implies 180º to result in a correctly aligned image.
The wavelength band throughout which the Image Intensifier is sensitive. The GaAs photocathode of the Gen III Image Intensifier is sensitive from 550 - 930 nm.
Thermal Imaging(sometimes called Infrared Imaging) is also considered Night Vision, though a thermal imaging camera works equally well during the day as it does at night, and has many uses other than those usually associated with night vision devices. All things emit thermal (or infrared) energy, which is invisible to the naked eye. A Thermal Imaging camera will visualise the heat emitted by objects in a viewed scene. Objects stand out because they are warmer of colder than the background they are viewed against. This contrast is the "thermal image" seen on a display.
Every intensifier tube is hand made, and every tube has different characteristics. This fact requires testing for every Mil-Spec tube that is made, in order for the military to accept it for use. The testing process identifies the exact performance of a tube, and determines if MILSPEC (Military Spec) or COMSPEC (less-than-Mil-Spec) quality has been achieved. Litton issues a Birth Certificate (tube sheet) for every Gen 2 and Gen 3 tube they make, be they destined for military or commercial applications. ITT issues tube certificates only to their military clients. A very brief description of various tube grades is as follows:
- Standard Spec: a new tube without a factory tube certificate. Typically these are production over runs that are bought at a discount, and are tested for compliance to specifications before installation in a device.
- COMSPEC: new commercial spec tubes with tube certificates that meet nearly all categories of inspection for MILSPEC tubes, but fall short in one or more category. These COMSPEC tubes will be categorised by photocathode response and tube resolution. ·
- MILSPEC: a new military grade tube with tube certificates, that meets the military OMNI III specifications in ALL and EVERY category. · COMSPEC OMNI IV: a new commercial grade tube with tube sheet that meet or exceed almost all categories of the OMNI IV military specifications, but fail one or more parameters. These COMSPEC OMNI IV tubes will be categorised by photocathode response and tube resolution.
- MILSPEC OMNI IV: a new military tube with tube certificates that meets ALL and EVERY category of inspection listed in the OMNI IV classifications.
The important performance parameters are:
- Signal-to-Noise Ratio
- Resolution and MTF
- Life time
Signal-to-Noise Ratio (SNR)
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 mlx. 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.
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 sharp images with a good contrast.
The life time of an Image Intensifier Tube is an extremely important parameter for Night Vision applications. A number of different definitions are used depending on the manufacturer. European Image Intensifiers typically have life time of 15000 hours as an expected life time, which is defined as the time after which still 50 % of the original sensitivity is left.