Night Vision: Image Enhancement - How Night Vision Works
Image enhancement is the process that most people associate with the green screen images of Night Vision. This works by collecting the tiny amounts of light, including the lower portion of the infrared light spectrum, that are present but may be imperceptible to our eyes, and amplifying it to the point that we can easily observe the image. Below are some of the details about image enhancement and the image-intensifier tubes used.
Image Enhancement: The Science
- A conventional lens, called the objective lens, captures ambient light and some near-infrared light. The gathered light is sent to the image-intensifier tube.
- The image-intensifier tube has a photocathode, which is used to convert the photons of light energy into electrons. The tube has high voltage, about 5,000 volts, for the image-tube components.
- As the electrons pass through the tube, similar electrons are released from atoms in the tube, multiplying the original number of electrons by a factor of thousands through the use of a microchannel plate (MCP) in the tube. An MCP is a tiny, glass disc that has millions of microscopic holes (microchannels) in it, made using fiber-optic technology. The MCP is contained in a vacuum and has metal electrodes on either side of the disc. Each channel is about 45 times longer than it is wide, and it works as an electron multiplier.
- When the electrons from the photocathode hit the first electrode of the MCP, they are accelerated into the glass microchannels by the 5,000-V bursts being sent between the electrode pair. As electrons pass through the microchannels, they cause thousands of other electrons to be released in each channel using a process called cascaded secondary emission. The original electrons collide with the side of the channel, exciting atoms and causing other electrons to be released. These new electrons also collide with other atoms, creating a chain reaction that results in thousands of electrons leaving the channel where only a few entered. An interesting fact is that the microchannels in the MCP are created at a slight angle (about a 5-degree to 8-degree bias) to encourage electron collisions and reduce both ion and direct-light feedback from the phosphors on the output side.
- At the end of the image-intensifier tube, the electrons hit a screen coated with phosphors. These electrons maintain their position in relation to the channel they passed through, which provides a perfect image since the electrons stay in the same alignment as the original photons. The energy of the electrons causes the phosphors to reach an excited state and release photons. These phosphors create a green image on the screen that has come to characterize night vision.
The market is flooded with information on the characteristics and qualifications of the different generations of image intensifier tubes. Often, this information is biased by the company that has published it. The information provided below dispels many of the myths on the market and provides cold hard facts about night vision image intensifiers.
Generation I - Often called Starlight scopes, Gen I image intensifiers were developed by GE in the 1960s. They were originally a form of television tubes, a simple three-stage photomultiplier that would amplify ambient light about 500 times. Although crude, they were many times better than anything that was currently on the market. The typical tube was 40 mm, much larger than the 18mm tubes used today. Characteristics of the Gen I tube included a blurry image due to geometric distortion, a high pitch whine when the system was turned on, and a residual green glow after the system was powered off. There are still some functioning starlight scopes on the market, but they are no longer in production.
Generation II - The original Gen II tubes were developed in the early 1970s. Their design included the introduction of the microchannel plate, which acted in place of the electron multiplier. Early Gen II tubes had issues with image distortion and tube life but were a significant improvement over Gen I technology. Magnifying ambient light approximately 20,000 times, the image was considerably more detailed than that available with a Gen I tube. They are still produced worldwide and are used by foreign militaries, law enforcement, civilians, and in some commercial applications.
Generation II Plus - When US Manufacturers started to develop Gen III image intensifiers, European manufacturers chose to continue to develop and improve Gen II technology, resulting in Gen II +. Manufactured in Europe, the Gen II+ tube utilizes an autogated power supply, improved microchannel plate, and different types of phosphor to increase the performance characteristics and life span of the tube. Light magnification is increased to 30,000 times, producing a crisper image. Gen II+ is significantly improved from Gen II; at times, it is nearly comparable to Gen III image intensifier performance except during low light conditions such as during the several days preceding and following a new moon.
Generation III - Manufactured only in the United States by L-3 EOS and ITT Night Vision, Gen III image intensifiers were developed in the 1980s and are the best option available for foreign militaries and law enforcement. With the addition of the gallium arsenide photocathode and the protective ion barrier, Gen III performance is notably increased in low light conditions. Typical light amplification ranges from 40,000 to 70,000 times. Export regulations restrict the performance of Gen III allowable for export based on its characteristics. (FOM, HALO, and power supply). The export qualified Gen III tubes’ performance is far superior to the Gen II and Gen II+ performance in low light conditions.
Generation III Plus - In the early 21st century, L-3 EOS (then Litton EOS) developed what were originally considered Generation IV image intensifiers. This tube was produced without the ion barrier and with an autogated power supply, which together greatly increased the target detection range and resolution. Later developments significantly reduced the image halo, which can be described as the haze seen around a street lamp during a foggy night. ITT’s development team created a similar tube which featured a thin ion barrier film and autogated power supply. The US Government later changed the designation to Gen III+, which is used to describe unfilmed or thin filmed autogated tubes. These tubes are not available for export.
The Cold Hard Facts
Generation II+ Image Intensifiers perform significantly better than Generation II. Still, they do not outperform Generation III in low light conditions. Generation III Image Intensifiers are manufactured only by L-3 EOS and ITT Night Vision. There are no other producers - European or Domestic Generation IV Image Intensifiers are not an accepted classification for image intensifiers by the US Department of Defense
- Gain: the number of times a night vision system amplifies light.
- Signal to Noise Ratio: number of good pixels as opposed to the number of interference pixels (this can be compared to the snow in an image on a TV channel).
- Halo: the refractance ring around a bright light source (this is demonstrated by the hazy ring around a street light on a foggy night).
- Resolution: visible line pairs per millimeter measured using US Air Force Resolution Test Chart below.
Figure of Merit (FOM): export performance guidelines. It’s calculated by multiplying the Signal to Noise Ratio by the Resolution.
- Micro Channel Plate (MCP): electron amplifier.
- Phosphor: the chemical agent that emits light to form an image.