A Brief History of Night Vision Devices
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range. Humans have poor night vision compared to many animals, in part because the human eye lacks the following types of ranges
- Spectral range – Night-useful spectral range techniques can sense radiation that is invisible to a human observer. Human vision is confined to a small portion of the electromagnetic spectrum called visible light. Enhanced spectral range allows the viewer to take advantage of non-visible sources of electromagnetic radiation (such as near-infrared or ultraviolet radiation). Some animals can see using much more of the infrared and/or ultraviolet spectrum than humans.
- Intensity range – Sufficient intensity range is simply the ability to see with very small quantities of light. Although the human visual system can, in theory, detect single photons under ideal conditions, the neurological noise filters limit sensitivity to a few tens of photons, even in ideal conditions.
Many animals have better night vision than humans do, the result of one or more differences in the morphology and anatomy of their eyes. These include having a larger eyeball, a larger lens, a larger optical aperture (the pupils may expand to the physical limit of the eyelids), more rods than cones (or rods exclusively) in the retina, a tapetum lucidum.
Enhanced intensity range is achieved via technological means through the use of an image intensifier, gain multiplication CCD, or other very low-noise and high-sensitivity array of photo detectors.
Night vision technologies
Night vision technologies can be broadly divided into three main categories:
- Image intensification – Image intensification technologies work on the principle of magnifying the amount of received photons from various natural sources such as starlight or moonlight. Examples of such technologies include night glasses and low light cameras.
- Active illumination – Active illumination technologies work on the principle of coupling imaging intensification technology with an active source of illumination in the near infrared (NIR) or shortwave infrared (SWIR) band. Examples of such technologies include low light cameras.
- Thermal imaging – Thermal imaging technologies work by detecting the temperature difference between the background and the foreground objects.
Night goggles are telescopes or binoculars with a large diameter objective. Large lenses can gather and concentrate light, thus intensifying light with purely optical means and enabling the user to see better in the dark than with the open eye alone. Often night goggles also have a fairly large exit pupil of 7 mm or more to let all gathered light into the user’s eye. However, many people can’t take advantage of this because of the limited dilation of the human pupil. To overcome this, soldiers were sometimes issued atropine eye drops to dilate pupils. Before the introduction of image intensifiers, night goggles were the only method of night vision, and thus were widely utilized, especially at sea. Second World War era night goggles usually had a lens diameter of 56 mm or more with magnification of seven or eight. Major drawbacks of night goggles are their large size and weight.
Active infrared – Imaging results with and without active-infrared
Active infrared night vision combines infrared illumination of spectral range 700-1000nm (just below the visible spectrum of the human eye) with CCD cameras sensitive to this light. The resulting scene, which is apparently dark to a human observer, appears as a monochrome image on a normal display device.
Because active infrared night vision systems can incorporate illuminators that produce high levels of infrared light, the resulting images are typically higher resolution than other night vision technologies. Active infrared night vision is now commonly found in commercial, residential and government security applications, where it enables effective night time imaging under low light conditions. However, since active infrared light can be detected by night vision goggles, there can be a risk of giving away position in tactical military operations.
Laser range gated imaging
Laser range gated imaging is another form of active night vision which utilizes a high powered pulsed light source for illumination and imaging. Range gating is a technique which controls the laser pulses in conjunction with the shutter speed of the camera’s detectors. Gated imaging technology can be divided into single shot, where the detector captures the image from a single light pulse to multi-shot, where the detector integrates the light pulses from multiple shots to form an image.
One of the key advantages of this technique is the ability to perform target recognition as opposed to detection with thermal imaging.
Thermal imaging cameras are excellent tools for night vision. They detect thermal radiation and do not need a source of illumination. They produce an image in the darkest of nights and can see through light fog, rain and smoke. Thermal imaging cameras make small temperature differences visible. Thermal imaging cameras are widely used to complement new or existing security networks.
The image intensifier is a vacuum-tube based device that converts visible light from an image so that a dimly lit scene can be viewed by a camera or the open eye. While many believe the light is “amplified,” it is not. When light strikes a charged photo cathode plate, electrons are emitted through a vacuum tube that strike the micro channel plate that cause the image screen to illuminate with a picture in the same pattern as the light that strikes the photo cathode, and is on a frequency that the human eye can see. This is much like a CRT television, but instead of color guns the photo cathode does the emitting.
The image is said to become “intensified” because the output visible light is brighter than the incoming IR light, and this effect directly relates to the difference in passive and active night vision goggles. Currently, the most popular image intensifier is the drop-in ANVIS module, though many other models and sizes are available at the market.
Night vision devices
A night vision device (NVD) is a device comprising an image intensifier tube in a rigid casing, commonly used by military forces. Lately night vision technology has become more widely available for civilian use, for example, EVS, or enhanced vision systems, which are included in the latest avionics packages in cirrus and Cessna planes to help pilots with situational awareness and avoid accidents. eVS is also available for rotary wing operators.
A specific type of NVD, the night vision goggle (or NVG) is a night vision device with dual eyepieces; the device can utilize either one intensifier tube with the same image sent to both eyes, or a separate image intensifier tube for each eye. Night vision goggle combined with magnification lenses constitutes night vision binoculars. Other types include monocular night vision devices with only one eyepiece which may be mounted to firearms as night sights. NVG and EVS technologies are becoming standard operating products on helicopter operations to improve safety. The NTSB is considering EVS as recommended equipment for safety features.