Night Vision Devices
Forensic investigations are not always conducted in well-lit settings or during daylight. When lighting conditions are diminished, assistance in maximizing the available light using night vision technology can be important in inspecting the scene of an accident or death. Night vision devices have also proved useful in conjunction with lasers to identify altered, obliterated, or over-written documents.
Night vision technology can also be part of surveillance systems. Analyzing the recordings from surveillance cameras can reveal aspects of a crime or accident scene before and during the incident that would otherwise not be available.
Night vision scopes are devices that enable machines or people to "see in the dark," that is, to form images when illumination in the visible band of the electromagnetic spectrum is inadequate. Although it is not possible to form images in absolute darkness (in the absence of any electromagnetic radiation), it is possible to form images from radiation wavelengths to which the human eye is insensitive, or to amplify visible-light levels so low that they appear dark to the human eye.
There are two basic approaches to imaging scenes in which visible light is inadequate for human vision:
In the first approach, low-level visible light that is naturally present may be amplified and presented directly to the viewer's eye. (Light in the near-infrared part of the electromagnetic spectrum [%.77–1.0 microns], either naturally present or supplied as illumination, may also be amplified and its pattern translated into a visible-light pattern for the viewer's benefit.) This technique is termed image intensification.
In the second approach, light in the infrared part of the spectrum (>.8 microns) that is emitted by all warm objects may be sensed by electronic devices. A visible-light image can then produced on a video screen. This technique is termed thermal imaging.
Image intensification is the method used for the devices termed night-vision scopes, which exist in a variety of forms that can be mounted on weapons or vehicles or worn as goggles by an individual. Image-intensification devices have been used by technologically advanced military organizations since the 1950s. In a modern, high-performance light amplifier, light from the scene is collimated—forced to become a mass of parallel rays—by being passed through a thin disk comprised of thousands of short, narrow glass cylinders (optical fibers) packed side by side. The parallel rays of light emerging from these optical fibers are directed at a second disk of equal size, the microchannel plate. The microchannel plate is also comprised of thousands of short, narrow cylinders (.0125–mm diameter, about one fourth the diameter of a human hair), but these microchannels are composed of semiconducting crystal rather than optical fiber. A voltage difference is applied between the ends of each microchannel. When a photon (the minimal unit of light, considered as a particle) strikes the end of a microchannel, it knocks electrons free from the atoms in the semiconducting crystal. These are pulled toward the voltage at the far end of the microchannel, knocking more electrons loose as they move through the crystal matrix. Thousands of electrons can be produced in a microchannel by the arrival of a single photon. At the far end of the microchannel, these electrons strike a phosphor screen that is of the same size and shape as the microchannel disk.
The phosphor screen contains phosphor compounds that emit photons in the green part of the visible spectrum when struck by electrons; thus, that part of the phosphor disk affected by a single microchannel glows visibly, the brightness of its glow being in proportion to the intensity of the electron output of the microchannel. (Green is chosen because the human eye can distinguish brightness variations in green more efficiently than in any other color.) The phosphor-disk image is comprised of millions of closely packed dots of light, each corresponding to the electron output of a single microchannel. The light from the phosphor disk is collimated (made parallel) by a second fiber-optic disk and presented to the viewer's eye through a lens. The function of the lens is to allow the user's eye to relax (i.e., focus at infinity), rather than straining to focus on an image only an inch or so away. Alternatively, the phosphor-disk image can be filmed by a camera.
Either a pair of night-vision goggles may contain two such systems, one for each eye, or, as in the case of the U.S. Army's AN/PVS-7B night vision goggles, a single image may be split into identical copies and presented to both the user's eyes simultaneously.
A "third generation" image intensifier has been described above; several other image-intensification technologies remain in the field. All, however, operate by using photons to liberating electrons, amplifying the resulting electron current, and using the amplified electron current to liberate visible photons.
Infrared imaging systems are bulkier and more expensive than image intensification systems. However, they work even in a complete absence of illumination (since all scenes "glow" in infrared) and can detect otherwise invisible phenomena, such as hot, nonsmoky exhaust plumes, that may be of forensic interest. Infrared imagers are also used for a wide variety of forensic and industrial purposes, as they can reveal chemical compositional differences not evident in visible light.