Night Vision

[Note to reader: topic sentences are in green; remaining weakness in red.]

ABSTRACT

Night Vision technologies encompass techniques of image conversion, image intensification and thermal imaging. Each has distinct characteristics and capabilities. The current image intensification technology -- utilizing photocathodes, phosphor screens and microchannel plates -- illustratres the technology of light amplification and image resolution.

Introduction

[No topic sentence in this introduction. Suggests for jazzing it up appear later.] Night vision technology, by definition, literally allows one to see in the dark. Originally developed for military use, it has provided the United States with a strategic military advantage, the value of which can be measured in lives. Federal and state agencies now routinely utilize the technology for site security, surveillance as well as search and rescue. Night vision equipment has evolved from bulky optical instruments in lightweight goggles through the advancement of image intensification technology.

Types of Night Vision

Night Vision technology is a term that presently encompasses three distinct technologies. The first night vision equipment was developed during the Korean War. Now sometimes referred to as generation-zero, this equipment employed image converter technology to transform infrared to visible light. The equipment operated in an active mode in that it required the subject of observation to be illuminated with an infrared light source. Lasers and filtered flashlights were utilized for this purpose. Resolution of the detector was constrained by the available processing technology at the time, limiting practical military engagement distances to within a few hundred meters. This active system also had the drawback of being easily detected by others with viewing equipment.

The subsequent principal technological development for night vision is thermal imaging. Thermal imagers are passive systems that respond to available infrared light at wavelengths in the 8-12 micrometer range. These wavelengths are readily emitted by all blooded animals as well as soil and plant life, warmed during the daytime hours. Infrared light is thus generated and available continuously, during both the day and night, so that these viewers have the distinction of performing equally well in both environmental lighting extremes.

The vast majority of night vision equipment and what most people think of when the are referring to ``Night Vision'' are devices that utilize image intensification technology. These devices are presently in their third development phase. They are passive devices that operate using naturally available light. Incoming light is converted to electrons, which are amplified and converted back into visible light.

Image Intensifier Principles of Operation

Figure 1 illustrates first-generation night vision. [Not a great topic sentence but it does has the advanage of calling attention to the figure.] Incoming light is collimated by fiber optic plates before impacting a photocathode t which releases electrons, which in turn impact a phosphor screen. The excited screen emits green light into a second fiber optic plate, and the process is repeated. The complete process is repeated three times providing an overall gain of 10,000.

Figure 1. First-generation image intensifiers employ three stages of photocathode adn phosphor screen light amplification.

Second-generation image intensification significantly increased gain and resolution by employing a microchannel plate. Figure 2 depicts the basic configuration. [These two sentences could have been combined: "Figure2 depicts how second-generation image ... plate."] The microchannel plate is composed of several million microscopic hollow glass channels fused into a disk. Each channel, approximately 0.0125 mm in diameter, is coated with a special semiconductor which easily liberates electrons. A single electron entering a channel initiates an avalanche process of secondary emission, under influence of an applied voltage, freeing hundreds of electrons. These electrons, effectively collimated by the channel, increase the resolution of the device. With additional electron optics, details as fine as 0.025 mm can be realized (half the diameter of a human hair).

Figure 2. In second-generation night vision the addition of the microchannel plate (MCP) collimated electron flow and increased the light-amplification gain.

Current image intensifiers incorporate their predecessor's resolution with additional light amplification. The multialkali photocathode is replaced with a gallium arsenide photocathode; this extends the wavelength sensitivity of the detector into the near infrared. The moon and stars provide light in these wavelengths, which boosts the effectively available light by approximately 30%, bringing the total gain of the system to around 30,000.

[No topic sentence. Indeed one might have moved this material to the front in a more dramatic way, perhaps by calling attention to the movie `Silence of the Lambs.'] slight green tint similar to some sunglasses. The apparent lighting of the landscape on a dark night is comparable to what the unaided eye would see on a clear winter night with fresh snow on the ground and a full moon.

Conclusion

Although the term ``night vision'' currently encompasses three distinct technologies, it is the evolution of image intensification technology that first made devices practical and widely used. Their success was the result of advancements in light amplification and resolution techniques.

References


[1] Figure from ``Scoping Out Night Vision,' National Law Enforcement
    and Corrections Technology Center (March 1996)

[2] L.S. Barrell (No date). Night Vision -- An Overview [Online].
    Available: http://home.megalinx.net/~eplurib/vision.htm 
    [1997, April 24].

[3] [No date]. About Night Vision [Online]. Available: 
    http://www.keyinfo.com/cat/jdh/jdh~00012.html [1997,March 10].

[4] [No date]. Microchannel Plates Principles of Operation [Online].
    Available: http://hea-www.harvard.edu/HRC/mcp/mcp.html [1997, 
    April 29].

Your comments and suggestions are appreciated.

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Edited by: wilkins@mps.ohio-state.edu [September 1997]