Operating life expectancy of Generation I image intensifier tubes was about 2000 hours.
Generation II tubes have a life expectancy from 2,500 hours to 4000 hours.
Generation III tubes to 10,000 hours.
This makes tube replenishment for the system virtually unnecessary. This is an important consideration when the intensifier tube normally represents 50% of the overall cost of the night vision system.
The early 1960's was witness to the beginning of passive night vision. Technological improvements included vacuum tight fused fiber optics for good center resolution and improved gain, multi-alkali photocathodes and fiber optic input & output windows. GEN I devices lacked the sensitivity and light amplification necessary to see below full moonlight, and were often staged or cascaded to improve gain. As a result, GEN I systems were large and cumbersome, less reliable, and relatively poor low light imagers. They were also characterized by streaking and distortion.
The development of the Microchannel Plate (MCP) led to the birth of Generation II devices in the late 1960's and early 1970's. Higher electron gains were now possible through smaller packaging, and performance improvements made observation possible down to 1/4 moonlight. The first proximity focused microchannel plate (MCP) image intensifier tube was an 18mm used in the original AN/PVS-5 NVG. Generation II+ provides improved performance over standard Gen II by providing increased gain at high and low levels. Generation II+ equipment will provide the best image under full moonlight conditions and is recommended for urban environments.
Amplification: 30,000 - 50,000x
The current state-of-the-art, the Generation III intensifier multiplies the light gathering power of the eye or video receptor up to 30,000 times. Requiring over 460 manufacturing steps, the GEN III intensifier is typically characterized by a Gallium Arsenide (GaAs) photocathode, which is grown using a metal organic vapor-phase epitaxy (MOVPE) process. The photon sensitivity of the GaAs phtocathode extends into the near-infrared region, where night sky illuminance and contrast ratios are highest. Sealed to an input window which minimizes veiling glare, the photocathode generates an electron current which is proximity focused onto a phosphor screen, where the electron energy is converted into green light which can then be relayed to the eye or sensor through an output window.
The GEN III Gallium Arsenide (GaAs) photocathode is uniquely sensitive beyond 800 nanometers, considered to be the critical near-infrared region where night sky illuminance levels are greatest. This spectral response shift to the red region results in improved Signal-to-Noise Ratios over GEN III's predecessors, delivering a three-fold improvement in visual acuity and detection distances.
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