The human eye has, as we all know, very limited night vision (NVIS) capability. Night vision goggles (NVG) were developed for military applications to permit night- time missions under the cover of dark, with particular issues arising in their use in aircraft and other vehicles.
In effect at night time, there remains a very low level of natural celestial illumination, the reflection of which from the landscape is imaged by the NVG.
NVG are based on image intensifiers, whereby a photon of light, falling on a photocathode, liberates an electron. This single electron goes through an electron amplifer, the resultant electrons then falling on a phosphor screen. Using lenses to image the view of the user, one may obtain on the phosphor screen a typical green- black image of the landscape.
It is important to ensure that such devices are not illuminated by artificial sources, such as display screen in aircraft and vehicle cockpits, else their operation may be entirely compromised.
Measurement for NVG compatibility are centred round two main themes, and are stated in document MIL-STD 3009 Lighting, aircraft, night vision imaging system (NVIS) compatible.
＆#8226; Measurement of the radiance of NVIS sources over the spectral range 380-930nm
＆#8226; Measurement of the transmission of source filters intended for NVIS use over the range 380-930nm.
In essence, both measurements seek to ensure that either no output is seen in the wavelength range at which the NVG is responsive, typically ~600-930nm.
This measurement requires an instrument with high sensitivity and high dynamic range, at the core of which is a double monochromator and cooled photomultiplier detector. Coupled to this is an easy to use control software which reports directly NVG results to determine pass or fail.
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