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NVIS-compatible Lighting
A pilot flying with NVIS must also be able to read the illuminated displays in the crew station with his unaided, normal vision. Likewise, an infantryman must still be able to read and operate communication or control equipment. Displays must be visible to the unaided eye while emitting very low levels of IR. If a display emits small amounts of radiant energy within the response range of the IR-sensitive photocathode, it may degrade the performance by impairing the resolution or sensitivity of the imaging system.
NVIS-compatible illuminated displays don't interfere with NVIS and remain readable to the unaided eye. NVIS compatibility also means that enemy NVIS find it difficult or impossible to detect the light source at a distance.
The success of an NVIS application, particularly in aviation, depends as much on the performance of the NVIS-compatible displays as on the NVIS itself. That's why Korry takes such care in designing filters, legends, light sources, and mechanical packaging to ensure the highest degree of NVIS compatibility.
A normal incandescent, EL (electroluminescent), or LED (lighting emitting diode) light source interferes with NVIS because it emits significant IR radiation within the response range of NVIS. This excess IR causes interference in several ways:
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IR reflections from the crew station canopy produce ghost images in the NVIS.
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IR can bloom the goggles, creating a halo-like glow around the displays and reducing visibility.
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High levels of IR activate the automatic gain control (AGC) and further degrade sensitivity and resolution.
The AGC feedback circuitry optimizes the gain, or level of NVIS intensification. A sudden increase in radiant energy within the NVIS response increases the electron current flow in the intensifier tube. The AGC detects this increase and compensates by reducing the acceleration voltage, lowering intensification so that the phosphor image dims and loses contrast. This protects the NVIS user from being blinded and disoriented by the flash of an explosion or flare. As soon as the radiant energy decreases, intensification goes back up. AGC also increases intensification when ambient irradiation is low, as on a moonless night.
An unfiltered incandescent bulb being turned on nearby has the same visual effect through the NVIS as a flashbulb to the unaided eye, followed by loss of image definition and poor visibility. The goggles actually shut down after the initial burst.
An improperly designed or manufactured NVIS-compatible display may still activate the AGC and reduce sensitivity and target resolution, or cause persistent ghosting or blooming.
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