Filters

As night vision has a lot to do with optics, it is only natural that optical filters find several applications there.

Ocular Filters

Many night vision devices offer a threading in front of the eyepiece that can be used for various accessories. One of the possible use cases are colored filters that change the hue of the light coming from the phosphor screen by filtering out certain colors.

Amber Filters

Amber filters modify the output of a green phosphor screen to an amber color. For some users, this decreases eye strain. The loss of brightness is not very large, as green and yellow are relatively close on the light spectrum.

Amber filters have even be issued with night vision devices in the military, for example in the USA Armed Forces.

White Phosphor Filters

Comparison between an Elbit F9800 XLSH image intensifier with and without the use of a white phosphor filter. All images are taken with F/2.4, ISO-3200. The top row has an exposure of 1/20 seconds, the bottom row has an exposure of 1/8 seconds.

White phosphor filters modify the output of a green phosphor screen to a much more desaturated, white-green color. The loss of brightness is much more extreme, as the filter has to remove much of the green light. These filters are generally regarded to as gimmicks, with the only real use being the ability to convey how white phosphor looks like without a white phosphor tube.

Objective Filters

Many night vision devices offer a threading in front of the Objective. Here its common to thread in protective filters.

Light-Interference-Filters

A Light-Interference-Filter (LIF) filter works by filtering specific parts of the incoming light. This is achieved with interference.

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Common LIF filters filter a specific part of the green light spectrum, as many military laser range finders and laser designators use that spectrum. Furthermore, LIF filters can be used to effectively convert ground-spec systems into aviation-spec systems, as they subdue the light emitted by the instruments in the cockpit.

Daylight filters

Daylight filters often work like strong sunglasses and reduce the incoming light. They are sometimes used in military and law enforcement to train with night vision during daylight without damaging the image intensifiers. Those filters are either semi-permanently affixed to the objective lens of the device or sometimes integrated into the lens cap, making them easier to remove.

Solar-blind filters

Solar-blind filters filter out the light spectrum emitted by the sun, leaving only artificial light sources for the device to detect.

UV Filter

The UV filter is often used as a de-mist shield. That means that its purpose is to protect the lens of the objective from mechanical damage.