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 light spectrum, to protect exposure from for example powerful military laser range finders and laser target designators.

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.

Minus-blue / NVIS compatibility

Minus-blue filters are used in aviation with the main purpose of reducing aircraft interior lights from interfering with night vision goggles. NVIS compatibility is required from the aircraft with interior lights modified or designed to only use a specific set of colors and brightness values. The common minus blue filters block light which has a shorter wavelength than a defined threshold. The most common types can be divided into:

• Class A
  • 625nm minus blue objective lens filter. Not compatible with red cockpit lights
• Class B
  • 665nm minus blue objective lens filter. Is compatible with NVIS Red cockpit lights
• Class C
  • 665nm "leaky green" minus blue objective lens filter. Compatible with NVIS Red and a narrow spectrum of green for holographic HUD compatibility

NVIS compatible aircraft have detailed and specific requirements for lights. Exterior lights are also included, for example, in "IR Mode" these cannot be visible to the dark adapted eye beyond a certain distance.

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.