Cockpit displays used alongside night vision goggles require careful control of their spectral output. A display can appear perfectly acceptable to the unaided eye while emitting sufficient energy within wavelengths detected by night vision equipment to interfere with its operation.
NVIS filters are used to control this unwanted output while retaining the visible light required for display readability, colour and symbology. Rather than simply reducing brightness, the filter is designed to provide a defined spectral response: transmitting the required visible wavelengths while attenuating wavelength regions that could interfere with the night vision system.
This becomes particularly important in modern aerospace display systems using high-brightness displays, LED backlighting and wide viewing angles. The challenge is to achieve the required NVIS compatibility without unnecessarily reducing visible display performance.
Depending on the application, NVIS filtering may also form part of a wider optical assembly incorporating anti-reflective coatings, conductive layers, transparent heating or EMI/RFI shielding.
What Does NVIS Mean?
NVIS stands for Night Vision Imaging System. The term is used in relation to night vision equipment and the systems, displays and illuminated components that must operate alongside it.
Night vision goggles amplify low levels of available light to improve visibility in dark operating conditions. Their spectral sensitivity extends beyond the response of the human eye, which means optical energy that is relatively inconspicuous to an unaided observer can appear much more significant when viewed through night vision equipment.
This creates a particular problem for illuminated cockpit displays and controls.
The visible output still needs to provide the luminance, contrast and colour required by the operator, but spectral output capable of interfering with the night vision system must be controlled.
NVIS compatibility is therefore a property of the complete optical and display system rather than simply an indication that a display has been made dim enough for night-time use.
Why Can Displays Interfere with Night Vision Goggles?
Electronic displays and illuminated controls produce light across a spectral range determined by their light source, display technology and optical construction.
Some of that output may extend into wavelength regions to which night vision goggles remain sensitive. If sufficient unwanted energy reaches the goggles, it can be amplified along with the external scene.
This can produce excessive apparent brightness or blooming and reduce the contrast available through the night vision system. The operator may then find it more difficult to view the external environment while simultaneously using illuminated cockpit information.
Simply reducing the overall brightness of the display is not necessarily an effective solution. The visible output may become too low for satisfactory display readability before the unwanted spectral component has been sufficiently controlled.
NVIS filtering addresses the problem spectrally rather than relying solely on overall brightness reduction.
How Do NVIS Filters Work?
An NVIS filter controls the spectral distribution of light leaving the display.
The required visible wavelengths are transmitted at levels appropriate to the display, while wavelengths capable of contributing excessive radiance within the response of the night vision equipment are attenuated.
Precision NVIS filter systems can use multilayer thin-film coatings on optical substrates to produce the required visible transmission and infrared attenuation characteristics.
These coatings use optical interference to create wavelength-dependent transmission and rejection. By controlling the materials and optical thicknesses within the multilayer structure, the spectral response can be designed so that selected visible regions remain transmissive while unwanted longer-wavelength output is strongly reduced.
The transition between these regions is important. Excessive attenuation extending too far into the visible spectrum can reduce display luminance or alter colour, while insufficient rejection at longer wavelengths can leave the display incompatible with the night vision system.
An effective NVIS filter is therefore not simply an infrared-blocking filter. It has to balance visible transmission and colour performance with the level of attenuation required within NVG-sensitive spectral regions.
The required balance depends on the display, illumination source, night vision equipment, viewing conditions and applicable system requirements.
NVIS Compatibility and Display Systems
NVIS filtering needs to be considered as part of the complete display architecture.
The spectral output of the light source is particularly important. Different display and backlight technologies can produce substantially different spectral distributions even where their visible appearance is similar.
LED-based systems can present particular challenges because their spectral characteristics depend on the LED technology, phosphors, display construction and operating conditions. A filter suitable for one display cannot therefore be assumed to provide the same NVIS performance when transferred to another.
Higher display brightness can introduce further considerations. Bright output is valuable for daylight readability, but the complete spectral distribution still needs to remain compatible with night vision operation when the display is used under low-light conditions.
The filter also becomes part of an optical stack that may include cover glass, anti-reflective coatings, touch interfaces, bonding materials, shielding structures and other display elements. Each can influence the final transmission, reflection and colour characteristics seen by the operator.
For this reason, NVIS filtering is most effectively developed around the complete display system rather than treated as an isolated optical component.
How Is NVIS Compatibility Assessed?
NVIS compatibility cannot be established simply by looking at a display or by confirming that its infrared output has been reduced.
The spectral output of the display needs to be considered in relation to the response of the night vision system and the applicable requirements for the intended platform or programme.
NVIS radiance is an important part of this assessment because it relates the spectral output of the illuminated component to the sensitivity of the night vision equipment. Controlling this radiance helps prevent the display from producing excessive response through the goggles.
Visible performance remains important at the same time. Luminance and chromaticity requirements can be used to establish whether the display continues to provide the required brightness and colour characteristics after filtering.
The result is a balance between two different optical requirements: suppressing spectral output that could interfere with night vision equipment while retaining the visible information required from the display.
The applicable limits and measurement conditions depend on the NVIS requirements of the particular system or programme. Compatibility should therefore be assessed against the relevant specification rather than treated as a single universal transmission target.
Optical Performance and NVIS Filter Design
The spectral transmission curve is central to NVIS filter performance, but it is not the only optical characteristic that matters.
Visible transmission needs to remain sufficient for the intended display, while colour changes introduced by the filter must remain within the requirements of the system. The spectral transition into the blocking region must provide the required attenuation without unnecessarily compromising visible output.
Reflection also needs to be controlled. Cockpit displays can be viewed under widely varying ambient illumination, from dark night-time conditions to bright daylight. Surface reflections can reduce display contrast and readability even where the spectral filtering itself performs correctly.
Anti-reflective performance may therefore be incorporated into the optical assembly alongside the NVIS filtering function.
Uniformity is another consideration, particularly for larger display formats. Spectral or optical variation across the active viewing area can produce differences in brightness or colour that become increasingly noticeable as display dimensions increase.
These requirements demonstrate why the performance of an NVIS filter cannot be represented by a single transmission or blocking value. The complete spectral and optical behaviour needs to correspond to the requirements of the display system.
Viewing Angle and Angular Performance
Viewing angle can be particularly important for interference-based NVIS filters.
The spectral response of a multilayer interference coating changes as the angle of incidence increases. Spectral features can shift towards shorter wavelengths, altering both the visible transmission region and the position of the transition into the blocking region.
This behaviour becomes important in cockpit displays because the screen is not necessarily viewed from a single position directly normal to its surface.
Larger integrated displays and wide-angle viewing requirements can expose the optical assembly to a range of viewing geometries. The NVIS response therefore needs to remain appropriate across the angular range required by the application rather than only at normal incidence.
Angular performance should consequently be considered during filter design alongside the nominal spectral requirement.
NVIS Filters in Aerospace Applications
NVIS-compatible optical systems are used in aerospace applications where illuminated displays and controls need to operate alongside night vision equipment.
These can include multifunction cockpit displays, avionics interfaces, mission displays and illuminated control panels.
The exact optical requirement varies between systems. Display technology, brightness, viewing geometry, night vision equipment and integration requirements can all affect the filter specification.
Modern cockpit architectures have also moved towards larger integrated displays. This places greater emphasis on maintaining spectral performance, colour and uniformity across wider active areas and viewing angles.
NVIS filtering therefore needs to develop alongside changes in display technology rather than being treated as a fixed optical solution that can be applied identically to every aerospace display.
Combining NVIS Filtering with Other Display Functions
A cockpit display window may need to perform several optical and electrical functions simultaneously.
Alongside NVIS filtering, an assembly may require anti-reflective performance to reduce surface reflections, EMI/RFI shielding to control electromagnetic interference, or transparent heating to manage condensation, icing or low-temperature operating conditions.
These functions can potentially be incorporated within a multi-layer optical assembly rather than being treated as completely separate components.
Integration has to be considered carefully because each additional material, coating or interface can influence transmission, reflection, colour and overall optical performance.
The final display window therefore needs to be engineered as a complete optical structure in which the different functions operate together without compromising the spectral requirements of the NVIS system.
Environmental Requirements for Aerospace NVIS Filters
Aerospace optical components can operate under conditions considerably different from those encountered by laboratory optics.
Temperature variation, humidity, vibration, abrasion and other environmental conditions can influence both the optical assembly and its coatings over time.
Temperature is particularly relevant to multi-layer structures because materials within the assembly can have different thermal and mechanical properties. Changes in operating conditions therefore need to be considered alongside the spectral design.
Exposed display surfaces may also require sufficient durability to withstand handling and cleaning without unacceptable deterioration of their optical performance.
The relevant environmental requirements depend on the platform, installation and applicable programme specification. As with spectral performance, the filter and display assembly should therefore be designed around the conditions it will actually encounter in service.
Custom NVIS Optical Systems
NVIS filter systems are rarely identical from one application to another.
Display technology determines the initial spectral output. The required visible luminance and colour characteristics establish what needs to be preserved, while the night vision system and applicable compatibility requirements determine which spectral regions require attenuation.
Viewing angle, display dimensions and the surrounding optical stack introduce further constraints.
Where additional functions such as EMI/RFI shielding, transparent heating or anti-reflective performance are required, these also need to be considered as part of the complete display window rather than added without reference to the existing optical design.
This systems-based approach allows the spectral filter, substrate, coatings and additional functional layers to be developed around the requirements of the actual display application.
NVIS Filter Requirements in Practice
Brinell Vision develops NVIS display filter solutions for aerospace and specialist display applications, including systems where spectral filtering needs to be integrated with other optical or electrical functions.
The Brinell Vision NVIS-A1000 series is designed to combine high visible transmission with a steep transition into near-infrared blocking. A1000k variants provide additional angular blocking and extended infrared attenuation where required by the application.
NVIS filter requirements can also be considered alongside anti-reflective coatings, EMI/RFI shielding, transparent heaters and other elements of the display optical assembly.
For further information about available configurations and application-specific requirements, see our NVIS Display Filters.
NVIS Filters – FAQs
What does NVIS mean?
NVIS stands for Night Vision Imaging System. In display applications, NVIS compatibility refers to controlling the optical output of illuminated displays and instruments so that they can be used alongside night vision equipment without producing unacceptable interference.
Why can cockpit displays interfere with night vision goggles?
Night vision equipment is sensitive to wavelengths beyond those used by the human eye for normal display viewing. A display can therefore appear acceptable to the unaided eye while emitting sufficient energy within NVG-sensitive spectral regions to interfere with night vision performance.
Are NVIS filters simply infrared-blocking filters?
No. Infrared attenuation is important, but an NVIS display filter also has to preserve the visible optical performance required from the display. The spectral design therefore needs to balance NVG compatibility with visible transmission, colour performance and the requirements of the display system.
Do LED displays require NVIS filtering?
LED and LED-backlit displays can require spectral control where their output is incompatible with the night vision equipment used alongside them. The required filtering depends on the spectral output of the display, its operating conditions and the NVIS requirements of the complete system.
Does viewing angle affect NVIS filter performance?
It can. The spectral response of interference coatings can shift as angle of incidence changes. This becomes particularly important in wide-angle cockpit displays, where NVIS compatibility and visible display performance may need to be maintained across a defined viewing range.
Can NVIS filtering be combined with EMI shielding and anti-reflective coatings?
Yes. Depending on the display requirements, NVIS spectral filtering can form part of a wider optical assembly incorporating functions such as anti-reflective performance, EMI/RFI shielding and transparent heating.