Light-emitting diode (LEDs) Used in Night Vision Imaging System (NVIS) Compatible Lighting
A night vision device (NVD) comprises of an Infrared (IR) image intensifier tube in a rigid casing, commonly used by military forces and other non-civilian sectors; however, night vision technology has become more widely available for civilian use, for example, EVS, or enhanced vision systems are finding their way into private aircraft and vehicles. Infrared light is electromagnetic radiation with a wavelength between 0.7 and 300 micrometers, which equates to a frequency range between approximately 1 and 430 terahertz (THz).
Night vision goggles (NVG) combined with magnification lenses constitutes night vision binoculars. Other types include monocular night vision devices with only one eyepiece, which may be mounted to firearms as night sights. NVG and EVS technologies are becoming standard operating products on helicopter operations to improve safety. Light emitting diodes used in Night Vision Imaging Systems must provide an environment that will not have near infrared (NIR) noise, which would interfere with the nighttime sensitivity of the NVGs. It is important to note that night vision compatibility (NVC) is only achieved when the design of the lighting equipment allows for proper use with and without the NVIS, at night or during the day.
NVIS are passive systems, which have a very high sensitivity to radiation in the approximate region of 600nm to 930nm (orange to near infrared). The NVIS work by converting photons from the outside night scene onto a micro-displayed visible image. The NVIS will amplify the nighttime scene approximately 2000 times. To protect the image intensifier assembly, the systems are equipped with an automatic gain control (AGC), which will aperture down the NVIS when exposed to bright lights in the region of approximately 600nm to 930nm. If displays or light sources are not NVIS compatible, the automatic gain control will activate and the NVIS will become proportionally less sensitive to nighttime objects outside of the cockpit.
The Department of Defense (United States) published the MIL-STD-3009 Standard for lighting, aircraft, night vision imaging system compatibility, which specifies the interface and performance requirements for aircraft lighting and display equipment that is intended to be used along with NVIS. This specification defines aircraft interior lighting standards for sources such as cockpit displays and caution / warning lights, for both day and nighttime operating conditions. NVIS filters designed for avionic applications must incorporate NIR attenuation properties, chromaticity, contrast for daylight readability and often EMI/RFI shielding. The MIL-STD-3009 superseded MIL-L-85762A standard. This standard also includes provisions for white light sources and for "leaky green" requirements.
The light emitting diodes in a display heavily influence its color, contrast, and NVIS radiance (NR) properties. White, green, and red light emitting diodes with certain spectral emission can be adjusted with filters to produce displays that comply with various NVIS color coordinates and NVIS Radiance (NR) specifications described in MIL-STD-3009. Meeting both color and Night Vision Imaging System (NVIS) Radiance limits can be challenging. Each application is unique and always involves certain constraints, such as space limitations or production methods that may require a specific approach in order meet compliance.
Filters are ideal for many commercial and military applications, including crew-station displays for aircraft and ground vehicles, as well as portable display systems in personal digital assistants (PDA), palmtops, laptops, etc. Plastic Night Vision Imaging System (NVIS) filters are the ideal method for making virtually any light source compatible with NVIS, NVG, NVD, or any other night-vision apparatus. Plastic filters are extremely durable. Where all-glass filters might fracture, lightweight plastic material survives the most demanding real-world environments.