Covert lighting on military aircraft is increasingly being implemented on existing airframes to create a compatible environment for pilots using night vision goggles during flight. This retrofitting procedure requires that new and improved lighting devices, such as LEDs, fit within the parameters defined by the original lamp illuminators. The area is often smaller than what is optimal, or the limited space may not have enough room for airflow to use individual LEDs in component form.
A chip-on-board solution is often a feasible approach, where individual chips in the near infrared (NIR) are placed close together. In this case, the total amount of power per mm² will increase, but the angle of illumination will be spread over a 120-degree cone angle. This emission pattern works quite well for anti-collision lights, but it does not produce enough straight-ahead intensity for covert-landing lights. Most covert units employed today will use optical lenses in front of the LEDs to collimate the light angle to somewhat less than 30 degrees. The disadvantage to creating this tight optical collimation is that the LED features will be incorporated into the illumination pattern and create unwanted bright spots and dark holes within the illumination field. For fast jet aircraft, the collimation of the light is very tight (at less than a 10-degree cone angle) and collimating the light via optical lenses is still the only viable option.
In propeller-driven aircraft and helicopters, the demand for highly collimated light is lessened and emission cone angles up to 30 degrees or more is optimal. In helicopters, the 30-degree emission angle is desirable because the pilots are also often looking for obstructions that may be surrounding their landing zone. However, a very wide emission chip-on-board solution will still require additional lenses or expensive reflectors placed around the array of LEDs.
To meet this covert aircraft lighting challenge, Opto Diode Corporation has developed a high power, 850nm IR LED array that emits at a 30-degree cone angle and collimates the light using reflection rather than optical lenses. Using reflection, this new light becomes a very homogenous beam without bright spots of light or dark areas. It compares favorably to a landing light utilizing a tungsten or similar lamp. The thermal resistance of the device is just 0.8C/Watt and is electrically isolated, so the heat can be dissipated easily through the metal chassis without need for electrically- isolating thermal pads. Just as important is the compact size of the array (only 2 × 2 inches), yet it has a total output of 16W of optical power. The relatively small footprint allows either a single array or multiple arrays to be retrofit within existing landing light housings without significant modifications.
For more information, visit Opto Diode at http://info.hotims.com/40431-302.