Solid-state sources tackle night-vision applications
By extending visible LED and laser technology into the near-infrared (NIR) spectrum, developers of solid-state lighting systems are also expanding the capabilities of night vision system for applications in areas such as automotive safety, infrastructure security, and search and rescue.
NORTHVILLE, MI - By extending visible LED and laser technology into the near-infrared (NIR) spectrum, developers of solid-state lighting systems are also expanding the capabilities of night vision system for applications in areas such as automotive safety, infrastructure security, and search and rescue. Departing from the far IR wavelengths and ghost-like imagery of traditional night vision systems, NIR vision systems can use off-the-shelf CCD and CMOS camera technology to provide imagery similar in quality to what you would expect to see at visible wavelengths through a camcorder, according to Sevugan Nagappan, a marketing manager for infrared and laser products at OSRAM Opto Semiconductors.
The sensitivity of CMOS and CCD detectors tends to fall off in moving from the visible to the IR, and near IR illumination sources used to extend the spectral range for night vision include halogen bulbs, infrared LEDs, and infrared lasers. Halogen bulbs can be obtained at the lowest cost, but they are only 5% efficient in producing light. And since it includes light in the visible spectrum, the efficiency falls a good bit further in the process of filtering down to the NIR component. So while the purchase price of a halogen bulb may be only a few dollars, the overall cost grows by an order of magnitude and becomes comparable to the cost of LED based illumination sources, when one factors in the illumination efficiency and expensive visible light filters.
Solid-state illumination sources, on the other hand, are more energy efficient in producing light overall and can also yield relatively narrow illumination bands, specifically within the NIR range. This is obviously important in automotive applications where a primary goal is to increase the driver’s viewing distance without increasing the visible light output beyond established legal limits, established to avoid flash blinding hazards to oncoming traffic. It is also important in covert spotlight systems intended for infrastructure security, where the goal is to monitor an area using illumination sources that are not visible to the naked eye. In addition to better energy efficiency, the relative compactness of solid-state sources actually facilitates applications requiring compactness, such as mounting in automotive night vision systems and portability for search and rescue applications.
At OSRAM, developmental efforts are focused on LED illumination systems at 850 nm and laser illumination systems as 808 nm, both intended to come as close to the top of the visual spectrum at 780 nm as possible to operate at the highest CCD and CMOS sensitivities in the NIR. The LED effort is based on a thin-film approach that enables scaling to large chip sizes. While high brightness can be obtained by collecting numerous small chips, reasonable illumination distances cannot be attained even with the use of complex focusing optics, which also take a toll on illumination efficiency, according to Nagappan. Typically NIR LED chip sizes are on the order of 300 µm by 300 µm and provide about 40 mW of light at 100 mA drive currents. OSRAM has already developed a 50 mW NIR chip, but night vision applications require increasing that power output by a couple of orders of magnitude.
The company is currently developing a 1-mm by 1-mm sized NIR chip emitting at 850 nm to provide a half-watt of light output with input currents on the order of 1 amp. The ability for scaling comes from a thin-film approach in which all of the light is radiated vertically from the face of the chip rather than having a significant amount absorbed by the substrate and emitted through the sides.
Another high-power LED configuration, introduced late last year in the visible range and under development for use in the NIR, combines an array of large thin-film LED chips in one package to increase output by another order of magnitude into the 5-W range, which could provide NIR illumination out to 200 m (about twice the distance viewable with high-beam headlights). But for distances out to 500 yards or so, an additional order of magnitude of power would be useful (20 W CW or more) and would require lasers.
While all three technologies-bulbs, LEDs and lasers-could provide, say, 200 m of visibility in auto applications, technologically the laser might be the preferable solution because the single wavelength could be exactly matched to the camera system for most efficient operation and eliminate image blooming due to oncoming vehicles, according to Nagappan. Cost-wise, however, an LED based system or halogen illumination might be preferable in less efficient designs or systems with slightly diminished performance. For a covert illumination application with only a relatively short distance requirement, the LED system, with no need for either band-pass filters or diffusing optics, might prove most efficient and cost effective.
Hassaun A. Jones-Bey