By extending visible light-emitting-diode (LED) and laser technology into the near-IR spectrum, developers of solid-state lighting systems are also expanding the capabilities of night vision for applications 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, near-IR vision systems can use off-the-shelf CCD and CMOS cameras 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 (Northville, MI), which is currently participating in the European EDEL (Enhanced Driver’s Perception in Poor Visibility) project.
The sensitivity of CMOS and CCD detectors tends to fall off in moving from the visible to the IR. Near-IR illumination sources used to extend the spectral range for night vision include halogen bulbs, IR LEDs, and IR lasers. Halogen bulbs cost the least, but they are only 5% efficient in producing light. And because the bulb also produces visible light, the efficiency falls a good bit further when filtered to the near-IR component. So while a halogen bulb can cost only a few dollars, the overall cost grows by an order of magnitude and becomes comparable to the cost of LED-based illumination sources, Nagappan said.
Solid-state illumination sources, on the other hand, in addition to being more energy efficient in producing light overall, can also yield relatively narrow illumination bands within the near-IR range. This is important in automotive applications in which a primary goal is to increase the driver’s viewing distance without increasing visible-light output beyond legal limits. It is also important in covert spotlight systems for infrastructure security to monitor an area using illumination sources that are not visible to the naked eye. In addition, to improve energy efficiency, the compactness of solid-state sources facilitates applications requiring miniaturization, such as automotive night-vision systems and search-and-rescue equipment.
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 long-wavelength end of the visual spectrum at 780 nm as possible. This allows operation at the highest CCD and CMOS sensitivities in the near-IR.
Thin-film LEDs
The LED effort is based on a thin-film approach that enables scaling to large chip sizes. While high brightness can be obtained by amassing numerous small chips, reasonably large illumination distances cannot be attained even with the use of complex focusing optics, which also take a toll on illumination efficiency, Nagappan said. Typically near-IR-LED chip sizes are on the order of 300 × 300 µm and provide about 40 mW of light at 100-mA drive currents. Osram has already developed a 50-mW near-IR chip, but night-vision applications require increasing that power output by a couple of orders of magnitude.
The company is currently developing a 1 × 1-mm-size near-IR chip emitting at 850 nm to provide a 0.5-W light output with input currents on the order of 1 A. 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.
In fabricating thin-film devices, a light-emitting epitaxy (active) layer is grown on a substrate as with regular LEDs, but the active layer is removed and flipped onto a metal-coated substrate that instead of absorbing light like a normal substrate actually reflects all of it back outward through the top of the active layer. Because light is not lost through lateral substrate emissions, the fabrication process is scalable to larger dimensions while retaining very high efficiency, such that radiated output increases almost in direct proportion to the size of the chip.
Another high-power-LED configuration, introduced late last year in the visible range and under development for use in the near-IR, 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. Five watts of power could conceivably provide near-IR illumination to a distance of about 200 m-about twice the distance viewable with high-beam headlights. For even further distances out to 500 m or so, an additional order-of-magnitude increase in power would be useful (20 W or more) and would require lasers. With these types of applications in mind, Osram has developed a 30-W, 808-nm self-cooled laser-diode bar.
The relatively low power requirements, high output power, and compact size of lasers would seem to make them well suited for applications such as covert illumination. Lasers can illuminate at large distances and with their monochromaticity can be very efficiently tailored to specific sensing systems. But lasers are more expensive than LEDs, and they also require diffusing optics to make them eye-safe and to adapt beam width for wide viewing angles.
But the laser might be the preferable solution for automotive use, because the single wavelength can be exactly matched to the camera system, eliminating image blooming caused by headlights of oncoming vehicles, Nagappan said. For a covert-illumination application with only a relatively short distance requirement, on the other hand, an LED system with no need for either band-pass filters or diffusing optics might prove most efficient and cost-effective.