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Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Near infrared light is electromagnetic radiation of a wavelength slightly longer than that of visible light and cannot be detected by the human eye. Therefore an IR-sensitive camera, typically a CMOS camera, is used to detect the image.
To date, the most widely used IR vision system technology is thermal IR imaging. Developed more than 50 years ago, this technology does not require visible light or other light to illuminate a scene. Thermal IR imaging shows the thermal patterns emitted from, or reflected off of, a target. These devices are "passive" and they sense warm objects rather than actually illuminating targets and capturing detailed visual images.
Thermal IR imaging is especially useful in cooler environments and over a long range. It has been used extensively in security applications, mainly for military night vision systems, for decades. However, this technology has a number of limitations. Thermal IR devices yield low-resolution images (with typically 320 x 240 pixels), which is at least 5 times less than CCTV cameras and therefore works well only for locating warm targets. They cannot "see" through windows or other translucent materials and cannot usually decipher between friend and foe at long or short range. Additionally, they cannot read the lettering on a vessel, the license plate on a car, or other key identifying numbers. They also are less effective when used in non-optimal conditions such as hot deserts or in rainy, foggy environments. All of these limitations sparked the demand for a new and more advanced IR technology.
Active-IR infrared imaging technology has been developed over the last few years. It uses light in the near-infrared range with a wavelength typically of 850nm to illuminate a scene or subject and capture it using a CCTV-type camera at wavelengths invisible to the human eye. New industries, such as night-time surveillance and automotive night vision, have grown substantially since the advent of active-IR imaging.
The limitations of thermal-IR imaging are being addressed by active-IR technology, which is more effective and provides lower system integration costs. Active-IR devices can eliminate shadows and reveal identifying lettering, numbers and objects to complement or outperform thermal-IR devices. The active-IR devices show none of the aforementioned limitations of thermal-IR devices and can be integrated in night vision goggles, helping to differentiate between friend and foe, and provide visibility through windows (as well as snow, fog and mist). It eliminates shadows and provides a much clearer and detailed image.
There are a variety of active-IR solutions for infrared illumination that include high-power infrared emitters based on Thinfilm technology and continuous-wave IR lasers in combination with CCTV-type cameras. The packages are available in a broad portfolio, in single-chip or array configurations, with SMT or through-hole mounting options, meeting the different needs of the various applications. Their optical output power can range from 50mw up to 15w continuous wave (CW), passively cooled.
With active-IR, illumination distance can be arbitrary, depending on the vision system receiver. So-called "short-range" (less than 50 mw to 50 mw optical power) illumination applications include mobile electronics, photoelectric sensors, garage door openers, remote controls, CCTV cameras used in security applications, ATM security illumination, biometric identification and medical imaging.
In the medical area, high-power infrared emitters such as the IR Golden DRAGON® provide an infrared light source with high efficiency in a very small package (1 x 1 mm2 emitting area). These devices are being used in medical imaging applications such as a new vascular viewing technology that uses infrared light to visualize both superficial veins and more deeply located arteries, greatly facilitating the insertion of a needle or catheter.
These applications increasingly are using the advantages of active-IR LED devices with Thinfilm technology, which provides brighter light over a longer lifetime. This pure surface emitting and scalable chip technology, developed by OSRAM Opto Semiconductors, provides increased optical output, better optical coupling, lower forward voltages and improved thermal performance compared with previous chip construction. Conventional LED chip technologies have the drawback of side emission losses that can reduce the light output and diminish thermal performance.
The Thinfilm chip technology can be used in a variety of packages, for example in a series of very small 3 mm and 5 mm radial packages. The tiny size of these devices delivers significant mW/steradian from a very compact package. The same can be achieved in SMT components.
Applications involving larger-area illumination such as perimeter protection for security systems (e.g., parking lots or large private homes) often will use "medium range" optical power devices (typically about 500 mW). The key is wide-area illumination provided by very small point light sources, such as high-power, high-current IR-emitters picked up by CCD/CMOS cameras. These point light sources will provide the infrared light for a "hidden" camera that is not visible to an intruder.
The first available IR-high-power LED in an SMT package, the IR Golden DRAGON® is optimized for CCD and CMOS cameras, making it suitable for automotive night-vision systems and for security system illumination. Designed for high power output in a small footprint, it allows designers to illuminate broad areas and eliminates the need for complex optics. It also provides the ability to redesign existing systems with fewer parts, which reduces cost and system space.
For special security applications, mainly in the military, there is a 940nm version that offers enough power for the camera sensor but, at the same time, does not emit any visible red glow as in the case of 850nm emitters. For example, one stationary CCTV camera with Golden DRAGON® IR illuminators mounted on a parking lot lamp post can illuminate the entire area with no dead spots. Automatic license plate recognition in airport parking lots is becoming a very big application.
"Long-range" IR illumination applications require optical power in the greater than 5W range and include high-powered night vision illumination and communication, vehicle/helicopter night vision, remote monitoring, nighttime aircraft runway lighting, landing zone security, and identification by pilots of objects on the ground.
The new LED technology based on Thinfilm technology offers the most powerful infrared LED arrays in the world for CCTV applications and night vision equipment. OSTAR®-Observation LED arrays, for example, deliver power of up to 5W and provide an optimal solution for covering long distances and monitoring extensive areas with infrared light, particularly in applications that require continuous operation and/or high ambient temperatures. They provide exceptionally low thermal resistance, and are certified for automotive applications at temperatures up to 125 °C. Only 3.3 x 1.35 cm in size, they emit IR light with a wavelength of 850 nm or 940nm. The compact, flat design and high radiance of these LED arrays allow them to be easily integrated into a variety of applications, including night vision and surveillance cameras, covert runway lighting, and perimeter security. In the automotive area, they are suitable for pre-crash sensors and active night vision systems.
Even more output power can be achieved from an IR-laser. The first surface-mountable CW (continuous wave) high-power laser with the world's smallest footprint (6.0 x 4.5 mm2) and an output of greater than 10W has recently been introduced. Its operating temperature range of -40º to +100º C, combined with its SMT capability and compatibility with modern production processes, make it well suited to volume applications for NVIS, CCTV and traffic monitoring.
The superiority of Active versus thermal IR imaging has been well established in the last few years. Its implementation in a wide range of advanced IR devices utilizing innovative Thinfilm technology has made possible major technological and performance improvements for military, security and automotive night vision imaging applications, as well as in the medical, biometric and other key sectors.
Karl Leahy is General Marketing Manager at OSRAM Opto Semiconductors, Santa Clara, Calif.