Infrared Detectors Image the Future

Anne Fischer Lent

The ability to see beyond the capabilities of normal human vision is vital in areas such as emergency services, defense, science, maintenance and surveillance, and all of these areas stand to gain by the latest advances in infrared imaging technology.

Objects at room temperature glow brightest in the wavelength range from 8 to 10 μm, so cameras that can see within that range are targeted for applications in security, surveillance, navigation and flight control, fire fighting and early-warning systems. Cameras with a wavelength range of more than 10 μm are used in ground-based telescopes, which can survey the Earth’s atmosphere, image distant stars and galaxies, and search for objects such as planets in orbit.

These images were captured with Raytheon’s low-power IR camera with a 160 x 120 array.

Today’s thermal infrared imaging cameras look like video cameras. They fit neatly in the palm of your hand or can be mounted on a helmet or pole for security, fire fighting and surveillance.

Close-up of BAE Systems’ 28-μm vanadium microbolometer pixel.

At the heart of an IR camera is the focal plane array, which is a two-dimensional matrix of pixels. Each pixel absorbs radiation from the object being viewed, generating heat, and converts that heat to an electrical signal. One recent technological advance that has increased the applicability of imaging cameras is the drive to larger arrays. The more pixels, the more discriminating the image will be. Arrays of 320 x 240 pixels were standard until a few years back when manufacturers came out with 160 x 120 so they could offer lower-cost products. Today some manufacturers, such as BAE Systems of Lexington, Mass., offer 640 x 480 microbolometer-based detectors.

As the technology behind the manufacture of detectors improves, resolution will continue to improve. According to Glen Francisco, product line manager for amorphous silicon bolometers, fire and security at Raytheon in Dallas, the technology is there to continue to shrink or grow the resolution of the array. “It just takes experience, knowledge and a little time,” he said.

Raytheon’s Control IR 2000 AS uses an uncooled, amorphous silicon bolometer.

The other components of an infrared imaging camera are the optics, which zoom and focus the scene, and the electronics, which produce the video signal. In the past, most thermal detectors used thermoelectric coolers to stabilize the temperature, although uncooled detectors are now populating the market. The drawbacks to using a cooler are that adding a device increases cost and size, and that it consumes power from the battery.

Commercially available IR detectors vary in thermal sensitivity and spectral responsivity, with requirements based on specific applications. Thermal sensitivity is determined by the noise equivalent temperature difference, which is equal to the temperature difference required in a scene for the camera to produce a signal-to-noise ratio of 1. Of course, this value depends on other things as well, such as different lenses or widely varying scenes.

BAE Systems’ camera with a 640 x 480 uncooled microbolometer focal plane captured this thermal image of a mall.

Spectral responsivity is the detector’s approximate wavelength range. In general, short-wave IR covers from 1 to 3 μm, mid-wave IR, from 3 to 5 μm, and long-wave IR, from 8 to 14 μm. Short-wave IR uses natural reflection and emission, and is targeted for night-vision applications that make use of available low light levels and sky glow. Mid-wave IR, which provides good thermal imaging from room-temperature to hot objects, is frequently used in predictive maintenance and fire-detection applications. Long-wave IR is suitable for colder conditions and where there is no solar reflection, and is used in surveillance, military and other applications where cold and solar glare are considerations.

Detector material

A microbolometer is a micromachined bridge that often is coated with vanadium oxide or amorphous silicon. These materials absorb the infrared radiation, and the change in temperature changes its resistance, which is sensed electronically. There are advantages and disadvantages to each, but the choice for the end user often boils down to finding a camera that will offer the resolution, size, flexibility, cost and other features needed for a particular application.

Infrared cameras are used in commercial, military and scientific applications; the demands of military applications drove much of the research over the past 10 years. The result is a lower-cost product that meets the needs of many commercial uses, such as predictive maintenance, process control, security, night vision in cars, fire fighting, testing and law enforcement.

To meet the needs of commercial markets, keeping costs down is paramount, Francisco said. He said that Raytheon uses amorphous silicon in its detectors for just this reason. Detectors manufactured with vanadium oxide have to be packaged one at a time and take more care to ensure the best performance, while silicon detectors are designed for easy setup and can be produced more quickly. High-volume production translates to lower cost and not necessarily to decreased sensitivity, he said.

The company’s new Control IR 2000 AS is a long-wave-IR video camera core targeted at OEMs supplying the commercial IR industry. The same camera core can be used in a variety of commercial applications simply by changing its housing or mount. Among the main users of such cameras are utility companies that perform preventive maintenance by monitoring lines and transformers for hot spots. Another increasingly popular use is as a fire fighting tool, although manufacturers must continue to drop prices below the current cost of about $20,000 before fire departments can put one of the cameras into the hands of each firefighter.

The cantilever approach

Sarcon Microsystems Inc. in Knoxville, Tenn., has taken a unique approach to infrared sensing that is based on microcantilever technology. The new technology results in an uncooled IR detector that approaches the theoretical limits of infrared sensitivity, said David J. Smith, vice president of sales and marketing. He said that the company’s new detectors are 10 times more sensitive than microbolometer-based detectors.

The difference lies in the fact that the microcantilever technology uses capacitance rather than resistance. The microcantilever arm is attached at one end to a fixed support structure, while the other end is allowed to bend. The surface of the detector is coated with an absorbent material, and when the infrared radiation converts to heat, the temperature of the arm rises. When the cantilever is heated, it bends, generating an electrical signal in the detector that is proportional to the intensity of the radiation absorbed by the sensor. These detectors can be stacked in rows and columns to produce a two-dimensional array of up to 320 x 240 pixels.

The new IR detectors will be sold to OEMs along with the supporting electronics. Smith said that they will be used in the same applications as other types of detectors, but will provide sensitivity 10 times greater than other technologies will, and at a lower cost.

Extended-wavelength cameras are a new trend for spectroscopy and military applications, and may soon trickle down to the commercial side. The cameras are built on standard sensor technology with additional processing of the detector material at the fabrication level. They operate over a wider spectral range than sensors built the standard way. One example of such a material is VisMir, developed by Santa Barbara Research Center in Goleta, Calif. This indium-antimonide sensor material is processed in such a way that it extends spectral response from the mid-IR (5.5 μm) down into the visible band.

Indigo Systems Corp., also in Goleta, has developed a material that extends the short-wave response of its standard InGaAs detectors, offering simultaneous wavelength capability for the visible and near-IR. Called VisGaAs, for visible indium gallium arsenide, the detectors are responsive down into the ultraviolet at 350 nm while maintaining response out to 1700 nm.

The benefits of this material, said Austin Richards, senior applications engineer, is that it provides a broad-spectrum capability that other sensors can’t achieve. The advantage is that VisGaAs is uncooled and uses readily available InGaAs as the starting material.

The trend continues

The trend toward smaller size, lower power and decreased cost will continue while performance improves and opens up new applications. At the same time, new demands, such as the implementation of devices for the homeland security initiative, will push manufacturers to continue to innovate.

Gabor F. Fulop of Maxtech International in Fairfield, Conn., said that it’s an exciting time for this market, which will continue to grow at a rate of about 20 percent a year. Coupled with evolutionary advances in electronics, such as more-powerful digital signal processors, infrared imaging will make important inroads into a variety of application areas.