Thermal Sensing with Short-Wave Infrared Detectors

Hank Hogan

Because glass is not transparent at mid- and long-infrared wavelengths, it stops current thermal detectors cold. Consequently, regular glass lenses cannot be used for these thermal imagers, and thermal imagers cannot peer through the window of a vehicle to look for interlopers or through an electrical box to find overheated switches.

Researchers built a short-wavelength IR detector using a thermoelectrically cooled indium gallium arsenide detector (photodiode) and commercial visible-wavelength-optimized optics (left). Measurements show that the readings of a 50 °C blackbody with the setup are stable (right). Courtesy of Howard W. Yoon of NIST. Reprinted with permission from Optics Express.

Now Howard W. Yoon and George P. Eppeldauer of the National Institute of Standards and Technology in Gaithersburg, Md., have demonstrated that short-wave infrared sensors operating from 2.0 to 2.5 μm can function as thermal detectors. When cooled to –85 °C, they detected objects that were not much hotter than the background, such as a human hand.

The detectors in the demonstration were built of either indium gallium arsenide or mercury cadmium telluride. In the past, the former has been sensitive from 0.9 to 1.7 μm, and the latter usually has been sensitive in the 10- to 12-μm range. However, both materials were engineered and fabricated to respond with a cutoff wavelength of 2.5 μm. This was possible thanks to compositional changes. Also, better materials that now are available enable high shunt resistance in the sensor circuitry, thereby lowering the noise.

The researchers chose the wavelength range of 2.0 to 2.5 μm, in part because there is an atmospheric window offering little loss at those wavelengths. In addition, standard optical glass transmits such wavelengths, producing a clear advantage.

A third benefit was that the Planck radiance law for blackbodies favors the use of the shortest possible wavelength. The ratio of radiance of a blackbody at 22 °C to one at 300 °C is greater than 19,000 at around 2.25 μm, while the ratio is only 370 at 4.0 μm. This difference results in higher discrimination of the target-to-background signal and lower sensitivity to the emissivity of an object when determining its temperature.

The researchers measured the performance of 1- and 3-mm indium gallium arsenide and mercury cadmium telluride devices sensitive to the wavelength range, cooling them with thermoelectric methods to –85 °C. Placing a hand — at 34 °C — in front of the 3-mm detector resulted in an output voltage roughly double that of room temperature objects that were 22 °C.

In the demonstration, the scientists constructed a prototype short-wavelength IR detector in a radiation thermometer configuration using a standard glass lens, a chopper wheel and a 3-mm sensor. When imaging an object at 50 °C, they found a noise-equivalent temperature difference of less than 3 mK. That figure climbed slightly at body temperature but remained below 10 mK. The performance was equivalent to that of a cryogenically cooled indium antinomide detector sensitive to thermal wavelengths.

However, their detector could be used with off-the-shelf glass optics. It also could achieve diffraction-limited performance, which could prove useful for target discrimination at long distances.

Along with its advantages, this short-wavelength IR detector does have one significant drawback. Because of its operating wavelength, it picks up reflected sunlight, moonlight and nightglow. That extra light will have to be accounted for when performing thermal imaging.

Yoon noted that advances might someday enable a one-size-fits-all imaging device. “With better materials, it might be possible to perform visible, near-infrared and thermal infrared measurements with just one set of sensors and lenses.”

Optics Express, Jan. 21, 2008, pp. 937- 949.