Electronic Design

Uncooled Thermal Imaging Has Mass-Market Appeal

With these temperature-tuned thin-film filters, inexpensive CCD/CMOS cameras can sense thermal radiation wavelengths of 8 to 15 µm.

Low-cost thermal imaging is here, and it's kicking the door open to mass-market applications. The culprit is a CMOS-compatible active thin-film technology platform devised by RedShift Systems Inc. The technology behind the platform is based on Princeton University's research in applying thin-film semiconductors to tunable optics.

Traditional microbolometers use the thermo-electric effect to detect infrared (IR) signals in a semiconductor. Yet RedShift's technology employs the thermo-optic effect. A focal-plane array (FPA) in the platform consists of thermally tunable thin-film filter membrane pixels. Each thermal pixel acts as a wavelength translator, converting far-IR radiation signals into near-IR signals that can be detected by off-the-shelf CCD or CMOS image cameras (Fig. 1).

The platform uses optical-filter and microelectromechanical-system (MEMS) technologies to build a low-cost passive long-wavelength IR FPA without electrical leads or active cooling. The probe signal source can be a tunable source with the same thermal stability as the FPA filters.

High-quality thermal-imaging capability is available in IR vision systems. But such systems require expensive cryogenic cooling, limiting thermal imaging to niche high-performance applications. Another technique uses the thermal-resistance or the thermal-polarization effect of pyroelectric or vanadium-oxide (VOx) materials. Yet these materials don't lend themselves to low-cost processing methods.

Some other recent thermal-imaging approaches use bi-layer micro cantilevers to reflect light onto CCD and CMOS sensors. However, these methods suffer from background radiation and cantilever fluctuation noise effects.

The lack of a need for a thermoelectric cooler plus the compatibility with a standard CMOS process in RedShift's approach are major drivers for low-cost, mass-market applications. Decoupling of the sensing and readout planes is another one.

"I am confident that we can deliver thermal-imaging modules at a price of less than $1000," says Matthias Wagner, RedShift's CEO and co-founder. That represents about a tenfold reduction in costs, compared with the approximate price of $10,000 for current IR imaging systems.

The thin-film process used by RedShift is a standard process commonly found in flat-panel displays. Fabricating the wavelength conversion circuitry from thermal energy to light is a simple process that results in high yields and low costs. It also allows for fabless manufacturing (Fig. 2).

RedShift's tunable optical filter is a Fabry-Perot structure. Two silicon-nitride/amorphous-silicon (SiNx/a-Si) mirror layers are sandwiched around an a-Si cavity. These provide a high thermo-optic coefficient index of 2.3 × 10­4/K at 300K and allow for a transmission spectra that's tunable at different temperatures from 22°C to 82°C (Fig. 3).

"Our platform's temperature sensitivity is comparable to that of what's available on the market, yet it is much lower in cost," adds Wagner. "Because of this low-cost advantage, we're looking at large-volume applications like video security and automotive active safety applications."

Automotive applications would include pedestrian detection and automatic dimming of oncoming-vehicle high beams. Other potential applications include law enforcement (to detect the absence or presence of individuals within homes and vehicles) and firefighting (to detect the beginning of fires before they go out of control).

RedShift plans to introduce its first mass-market product, a thermal imaging module, early next year. The company already is supplying the product to the telecom industry. The final product will be the size of a handheld security camera.

RedShift demonstrated a 16- by 120-pixel IR FPA packaged prototype with pixel interoperability of more than 99.9% (Fig. 4). With a fill factor of more than 92%, this prototype has a temperature coefficient of 6%/K and average absorption of 42% over the 8- to 15-µm range. Noise-equivalent temperature difference (NETD) values of 0.28K while operating at 22-Hz frame rates were achieved without the need for temperature control.

RedShift Systems Inc.

TAGS: Components
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