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The Incredible Shrinking IR Camera

Photonics Spectra
Apr 2002
Bill Meyer

Design efficiency is the reason that today's miniature cameras can provide viewing performance previously possible only with their larger counterparts.
Many interesting and important IR imaging applications remain limited by the practical and economic barriers of camera technology; namely, camera size, weight and power consumption. A typical example is in fire fighting, where ideally every firefighter would have a lightweight, long-life, smoke-penetrating IR vision system integrated into his or her helmet. Instead, many still share one handheld unit per team.

Compounding the problem is the fact that many applications are demanding in terms of performance. As a result, camera miniaturization and power reduction must be accomplished without compromising critical performance parameters. Systems must also be flexible or readily customizable to the needs of specific applications — even OEM applications that require cameras designed for low-cost volume manufacturing. The key to success lies in camera design.

Rethinking design

At Indigo Systems Inc. in Santa Barbara, Calif., for example, the development team for the Omega miniature IR camera began with aggressive design goals: to create a small, lightweight, low-power IR camera without compromising performance or features, and to create a technology that is economically viable for high-volume, low-cost applications. Achievement of these goals required a ground-up rethinking of virtually every component and algorithm.

The most critical IR camera component is the detection mechanism. Because the company designs and fabricates all of its mixed-signal readout integrated circuits and detector technologies (that together form the focal plane array), the design team was free to select the array material, size and configuration. The final choice was an optimized 164 x 129-pixel vanadium-oxide microbolometer array.

Although active cooling is not a requirement for operation, most microbolometer-based commercial cameras use a thermoelectric cooler to stabilize the temperature of the array and thereby minimize image nonuniformities and artifacts. However, the large power drain of the cooler at the edges of the temperature range effectively limits the camera’s operating temperature. The alternative was an approach that electronically compensates for temperature changes on the array. This dramatically lowers camera power consumption to 1.2 W; allows the camera to operate over a wide range of ambient temperatures, from 240 to +55 °C; and eliminates the slow turn-on time often associated with IR cameras with active cooling.

Another important consideration in the design was the microbolometer material, with options being vanadium oxide or alpha (amorphous) silicon. Although vanadium oxide offers significantly higher performance, some will argue that alpha silicon arrays are easier and less costly to manufacture in volume because the material could be transitioned to a standard silicon fabrication process. Reality, though, has shown that there is very little difference in transferability of the two processes to standard silicon foundries.

Ultimately, vanadium oxide translates directly into better image quality. The key figure of merit for camera sensitivity is its noise-equivalent delta temperature, the temperature difference that gives a signal equal to the noise floor of the camera. So far, the best-advertised delta temperature parameter for a commercial camera based on alpha silicon is 100 mK, when using an f/1.0 lens. By rethinking system design and material selection, engineers reduced the specification to 85 mK with an f/1.6 lens. When normalized to f/1.0, the resulting system can deliver 35-mK performance. End users can choose between higher-performance (f/1.0) optics and smaller, lower-cost optics (f/1.6) that provide a better depth of field.

Figure 1.
Combined with pixel interpolation, a 160 x 120-pixel array can provide more-than-sufficient spatial resolution for many thermal imaging applications.

Another important issue was the optimization of the array size. Although it is true that more pixels (larger arrays) provide more spatial detail and improve image quality, they also significantly affect system size, weight, power and cost. Thus, the camera designers selected the 160 x 120-pixel array format for this product — with focus on back-end processing and pixel interpolation — to generate sharp, high-quality RS-170 (or PAL) imagery (Figure 1). A smaller array size also translates to smaller data rates and frame sizes for applications that use the camera’s real-time 14-bit digital output.

Another internal aspect is video input. While applications such as industrial process monitoring and research and development benefit from having high-dynamic-range digital output, applications such as fire fighting and surveillance typically require video output. An analog video signal, though, is roughly equivalent to an 8-bit digital signal, and the conversion from digital allows the use of something the camera designers call “smart” image optimization. Rather than use a standard linear conversion to compress the 14 bits into 8-bit video, the circuitry analyzes the image intensity in real time, frame by frame, and performs the conversion nonlinearly using an algorithm optimized for each frame. This maximizes the contrast in darker (colder) parts of the frame, while avoiding washout of brighter (hotter) objects in the image frame (Figure 2).

Figure 2. By comparing the clarity of these images of a hot car engine, it is easy to see the efficacy of 14-bit to analog video data compression (right) as compared with linear conversion (left).

Thinking modular

Because many secondary features are often incorporated into an IR camera to optimize its use in diverse applications, the camera designers developed a modular approach to adding features. Their goals were to minimize camera size and cost by enabling end users to select only the options needed for their applications.

One example of this approach is a battery module that provides portability with two-hour battery life. Another is a display option with a monochrome liquid crystal display viewfinder that allows direct viewing of thermal images without using frame grabbers. For research and development applications where it is advantageous to stream the digital image data into a high-performance frame grabber, the option is available to add serial-to-parallel conversion for direct compatibility with industry-standard frame grabbers.

When the miniature camera is integrated into a fully functional IR imaging system, all camera settings are controlled through its RS-232 interface. Although these settings are nonvolatile to maximize portability, both portability and real-time access to camera settings may be necessary. This can be accomplished using a cable connector and a handheld palm-top computer. The software in this module allows the palm-top screen to become a virtual control panel for the camera.

If necessary, the camera will even operate in a sealed environmental enclosure to protect it from chemical vapors and dust.

Understanding applications

Potential applications for such thermal imaging cameras include detecting heat sources, sensing in environments with little ambient light, and viewing through smoke and other visually opaque materials.

This last requirement is especially critical for both fire-fighting and automotive applications. In a building fire, smoke will rapidly reduce visibility to a few inches because of light scattering. Long-wavelength IR is not scattered to this high degree and passes easily through thick smoke, enabling a firefighter to navigate and to find victims. In the automotive world, long-wavelength IR cameras can extend night vision beyond the headlights by projecting the resulting IR image onto the windshield or onto a head-up display unit.

In law enforcement applications, system users could even detect the presence and location of people within a dark or obscured scene, such as undergrowth, by the heat they generate (see sidebar). Other notable applications that would benefit from miniature IR cameras include search-and-rescue and unmanned reconnaissance vehicles where low weight and minimum power consumption are major assets.

The key to shrinking the IR camera is to provide versatility without adding cost or reducing performance. From machine vision and process monitoring to emerging applications in a host of industries, the continuing maturation of IR imaging technology will only expand users’ detection capabilities.

Meet the author

Bill Meyer is a systems engineer with Indigo Systems in Santa Barbara, Calif.

Hot-Shoe Connector Creates an Instant Video System

A complete portable IR imaging system requires not only the camera, but also the integration of several elements. The most important of these components are devices for image display and storage. To satisfy the needs of users who are interested only in easy access with a low-cost turnkey IR system, miniature cameras can work in sync with off-the-shelf personal video cameras.

The concept is simple. Personal video cameras contain complete image storage and display functions, as well as image manipulation and output capabilities. Moreover, volume manufacturing has lowered the cost of even high-performance cameras to only a few hundred dollars. The key to ready access is something called a “hot shoe” interface. A hot shoe is an active physical adaptor on top of most commercial camcorders that provides a solid clip-on physical mounting platform for accessories, and, in some cases, electrical power and video interconnects.

A key example of this is the Intelligent Accessory Shoe found on many Sony camera products. Engineers at Indigo Systems Inc. designed an optional module that allows rigid mounting of the firm’s miniature camera to a video camera without a single screw or bolt. More importantly, the video output from the IR camera is fed seamlessly into the video camera through the auxiliary input.

With this simple lightweight combination, an end user can switch instantly between visible and IR imaging, and can record IR imagery, visible imagery and voice on standard analog or digital videotape. This performance combination is expected to be particularly attractive for many law enforcement and surveillance applications.

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