INFRARED IMAGING: Focal-plane array's hot new electronics open up 8 to 14 μm region

A way to image in a range of mid-infrared (IR) wavelengths without cryogenic cooling may become widely available thanks to efforts by scientists from the Fraunhofer Institute of Microelectronic Circuits and Systems (IMS; Duisburg, Germany). The technique concerns the long-wave IR 8 to 14 μm wavelength range–a region that normally calls for thermoelectric or liquid cryogenic cooling.

Now, Dirk Weiler of the IMS and his colleagues have shown off a completely uncooled 640 × 480-pixel IR focal-plane array (IRFPA) for the 8 to 14 μm region.1 This wavelength regime is of specific interest because it encompasses the wavelengths corresponding to the body temperature of warm-blooded animals and, crucially, humans. As a result, significant technology in both cooled and uncooled detectors exists in the military sphere in the form of night vision.

But a more prosaic–one might even say "pedestrian"–application is the detection of animals and people from, say, a moving car. Another might be in firefighting, where an appropriate IR detector might help locate people in smoke-filled buildings.

"Fraunhofer IMS has over 25 years of design experience in low-noise and low-power CMOS circuits, in particular for sensor readouts," says Weiler. "In 2006, we started the development of IR sensors with a focus on microbolometers for automotive applications." Since the early 1990s, the IMS has been developing CMOS-compatible surface micromachining, such as for capacitive pressure sensors, says Weiler. The team's IRFPA comes from this combination of electronics expertise with precision machining.

At the heart of the device is the VGA-resolution array of microbolometers, with a 25 μm pitch. These take advantage of tiny changes in resistance that the array elements exhibit when exposed to mid-IR light, turning the changes through a number of steps into a digital signal.

Click to Enlarge
An uncooled focal-plane array captures IR images in the 8 to 14 μm wavelength region. (Courtesy of Fraunhofer IMS)

Resistance directly converted to digital

Normally these changes compete with IR radiation emitted by any nearby objects–hence the cryogenics. But the IMS team's array obviates the need for such cooling, largely because of a clever electronic trick in the form of a "sigma-delta" converter, which changes the way they gather a digital signal from each array element.

The conventional method is to first create an analog signal and then convert this into a digital signal using an analog-to-digital converter (ADC), says Weiler. However, the passing of the signal through the ADC adds unnecessary noise.

"The new approach is that we use a sigma-delta ADC to convert directly the resistance change of a bolometer into a digital signal," he notes. "Due to the direct conversion, we have the advantage of a better signal-to-noise ratio." Weiler says that the team has developed a complete detector device, comprising the microbolometer array, the readout circuit, and the vacuum package. The complete devices in chip-scaled packages are made on 8 in. CMOS wafers completely in-house at IMS.

Although one of the more prominent applications would be in mobile systems such as automobiles, the group is not working to integrate their detector in a full camera assembly. Instead, says Weiler, a new spin-out at the IMS will handle development and fabrication of the readout electronics, microbolometers, and vacuum package. His team, meanwhile, will work on ensuring the vacuum stability of the package as it is further refined, and improving the overall noise floor of the array.–Jason Palmer

1. D. Weiler et al., Proc. SPIE, 7660; DOI: 10.1117/12.849839.

More Laser Focus World Current Issue Articles
More Laser Focus World Archives Issue Articles

Most Popular Articles


Fracking, climate change, and lasers:  new tools to reduce fugitive methane emissions

This webcast, sponsored by Hamamatsu Corporation, covers recent developments and field deployments of compact quantum-cascade-laser (QCL)-based methane senso...

Opportunities in the Mid-IR

The technology for exploiting the mid-IR is developing rapidly:  it includes quantum-cascade lasers and other sources, spectroscopic instruments of many...

Fiber Optic Sensors – Fundamentals, Principles and Applications

In this webcast, sponsored by Nufern, we focus on optical fiber sensing technology.  Fundamental concepts will be presented first, followed by the under...

Infinite Possibilities – Easily Combining Scanner and Servo Motion

High precision motion control applications such as laser micromachining, 2-photon polymerization, glass panel and film patterning, and additive manufacturing...
Technical Digests

HIGH-POWER FIBER LASERS: Working in the kilowatt regime

High-power materials-processing fiber lasers are available in an increasing variety of forms, as ...
Sponsored by

Click here to have your products listed in the Laser Focus World Buyers Guide.


Phantom v1610

Phantom v1610 high-speed digital camera can shoot 1 million FPS.

Phantom v711

Phantom v711 high-speed digital camera

Evolve 128 EMCCD Camera

Quantitative high performance with extreme sensitivity for low-light applications.


Surface Optics Corp

Designs and manufactures hyperspectral and multispectral imagers operating from the ult...

Optics Balzers AG

Possesses comprehensive know-how in optical thin-film coatings and components, glass pr...

Cremat Inc

Manufactures and supplies charge-sensitive preamplifiers for use in nuclear and x-ray d...

Social Activity

Copyright © 2007-2014. PennWell Corporation, Tulsa, OK. All Rights Reserved.PRIVACY POLICY | TERMS AND CONDITIONS