The benefits of car night vision systems that enable drivers to see people or animals more clearly on dark, unlit roads have already started appearing in luxury cars. But these systems rely on near-infrared (NIR) radiation, which requires the cars to be fitted with infrared headlights to illuminate the road ahead. Falling into the “thermal imaging region”, Long-wavelength (LWIR) cameras require no such external light source but the sensors require constant cooling, adding to the cost and complexity of such devices. Researchers have now developed a new type of detector which functions at room temperature allowing it to be used in cars and other mobile applications.
Objects at roughly body temperature are luminous in the infrared region at a wavelength of around ten micrometers. Unfortunately infrared cameras for the wavelength range above five micrometers like it cold – the sensor has to be constantly cooled down to about minus 193 degrees Celsius (minus 315 Fahrenheit). Although uncooled imagers for the LWIR range do already exist, they are mainly used by the military. That is set to change with researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems (IMS) in Duisburg having succeeded in producing an imaging sensor for the LWIR range that functions at room temperature.
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At the heart of the IRFPA (Infrared Focal Plane Array) sensor is a microbolometer – a temperature-sensitive detector that absorbs long-wave infrared light. To produce a two-dimensional image, several microbolometers are combined to form an array. If the microbolometer absorbs light from a heat source, its interior temperature rises and its electrical resistance changes. A readout chip then converts this resistance value directly into a digital signal. Previously this was not possible without a further intermediate step – normally the electrical pulse is first translated into an analog signal and then digitized using an analog/digital converter.
“We use a very specific type of converter, a sigma-delta converter, in our imager. This has enabled us to produce a digital signal directly,” explains Dr. Dirk Weiler, a scientist at the IMS.
The researchers say that because the complex and costly cooling mechanisms are no longer required, the potential applications for LWIR technology include areas beyond the automotive sector. Weiler says that mobile devices in particular should benefit from the new development. Not only is weight saved due to the fact that the cooling mechanism is no longer needed, but the battery power available and therefore the operating time of the mobile device increase because no energy is needed for cooling.
One example Weiler proposes for the potential uses of mobile infrared cameras is firefighting, where they could detect hidden hotspots or locate people in smoke-filled buildings.