MWIR infrared gating imaging with uncooled PbSe FPAs for surveillance applications

Uncooled focal plane arrays have revolutionized the market of infrared technology in the last years, reducing the size, weight, power and cost of thermal detectors, and make them accessible to mass markets. This revolution has been lead by micro-bolometers, considered a mature technology for the long wavelength infrared range. However, there is a lack of uncooled detectors in the mid-wavelength infrared (2-5μm) range, one of the most interesting bands for gas identification and temperature measurements of hot objects. Here, we describe the development of a novel gating imaging system for the mid-wavelength infrared. The system is based on unique low cost uncooled PbSe focal plane arrays with a peak response around 3.6 um. The PbSe detectors present some challenges, such as low sensitivity (which limits their application for imaging objects at temperature above 200°C and low signal-to-noise ratio (mainly due to thermal drifts and electronic noise). On the other hand, the photonic detection technology enables high speed acquisition up to 10000 frames per second depending on the resolution. The proposed system combines a modulated aperture, turning the incident light on and off periodically, and a digital lock-in to demodulate the incoming signal and avoid noise in the unwanted region. Contrary to active lighting approaches, the whole scene is modulated at a high frequency and the signal processing is tailored to reduce the internal noise introduced by the detector, while keeping enough bandwidth to achieve video at 30 frames per second. Two imaging prototypes were built: a camera array with six detectors of resolution 32x32, and a camera array with three detectors with increased resolution (128x128) and sensitivity. The proposed technique significantly improves the SNR and reduces the thermal drifts of the PbSe detectors, enabling the imaging of objects at temperatures below 100°C. The camera array was tested in the field during a firefighting training, showing enhanced capabilities to see through fumes, detect fire hotspots and measure temperature robustly.