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"The broad response of this array, particularly in the far infrared - 8 to12 micrometers - is crucial for infrared spectroscopy," said Jhabvala. The advance was possible because quantum wells can be designed to detect light with different energy levels by varying the composition and thickness of the detector material layers. The new version can see infrared between 8 to 12 micrometers. NASA's original QWIP array could detect infrared light with a wavelength between 8.4 and 9.0 micrometers. Credit: NASA Print-resolution image (200 K jpg image) Also note the different hand temperatures of the people, some warm some cold. Note the hand visible inside the labcoat pocket and the very warm tongue of Denver, curled in a yawn. This image illustrates the slight difference in temperatures of the scene – dark red being coldest and orange the warmest. Image left: Another false color image with the QWIP camera of engineers and a seeing-eye dog (named Denver). A computer uses this information to create an image of the infrared source. If light with the correct energy hits one of the quantum wells in the array, the freed electron flows through a separate chip above the array, called the silicon readout, where it is recorded. Quantum wells employ the bizarre physics of the microscopic world, called quantum mechanics, to trap electrons, the fundamental particles that carry electric current, so that only light with a specific energy can release them. Each layer is extremely thin, ranging from 10 to 700 atoms thick, and the layers are designed to act as quantum wells. NASA's QWIP detector is a Gallium Arsenide (GaAs) semiconductor chip with over 100 layers of detector material on top.
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The more pixels that can be placed on a detector of a given size, the greater the resolution, and NASA's QWIP arrays are a significant advance over earlier 300,000-pixel QWIP arrays, previously the largest available. They are similar in principle to the detectors that convert visible light in a digital camera. A conventional infrared detector has a number of cells (pixels) that interact with an incoming particle of infrared light (an infrared photon) and convert it to an electric current that can be measured and recorded. Infrared light is invisible to the human eye, but some types are generated by and perceived as heat. Murzy Jhabvala of NASA's Goddard Space Flight Center, Greenbelt, Md., Principal Investigator for the project. "The ability to see a range of infrared wavelengths is an important advance that will greatly increase the potential uses of the QWIP technology," said Dr. Notice the thermal handprint left on her lab coat as she removes her hand from her pocket. Warmer temperatures are orange, cooler temperatures are dark red. Image right: This is a false color image of a Goddard engineer in the far infrared (8-12 micrometer IR spectral band) taken with the 1 megapixel GaAs QWIP camera. The new QWIP array is the same size but can now sense infrared over a broad range. However, at the time it could only detect a narrow range of infrared colors, equivalent to making a conventional photograph in just black and white. It was a low-cost alternative to conventional infrared detector technology for a wide range of scientific and commercial applications. The detector, called a Quantum Well Infrared Photodetector (QWIP) array, was the world's largest (one million-pixel) infrared array when the project was announced in March 2003. An inexpensive detector developed by a NASA-led team can now see invisible infrared light in a range of "colors," or wavelengths.