Two-color thermal detector with thermal chopping for infrared focal-plane arrays.
Micromachined thermal infrared (IR) detectors are emerging into the marketplace to provide high-performance thermal (IR) imagery at low cost. Thermal detectors can be improved when a tunable wavelength response is provided and when a thermal chopper is incorporated into the detector by use of microelectromechanical (MEM) elements. Most thermal detectors require a chopper, continuous synchronous chopping in the case of pyroelectric detectors, or asynchronous chopping in the case of staring microbolometers. Mechanical choppers are bulky and costly. We present the fundamental principles of micromachined thermal detectors that possess tunable wavelength or color response and a technique for thermal chopping. A micromirror, switching between two spatial positions under the detector, provides a response to two wavelength windows by tuning the optical resonant cavity. The image can then be integrated at the readout level to achieve a multicolor IR picture. A thermal MEM chopper can be used instead of a mechanical chopper to maintain the same video frame rate and to allow for an interlaced resetting of staring thermal arrays. Unlike the second generation of uncooled IR arrays, the actual temperature of objects can be obtained by a comparison of the response in two wavelength windows, in addition to the direct measurement of IR power that they radiate in the entire 8-14-microm spectral region.