The broad spectral response of these cameras makes them popular for uses as diverse as the military and the art world.
Art Stout, Electrophysics Corp.
Infrared cameras based on mercury-cadmium-telluride (MCT) focal plane arrays are finding their way into a growing number of imaging applications. Typical of these applications is the benefit of new levels of sensitivity in certain IR spectral regions not previously available with other types of IR detectors.
These new applications result from the continuing development of these staring arrays and the improvement in performance in the spectral range from 700 nm to above 11 μm. The improvements include increases in quantum efficiency and modulation transfer function, and decreases in thermal crosstalk and response time, while maintaining reasonable detector operating temperatures.
A big advantage in using an MCT detector as an IR sensor is its ability to “tune” the detector’s spectral response across a wide band. A detector fabricator with a single material can create devices that cover the IR spectrum from 1 to 12 μm. This spectral tuning is accomplished by adjusting the ratio of HgTe to CdTe in the starting melt during the crystal growth.
An important parameter for wavelength tuning is the detector’s operating temperature. As the temperature decreases, the cutoff wavelength gets longer. This effect is most pronounced for long-wavelength IR-doped devices and is negligible with short-wavelength IR focal plane arrays. For example, an MCT detector processed for an 11.2-μm cutoff at 77 K will have a cutoff of 12.0 μm at an operating temperature of 52 K.
MCT detectors exhibit low dark current characteristics at high operating temperatures and are linear with operating temperature (Figure 1). Our current 2.5-μm cutoff focal-plane array operates at 175 K and has a dark current value of 0.35 pA. In addition, the diode shunt resistance has been measured and typically exhibits values higher than 100 G½. This is desirable because it limits current leakage, thus enhancing the detector sensitivity.
Figure 1. The simulated cutoff equals f (focal plane array temperature) for a 30-μm2 photovoltaic diode processed at 11.2 μm.
MCT detectors also exhibit a very high modulation transfer function performance. Photons arriving at the MCT photodiode are converted to charge, which is swept into the contacts as current. The diffusion length (or how much the charge diffuses before collection) is smaller than other photodiode materials such as InSb, improving modulation transfer function and reducing thermal crosstalk.
Beyond traditional applications for IR focal plane arrays such as thermal imaging, there is growing interest in hyperspectral and spectroscopic applications. Hyperspectral cameras provide images of a target in a high number of spectral bins — typically more than 100 — with spectral resolution between 10 and 15 nm. The short-wavelength IR MCT technology is ideal for these types of applications because of its response out to 2.5 μm near the atmospheric absorption limit, where there are a significant number of water absorption wavelengths, low minimum in-put radiance and extremely high linearity (~0.5 percent over the full dynamic range). Applications include gas detection and analysis, target signature collection and agricultural monitoring.
Imaging spectroscopy provides simultaneous chemical and spatial information of samples in near real time. It is transitioning from space-based deployment to more down-to-earth applications, including pharmaceutical formulation development, plastics recycling and medical imaging. Replacing a single-point Fourier transform IR detector with a focal plane array detector creates an FTIR imaging system that produces a signal for an entire sample in one data collection, removing the necessity of moving the sample. Besides shorter collection times, FTIR imaging’s advantages over conventional FTIR mapping include a higher resolution down to diffraction limits.
From art to defense
Art restoration experts are using infrared cameras with a spectral response out to 2.5 μm to examine paintings for artifacts under the pigment, such as original line drawings made with charcoal pencil, mined black chalk and red chalk (Figure 2). IR reflectance spectroscopy has been widely employed for many years using CCD and near-IR vidicon cameras, both of which require significant energy input — a feature that may not be desirable in many delicate artwork examinations. Current MCT-based cameras require significantly less illumination, which also enables the use of very narrow spectral bandpass filters.
Figure 2. The image above of a section of Baptism of Jesus Christ, painted by Jan Pollack, shows the investigative benefit of extended spectral responsivity. On the sleeve of John the Baptist (whitish area), parallel hash marks can be seen beneath the IR-transparent paint layer. This technique is sometimes used by artists to give a shading effect. Courtesy of Los Angeles County Museum of Art.
Spectral signature data collection of military systems is enabled by the inherently high quantum efficiency of MCT and its broad spectral response. This is particularly true when making measurements in the long-wave spectrum. Today only MCT and quantum-well IR photodetector focal plane arrays are available for collecting spectral information in the long-wavelength IR (~10 μm) band. The latter have only a 1-μm bandpass and a typical quantum efficiency of less than 10 percent.
MCT detectors can be fabricated to cover the spectrum from 1 to 12 μm as well as to have quantum efficiency of greater than 80 percent. This enables narrow spectral data collection and requires much shorter integration times to image thermally dynamic events and fast-moving targets.
Advanced fabrication techniques have created high-performance two-dimensional IR focal plane arrays for both imaging and nonimaging applications.
Meet the author
Art Stout is vice president of business development at Electrophysics Corp. in Fairfield, N.J.