Dualband infrared focal plane arrays (FPA), developed for multi-spectral imaging applications, have advantages
over conventional multi-FPA sensor configurations in compactness and band-to-band pixel registration. These
FPAs have also enabled hyperspectral applications that employ gratings used in two orders, allowing high efficiency
hyperspectral imaging over very broad wavelength regions. As time progresses, multi-waveband FPAs are
expected to provide an increase in spectral information at the pixel level without the need for external, dispersive
optical elements. A variation on this approach, described here, uses detector material of fixed composition, with
waveband sensitivity achieved as a function of depth, made possible by the spectral dependence of the absorption
coefficient. An increase in the number of wavebands provides hyperspectral capability at the pixel level,
hereafter denoted hyperspectral pixel. This technology may someday become possible through advanced detector
array architectures, with photons of different wavelength continuously absorbed at different depths, and their
resulting photocurrents isolated with a vertical grid of contacts or an equivalent mechanism for transporting
depth-dependent signal photocurrent to a read-out circuit unit cell.
We reported previously on full-disk observations of the sun through a layer of black
polymer, used to protect the entrance aperture of a novel dualband spectrometer while
transmitting discrete wavelength regions in the MWIR & LWIR1. More recently, the
spectrometer was used to assess the accuracy of recovery of unknown blackbody temperatures2.
Here, we briefly describe MWIR observations of the full Moon made in Jan 2008. As was the
case for the solar observations, the Moon was allowed to drift across the spectrometer slit by
Earth's rotation. A detailed sensor calibration performed prior to the observations accounts for
sensor non-uniformities; the spectral images of the Moon therefore include atmospheric
transmission features. Our plans are to repeat the observations at liquid helium temperatures,
thereby allowing both MWIR & LWIR spectral coverage.
A dualband infrared focal plane array is the central component of a compact, low mass, multispectral imaging
spectrometer with perfect spectral registration. The prototype spectrometer design uses a grating blaze chosen to
be efficient over both 3.75-6.05 and 7.5-12.1 μm, although the mercury cadmium telluride focal plane array limits
the bandwidths with cutoff wavelengths near 5.2 and 10.5 μm. The spectrometer has been spectrally calibrated
with flooded blackbody illumination and offset and gain corrections have been performed. The wavelength
resolution is ±0.024 μm in the MWIR and ±0.083 μm in the LWIR, however this limitation is caused by the
calibration method and not by the design. The potential for determining the temperature of a blackbody or
greybody from the ratio of two narrow wavebands has been demonstrated.
A prototype of a compact, low mass, multispectral imaging spectrometer suitable for space-based applications
has been built utilizing a dual-band infrared focal plane array. The spectrometer design uses a grating blaze
chosen to be efficient at both 3.75-6.05 and 7.5-12.1 μm. The spectrometer had previously been spectrally
calibrated with flooded blackbody illumination and the mercury cadmium telluride focal plane array was found
to have cutoff wavelengths near 5.2 and 10.5 μm. The spectrometer was tested in the MWIR band by imaging a
blackbody at a distance of 100 m. Spectral images of the Sun were obtained in both MWIR and LWIR, allowing
comparison of the solar diameter at various IR wavelengths with tabulated visual widths. The performance of
the spectrometer is characterized at a deeper level as a result of these observations.
Dualband infrared focal plane arrays (FPA) were developed originally for multi-spectral imaging
applications, where their advantages in compactness and band-to-band pixel registration, relative to
conventional multispectral imagers, were recognized. As dualband FPA architecture is matured for
quantum well and mercury cadmium telluride focal plane arrays, and becomes within the grasp of
strained layer superlattice technology, applications in addition to multi-waveband imaging come to
mind. In various hyperspectral applications that employ gratings, the different grating orders can
sometimes be paired with the wavebands of the dual- (or multi-) waveband FPA, allowing high
efficiency hyperspectral imaging over very broad wavelength regions. Exploiting the "third dimension" of FPA detecting layers for dual- and multi-waveband capability proved its usefulness for
multi-waveband imaging; this paper will show similar advantages for hyperspectral applications and
describe such applications.
The contribution of a scene projector array to the nonuniformity of a test article's output image has been calculated. In addition to the inherent nonuniformity of the detector array, the output image nonuniformity is dependent upon the nonuniformity of the projector array and the relative positions of emitter images on the detector array as determined by the sampling ratio. In order to calculate the predicted output nonuniformity, a weighting function was developed that accounts for the different contributions of one emitter to different individual detector elements. It is through this weighting function that parameters such as the sampling ratio, the fill factor of the detector array, the optical blur of the emitters, and the alignment of the emitters with respect to the detectors affect the nonuniformity. A computer program has been written to numerically approximate the weighting function for a user- defined set of parameters. For realistic parameters, a significant contribution to the nonuniformity was found to occur when certain non-integer sampling ratios are used. This nonuniformity is based solely on the relative positions of the emitter images on the detector array and will occur even if the projector array is completely uniform.
The inherent non-uniformity of a Wideband Infrared Scene Projector (WISP) necessitates an analytical prediction of the contribution of the scene projector's non-uniformity to a test article's output image non-uniformity. A mathematical model has been developed to calculate this non-uniformity based upon a number of input parameters. The output image non-uniformity is dependent on both the non-uniformity of the scene projector and the test article, as well as a weighting factor that results from the relative contribution of the different emitters to the individual detector elements. It is through this weighting factor that parameters such as the sampling ratio, the optical blur of the emitters on the detector' focal plane array, the fill factor of the detector array, and the alignment of the emitters with respect to the detector elements affect the non-uniformity of the output image. Using this model, a theoretical limit for the maximum output image non-uniformity can be calculated for particular values of the scene projector's non-uniformity and the test article's non- uniformity. Realistic situations likely to be encountered during simulation testing were all found to be below the maximum. In order to make this model a useful tool for the laboratory environment, a computer program has been written that calculates the output image non-uniformity based on a given set of input parameters and a numerical approximation of the weighting factor.