Measuring the modulation transfer function (MTF) of digital imagers focused at or near infinity in laboratory or field settings presents difficulties because the optical path is longer than a typical laboratory. Also, digital imagers can be hindered by low-resolution detectors, resulting in the resolution of the optics surpassing that of the detector. We measure the MTF for a short-wave infrared hyperspectral imager developed by Resonon, Inc., of Bozeman, Montana, which exhibits both characteristics. These difficulties are overcome with a technique that uses images of building rooflines in an oversampled, tilted knife-edge-based MTF measurement. The dark rooftops backlit by a uniformly cloudy sky provide the high-contrast edges required to perform knife-edge MTF measurements. The MTF response is measured at five wavelengths across the imager's spectral band: 1085, 1178, 1292, 1548, and 1629 nm. The MTF also is observed at various distances from the roof to investigate performance change with distance. Optimum imaging is observed at a distance of 150 m, potentially a result of imperfect infinity focus and atmospheric turbulence. In a laboratory validation of the MTF algorithm using a monochrome visible imager, the roofline MTF results are similar to results from point-source and sine-card MTF measurements.
We describe our use of Digital Micromirror Devices (DMDs) for the performance testing, characterization, calibration,
and system-level data product validation of multispectral and hyperspectral imaging sensors. We have developed a
visible Hyperspectral Image Projector (HIP), which is capable of projecting any combination of many different
arbitrarily programmable basis spectra into each image pixel at up to video frame rates. For the full HIP, we use a
scheme whereby one DMD array is used in a spectrally programmable source, to produce light having the spectra of
materials in the scene (i.e. grass, ocean, target, etc), and a second DMD, optically in series with the first, reflects any
combination of these programmable spectra into the pixels of a 1024 ×768 element spatial image, thereby producing
temporally-integrated 2D images having spectrally-mixed pixels. The HIP goes beyond conventional Digital Light
Processing (DLP) projectors in that each spatial pixel can have an arbitrary spectrum, not just an arbitrary color. As
such, the resulting spectral and spatial content of the projected image can simulate realistic scenes that a sensor system
must acquire during its use, and can be calibrated using NIST reference instruments. Here we discuss our current HIP
developments that span the visible/infrared spectral range of 380 nm through 5400 nm, with particular emphasis on
DMD diffraction efficiency measurements in the infrared part of this range.
Deployment of compact hyperspectral imaging sensors on small UAVs has the potential of providing a cost-effective
solution for rapid-response target detection and cueing based on time critical spectral information collected at low
altitudes. To address this goal, a new compact hyperspectral imaging sensor is being developed with an anamorphic
optical system that partially decouples image formation along both the spatial and spectral axes found in conventional
push-broom hyperspectral imagers. This design concept benefits from a reduction in complexity over standard highperformance
spectrometer optical designs while maintaining excellent aberration control and spatial and spectral
distortion characteristics. The anamorphic optical system has the advantage of removing the spectrometer slit focus
along the spatial axis and in turn eliminates nearly all aberrations in the front-end optics, regardless of field angle or
aperture size. This paper presents results from the first prototype anamorphic imaging spectrometer, which weighs 4
pounds and is designed for operation in the Short Wave InfraRed (SWIR) spectral band over a wavelength range of 1
μm to 1.7 μm dictated by the uncooled InGaAs focal plane array used as the detector. The anamorphic system design will be discussed and results from characterization and field measurements will be presented.
Measuring the Modulation Transfer Function (MTF) of a hyperspectral imaging spectrometer focused at infinity requires
a longer optical path than is available in a typical laboratory. We describe a technique that uses images of rooflines on
buildings of opportunity in a knife-edge-based MTF measurement. This technique only measures the MTF along one
dimension. However, the hyper-spectral imaging systems characterized in this paper are particularly suited to a knife-edge
technique, as imaging only takes place in one dimension of the array and spectral separation takes place along the
other. The sharp edges needed in these measurements were provided by dark rooftops backlit by a uniformly cloudy sky.
We have applied this technique to hyperspectral imagers that operate in the visible-near infrared (VNIR) and short-wave
infraRed (SWIR) spectral bands. The data presented in this paper focuses on the characterization of the SWIR imaging
spectrometer developed by Resonon Inc.