The development and demonstration of a new snapshot hyperspectral sensor is described. The system is a significant
extension of the four dimensional imaging spectrometer (4DIS) concept, which resolves all four dimensions of
hyperspectral imaging data (2D spatial, spectral, and temporal) in real-time. The new sensor, dubbed "4×4DIS" uses a
single fiber optic reformatter that feeds into four separate, miniature visible to near-infrared (VNIR) imaging
spectrometers, providing significantly better spatial resolution than previous systems. Full data cubes are captured in
each frame period without scanning, i.e., "HyperVideo". The current system operates up to 30 Hz (i.e., 30 cubes/s), has
300 spectral bands from 400 to 1100 nm (~2.4 nm resolution), and a spatial resolution of 44×40 pixels. An additional
1.4 Megapixel video camera provides scene context and effectively sharpens the spatial resolution of the hyperspectral
data. Essentially, the 4×4DIS provides a 2D spatially resolved grid of 44×40 = 1760 separate spectral measurements
every 33 ms, which is overlaid on the detailed spatial information provided by the context camera. The system can use a
wide range of off-the-shelf lenses and can either be operated so that the fields of view match, or in a "spectral fovea"
mode, in which the 4×4DIS system uses narrow field of view optics, and is cued by a wider field of view context
camera. Unlike other hyperspectral snapshot schemes, which require intensive computations to deconvolve the data
(e.g., Computed Tomographic Imaging Spectrometer), the 4×4DIS requires only a linear remapping, enabling real-time
display and analysis. The system concept has a range of applications including biomedical imaging, missile defense,
infrared counter measure (IRCM) threat characterization, and ground based remote sensing.
The incentive for the 4D-IS concept was driven by the need to adequately resolve all four dimensions of data (2D spatial, spectral, and temporal) with a single, radiometrically calibrated sensor. Very fast changing phenomena are of interest; including missile exhaust plumes, missile intercept events, and lightning strikes, hypervelocity impacts, etc. Present sensor capabilities are limited to imaging sensors (producing spatial image), spectrometers (that produce a mean signature over an entire field of view with no spatial resolution), radiometers (producing in-band radiance over an entire FOV), or imaging spectrometers (or hyperspectral sensors, tunable filter type, pushbroom scanning, imaging Fourier Transform, Fabry-Perot, or CTHIS type) that produce a data cube containing spatial/spectral information but suffer from the fact that the cube acquisition process may take longer time than the temporal scale during which the event changes. The Computer Tomography Imaging Spectrometer (CTIS) is another sensor capable of 4D data collection. However, the inversion process for CTIS is computationally extensive and data processing time may be an issue in real-time applications. Hence, the 4D-IS concept with its ability to capture a full image cube at a single exposure and provide real time data processing offers a new and enhanced capability over present sensors.
The 4D-IS uses a reformatter fiber optics to map a 2D image to a linear array that serves as an input slit to an imaging spectrometer. The paper describes three such instruments, a VNIR, a MWIR, and a dual band MW/LWIR. The paper describes the sensors' architecture, mapping, calibration procedures, and remapping the FPA plane into an image cube. Real-time remapping software is used to aid the operator in alignment of the sensor is described. Sample data are shown for rocket motor firings and other events.