Clayton C. LaBaw

Multispectral imaging systems using acousto-optic tunable filter

This paper discusses recent activities of the Jet Propulsion Laboratory (JPL) in the development of a new type of remote sensing multispectral imaging instrument using acousto- optic tunable filter (AOTF) as a programmable bandpass filter. This remote sensor filter provides real-time operation; observational flexibility; measurements of spectral, spatial, and polarization information using a single instrument; and compact, solid state structure without moving parts. An AOTF multispectral imaging prototype system for outdoor field experiments was designed and assembled. Some preliminary experimental results are reported. The field system is used to investigate spectral and polarization signatures of natural and man-made objects for evaluation of the technological feasibility for remote sensing applications. In addition, an airborne prototype instrument is currently under development.

Optical design of an imaging spectrometer utilizing an acousto-optic tunable filter as a disperser

Designing an imaging spectrometer using an AOTF can be a difficult task since there is no software that can simulate the bulk diffraction that takes place in the AOTF material. In this paper we will describe the method used to simulate the effects of the AOTF using a refractive grating halfway along the crystal. Apart from the zero order we also collect the +1 and -1 orthogonally polarized orders produced by the AOTF, gaining additional information that may be used in certain applications. The +1 and -1 order are imaged into the same focal plane, eliminating the need of a separate focal plane for each order and resulting in considerable savings in cost and mass. The whole system is achromatized from 1.2 - 2.4 microns using only two types of glass, one of which is BK7. The system has been designed, built, and tested.

Thermal Infrared Imaging Spectrometer - An advanced optics technology instrument

Through the use of a special optical filter, the Thermal Infrared Imaging Spectrometer, an airborne multispectral IR imaging instrument operating in the thermal emission region (7.5-14 microns), will achieve signal-to-noise ratios greater than 600 with ambient temperature optics. This instrument will be used to do compositional surface mapping of the terrain, and will refine the ability to categorize rock families and types by providing much higher spectral resolution in the emission region than was previously available. Details of the optical system, the detector, the cooler system, and the support electronics are described.

Thermal Infrared Imaging Spectrometer (TIRIS) status report

The TIRIS is a pushbroom long wave infrared imaging spectrometer designed to operate in the 7.5 to 14.0 micrometer spectral region from an airborne platform, using uncooled optics. The focal plane array is a 64 by 20 extrinsic Si:As detector operating at 10 K, providing 64 spectral bands with 0.1 micrometer spectral resolution, and 20 spatial pixels with 3.6 milliradians spatial resolution. A custom linear variable filter mounted over the focal plane acts to suppress near field radiation from the uncooled external optics. This dual- use sensor is developed to demonstrate the detection of plumes of toxic gases and pollutants in a downlooking mode.

Design of multibandwidth frequency selective surfaces for near-infrared filtering

This paper describes the design of infrared filters using methods drawn from microwave and millimeter wave filters. Special note is made of approximations made in the infrared design, and ways to improve upon these approximations. Results from the design, manufacture and test of linear wedge filters built using microlithographic techniques and used in spectral imaging applications will be presented.

Performance and application of real-time hyperspectral imaging

Hyperspectral imaging is the latest advent in imaging technology, providing the potential to extract information about the objects in a scene that is unavailable to panchromatic imagers. This increased utility, however, comes at the cost of tremendously increased data. The ultimate utility of hyperspectral imagery is in the information that can be gleaned from the spectral dimension, rather than in the hyperspectral imagery itself. To have the broadest range of applications, extraction of this information must occur in real-time. Attempting to produce and exploit complete cubes of hyperspectral imagery at video rates, however, present unique problems for both the imager and the processor, since data rates are scaled by the number of spectral planes in the cube. MIDIS, the Multi-band Identification and Discrimination Imaging Spectroradiometer, allows both real-time here are the major design innovations associated with producing high-speed, high-sensitivity hyperspectral imagers operating in the SWIR and LWIR, and of the electronics capable of handling data rates up to 160 megapixels per second, continuously. Discussion of real-time algorithms capable of exploiting the spectral dimension of the imagery is also included. Beyond design and performance issues associated with producing and processing hyperspectral imagery at such high speeds, this paper also discusses applications of real-time hyperspectral imaging technology. Example imagery includes such problems as detecting counterfeit money, inspecting surfaces, and countering CCD.

Lunar Scout Infrared Detector (LSIRD): simple low-cost imaging spectrometer

A novel design for a compact, light weight, imaging spectrometer has been proposed for an orbiting Lunar mapping mission. Simple in design, its dual arm optical system employs a transmission grating and a dichroic mirror to provide continuous two-octave spectral response. The gratings first order wavelengths are reflected into the SWIR arm, while the second order wavelengths are transmitted to the VNIR arm. The instrument design is that of a push broom camera. It uses one of the detector(s) dimensions for spectral selection, the other detector(s) dimension for cross-track spatial selection, and the forward motion of the platform (in this case, a spacecraft) for down-track spatial coverage.

Development and operation of a material identification and discrimination imaging spectroradiometer

Many imaging applications require quantitative determination of a scenes spectral radiance. This paper describes a new system capable of real-time spectroradiometric imagery. Operating at a full-spectrum update rate of 30Hz, this imager is capable of collecting a 30 point spectrum from each of three imaging heads: the first operates from 400 nx m to 950 nm, with a 2% bandwidth; the second operates from 1.5 micrometers to 5.5 micrometers with a 1.5% bandwidth; the third operates from 5 micrometers to 12 micrometers , also at a 1.5% bandwidth. Standard image format is 256 X 256, with 512 X 512 possible in the VIS/NIR head. Spectra of up to 256 points are available at proportionately lower frame rates. In order to make such a tremendous amount of data more manageable, internal processing electronics perform four important operations on the spectral imagery data in real-time. First, all data in the spatial/spectral cube of data is spectro-radiometrically calibrated as it is collected. Second, to allow the imager to simulate sensors with arbitrary spectral response, any set of three spectral response functions may be loaded into the imager including delta functions to allow single wavelength viewing; the instrument then evaluates the integral of the product of the scene spectral radiances and the response function. Third, more powerful exploitation of the gathered spectral radiances can be effected by application of various spectral-matched filtering algorithms to identify pixels whose relative spectral radiance distribution matches a sought- after spectral radiance distribution, allowing materials-based identification and discrimination. Fourth, the instrument allows determination of spectral reflectance, surface temperature, and spectral emissivity, also in real-time. The spectral imaging technique used in the instrument allows tailoring of the frame rate and/or the spectral bandwidth to suit the scene radiance levels, i.e., frame rate can be reduced, or bandwidth increased to improve SNR when viewing low radiance scenes.

In-flight calibration verification of spaceborne remote sensing instruments

The need to verify the pei1ormaixc of untended instrumentation has been recognized since scientists began sending thnse instrumems into hostile environments to quire data. The sea floor and the stratosphere have been explored, and the quality and cury of the data obtained vified by calibrating the instrumentalion in the laboratoiy, both jxior and subsequent to deployment The inability to make the lau measurements on deep-space missions make the calibration vthficatkin of these insiruments a uniclue problem.