Mark Dombrowski

Handheld directional reflectometer: an angular imaging device to measure BRDF and HDR in real time

Many applications require quantitative measurements of surface light scattering, including quality control on production lines, inspection of painted surfaces, inspection of field repairs, etc. Instruments for measuring surface scattering typically fall into two main categories, namely bidirectional reflectometers, which measure the angular distribution of scattering, and hemispherical directional reflectometers, which measure the total scattering into the hemisphere above the surface. Measurement of the bi-directional reflectance distribution function (BRDF) gives the greatest insight into how light is scattered from a surface. Measurements of BRDF, however, are typically very lengthy measurements taken by moving a source and detector to map the scattering. Since BRDF has four angular degrees of freedom, such measurements can require hours to days to complete. Instruments for measuring BRDF are also typically laboratory devices, although a field- portable bi-directional reflectometer does exist. Hemispherical directional reflectance (HDR) is a much easier measurement to make, although care must be taken to use the proper methodology when measuring at wavelengths beyond 10 micrometer, since integrating spheres (typically used to make such measurements) are very energy inefficient and lose their integrating properties at very long wavelengths. A few field- portable hemispherical directional reflectometers do exist, but typically measure HDR only at near-normal angles. Boeing Defense and Space Group and Surface Optics Corporation, under a contract from the Air Force Research Laboratory, have developed a new hand-held instrument capable of measuring both BRDF and HDR using a unique, patented angular imaging technique. A combination of an hemi-ellipsoidal mirror and an additional lens translate the angular scatter from a surface into a two-dimensional spatial distribution, which is recorded by an imaging array. This configuration fully maps the scattering from a half-hemisphere above the surface with more than 30,000 angularly-resolved points and update rates to 60 measurements per second. The instrument then computes HDR from the measured BDR. For ease of use, the instrument can also compare both the BRDF and HDR to preset limits, generating a Pass/Fail indicator for HDR and a high-acceptable-low image display of BRDF. Beam incidence elevation is variable from normal incidence ((theta) equals 0 degrees) to 5 degrees off grazing ((theta) equals 85 degrees), while scattering is measured to nearly 90 degrees off normal. Such capability is extremely important for any application requiring knowledge of surface appearance at oblique viewing angles. The current instrument operates over the range of 3 micrometer to 12 micrometer, with extension into the visible band possible.

VNIR hypersensor camera system

The hypersensor camera operates with a unique multispectral imaging modality developed recently at Surface Optics Corporation. The Hypersensor camera is small, low cost, rugged, and solid state, using micro-optics and an array of spectral filters, which captures a complete multispectral cube of spatial and spectral data with every focal plane exposure. The prototype VNIR Hypersensor camera captures full cubes of 588x438 (spatial pixels) x 16 (spectral bands) at frame rates up to 60 Hz. This paper discusses the optical design of the Hypersensor camera, the measured performance, and the design and operation of a custom video-rate hyperspectral processor developed for this system.

Video-rate visible to LWIR hyperspectral imaging and image exploitation

Hyperspectral imaging provides the potential to extract information about objects in a scene that is unavailable to panchromatic imagers. This increased utility, however, comes at the cost of tremendously increased data. To have the broadest range of applications, extraction of the spectral information must occur in real-time. Attempting to produce and exploit complete cubes of hyperspectral imagery at video rates, however, presents unique problems, since data rates are scaled by the number of spectral planes in the cube. MIDIS (multi-band identification and discrimination imaging spectroradiometer) allows both real-time collection and processing of hyperspectral imagery over the range of 0.4 /spl mu/m to 12 /spl mu/m. We present the major design innovations associated with producing high-speed, high-sensitivity hyperspectral imagers operating in the VIS/NIR SWIR/MWIR and LWIR and of the electronics able to handle data rates up to 160 megapixels per second, continuously. Details of two realtime spectral imaging techniques used in MIDIS, dispersive and Fourier transform, are presented. Key to development of MIDIS are high-speed, high sensitivity arrays operating in the stated bands. Real-time algorithms able to exploit the spectral dimension of the imagery are also discussed. Beyond design and performance issues, the paper also discusses applications of real-time hyperspectral imaging technology, including problems such as mine detection, countering CC&D (camouflage, concealment, and deception), and counter terrorism applications.

Design of dual-band SWIR/MWIR and MWIR/LWIR imagers

Multispectral imaging is a well accepted technique for object discrimination. Hyperspectral imaging can result in highly complex optical systems that have frame rate limitations. For fast frame rate applications, dual band imaging can provide sufficient discrimination without sacrificing signal to noise ratio. The design of a fast frame rate (> 200 Hz) SWIR/MWIR and MWIR/LWIR camera is described. Two strategies for cooling the array are explored.

Compact CMOS multispectral/polarimetric camera

A novel, compact visible multispectral, polarimetric camera is under development. The prototype is capable of megapixel imaging with sixteen wavebands and three polarimetric images. The entire system encompasses a volume less than 125mm x 100mm x 75mm. The system is based on commercial megapixel class CMOS sensors and incorporates real time processing of hyperspectral cube data using a proprietary processor system based on state of the art FPGA technology.

Manufacturing and performance evaluation of a refractive real-time MWIR hyperspectral imager

Hyperspectral imaging in the 2-5 μm band has held interest for applications in detection and discrimination of targets. Real time instrumentation is particularly powerful as a tool for characterization and field measurement. A compact, real-time, refractive MWIR hyperspectral imaging instrument has been designed, and is undergoing testing. Using a combination of dispersive and corrective elements, the system has been designed for integration and preliminary test at room temperature with passive focus correction for the cryogenic elements. The F/1.75 design supports near diffraction limited performance from 2.5 μm to 5.0 μm. This paper will review the challenges in manufacturing such a system as well as the alignment and performance data.

Video rate visible to LWIR hyperspectral image generation and exploitation

Hyperspectral imaging is the latest advent in imagin 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 dimensions, 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 compete cubes of hyperspectral imagery at video rates, however, presents unique problems for both the imager and the processor, since data rates are scaled by the number of spectral planes in the cube. MIDIS allows both real-time collection and processing of hyperspectral imagery over the range of 0.4 micrometers to 12 micrometers .

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.

Object discrimination and optical performance of a real-time 2-5 μm hyperspectral imager

Hyperspectral imaging in the 2-5 um band has held interest for applications in detection and discrimination of objects of interest. Real time instrumentation is particularly powerful as a tool for characterization and field measurement. A compact, real-time, refractive MWIR hyperspectral imaging instrument has been designed and tested. The system has been designed for cryogenic operation to improve signal to noise ratio, reduce background noise, and enable real-time hyperspectral video processing. The system is a a 2-5 μm 32-band hyperspectral imager capable of collecting and processing complete hyperspectral image cubes at 15 cubes-per-second. Details of the system and object discrimination using this system are presented.

Performance and application of a very high-speed 2-12 μm ultraspectral FTIR imager

As an offshoot of hyperspectral imaging, which typically acquires tens to slightly more than 100 spectral bands, ultraspectral imaging, with typically more than 1000 bands, provides the ability to use molecular or atomic lines to identify surface or airborne contaminants. Surface Optics Corporation has developed a very high-speed Fourier Transform Infrared (FTIR) imaging system. This system operates from 2 μm to 12 μm, collecting 128 ×128 images at up to 10,000 frames-per-second. The high-speed infrared imager is able to synchronize to almost any FTIR that provides at least mirror direction and laser clock signals. FTIRs rarely produce a constant scan speed, due to the need to physically move a mirror or other optical device to introduce an optical path difference between two beams. The imager is able to track scan speed jitter, as well as changes in position of the zero path difference (ZPD) position, and perform real-time averaging if desired. Total acquisition time is dependent on the return stroke speed of the FTIR, but 16 cm -1 (1024 point) spectral imagery can be generated in less than 1/5 second , with 2 cm -1 (8192 point) spectral imagery taking proportionately longer. The imager is currently configured with X-Y position stages to investigate surface chemistry of varied objects. Details of the optical design, focal plane array, and electronics that allow this high-speed FTIR imager to function are presented. Results of using the imager for several applications are also presented.