Louis J. Denes

Telecentric confocal optics for aberration correction of acousto-optic tunable filters

A telecentric confocal optical arrangement is presented that greatly reduces the diffraction aberrations of the acousto-optic tunable filter (AOTF). Analytical expressions for the aberrations were identified based on the fundamental properties of Bragg diffraction, and additional aberrations due to focusing through the AOTF were also included. The analysis was verified by use of a geometrical ray trace optical code, and an experimental AOTF system was analyzed. Considerable improvement in the potential spatial resolution is predicted with confocal optics, which could accommodate large pixel-limited image fields of greater than 10(6) pixels. When the image quality of the experimental system was assessed, the resolution was found to be improved by the confocal optics and was diffraction limited. Higher resolution could have been obtained with the use of larger optics to increase the throughput before being limited by the aberrations.

Three-dimensional and thermal surface imaging produces reliable measures of joint shape and temperature: a potential tool for quantifying arthritis

IntroductionThe assessment of joints with active arthritis is a core component of widely used outcome measures. However, substantial variability exists within and across examiners in assessment of these active joint counts. Swelling and temperature changes, two qualities estimated during active joint counts, are amenable to quantification using noncontact digital imaging technologies. We sought to explore the ability of three dimensional (3D) and thermal imaging to reliably measure joint shape and temperature.MethodsA Minolta 910 Vivid non-contact 3D laser scanner and a Meditherm med2000 Pro Infrared camera were used to create digital representations of wrist and metacarpalphalangeal (MCP) joints. Specialized software generated 3 quantitative measures for each joint region: 1) Volume; 2) Surface Distribution Index (SDI), a marker of joint shape representing the standard deviation of vertical distances from points on the skin surface to a fixed reference plane; 3) Heat Distribution Index (HDI), representing the standard error of temperatures. Seven wrists and 6 MCP regions from 5 subjects with arthritis were used to develop and validate 3D image acquisition and processing techniques. HDI values from 18 wrist and 9 MCP regions were obtained from 17 patients with active arthritis and compared to data from 10 wrist and MCP regions from 5 controls. Standard deviation (SD), coefficient of variation (CV), and intraclass correlation coefficients (ICC) were calculated for each quantitative measure to establish their reliability. CVs for volume and SDI were <1.3% and ICCs were greater than 0.99.ResultsThermal measures were less reliable than 3D measures. However, significant differences were observed between control and arthritis HDI values. Two case studies of arthritic joints demonstrated quantifiable changes in swelling and temperature corresponding with changes in symptoms and physical exam findings.Conclusion3D and thermal imaging provide reliable measures of joint volume, shape, and thermal patterns. Further refinement may lead to the use of these technologies to improve the assessment of disease activity in arthritis.

Acousto-optic tunable filters in imaging applications

Milton S. GottliebBoris KaminskyCarnegie Mellon Research Institute700 Technology DrivePittsburgh, Pennsylvania 15230E-mail: ld2s@andrew.cmu.eduAbstract. We examine various physical characteristics of current-artacousto-optic tunable filters (AOTFs) for spectrally filtered imaging appli-cations. Imaging spectroscopy, where the goal is to obtain simulta-neously spatial and spectral information, is a topical application that ef-fectively utilizes the attractive features of the AOTF. Key measures ofperformance include image blur, relative contribution from sidelobes,contribution from background illumination, broadband scattering and dif-fraction efficiency.

Object detection with a field-portable spectropolarimetric imager

A relatively compact, lightweight, and programmable spectropolarimetric imager was used to collect spectral and polarization data from various objects and backgrounds, both in the laboratory and in field tests. This imager uses a tellurium dioxide (TeO2) acousto-optic tunable filter and a liquid-crystal retardation plate with a CCD camera. The spectral images were collected 450-1000 nm at 10- or 20-nm intervals at two or four polarization settings for each spectral interval. We analyzed a portion of these data to assess the effectiveness of this system for object detection. We present our measurements and discuss the analysis results.

Spectropolarimetric imaging for object recognition

We have built an all-electronic spectro-polarimetric imaging camera utilizing an acousto-optic tunable filter and a liquid crystal variable retardation plate. This combination of rapidly adjustable parameters allows operations at 30/sec. frame rate, and near real time adaptability to changing target signatures. The spectral capability of the AOTF permits us to apply simultaneous, multiple wavelength filtering which greatly increases selectivity. Electronically agile polarization analysis adds a valuable signature feature for many scenarios. The adjustable retardation gives the capability to analyze and display not only linear polarization, but more generally, elliptical polarization as well. We have developed background suppression algorithms based on spectral and polarization signatures so that a wide variety of targets may be displayed with greatly enhanced contrast.

Spectro-polarimetric imager for intelligent transportation systems

We have built a portable spectro-polarimetric machine vision system that operates at video frame rates. Our system contains only electronically controllable components, including an imaging acousto-optic tunable filter (AOTF), a phase retarder, acceptance and imaging optics, and a standard CCD-based camera. The device operates like an ordinary camera, except that a computer controls the spectral and polarization content of light to be viewed. For example, by sweeping the wavelength over the AOTFs range, one can obtain a spectral signature for each pixel in an image. Alternately, the camera can switch between two wavelengths, allowing for high-speed discrimination of closely matched colors in a scene. In addition to digitally controlling the wavelength, our imager uses a liquid crystal retarder to filter images based on polarization signatures of objects. We have implemented a number of algorithms to take advantage of the unique capabilities of our sensor, some of which can be applied to problems specific to transportation systems. We present two image processing applications that highlight the different methods we use to analyze scenes with our system. One application uses spectral processing to locate vegetation in a scene; the second uses polarization signatures to detect glare from hazardous road conditions such as water and ice.

AOTF polarization difference imaging

Recent work has indicated that polarization difference imaging has the potential to enhance the image quality of objects viewed in the presence of scattering media, such as fog and turbid water. We have utilized an AOTF spectro- polarimeter to implement this concept, and to expand its usefulness by incorporating real-time, adaptive, complex polarization and spectral filtering techniques into the system.

Hyperspectral imaging using acousto-optic tunable filters

Hyperspectral imaging holds great promise for object detection and recognition due to the richness of the spectral content in images from such objects. Ordinary broad-band imagers integrate the spectral information over the entire spectral band of coverage when used without any spectral filtering. In general, the spectral details in the images can be obtained by using an optical filtering element such as a filter wheel, a grating, or an acousto-optic tunable filter (AOTF). Since each task of detection and object recognition may require only a limited set of specific spectral bands based on the object as well as the background, it is best to choose a filtering optical element that has high-speed spectral selectivity with high resolution. Of all the optical filtering elements available, only an AOTF offers this capability. Such capability greatly reduces the amount of data collection and processing. In this paper, we present hyperspectral images obtained in the laboratory and from field tests, using visible-to-near-IR (VNIR) AOTF imagers. The imagers use a tellurium dioxide, TeO2, AOTF cell that covers the spectral band from 450 to 1000 nm with a spectral resolution of 10 nm at 600 nm, a charged coupled device (CCD) camera, image-forming optics, frame grabber board, rf electronics, and control and processing software. The imager used for outdoor testing is equipped with a variable phase retardation plate to obtain images with polarimetric signatures (patent pending). The spectral and polarimetric imaging capabilities of the AOTF imager were successfully tested to discriminate targets and backgrounds in various environments.

Image processing using acousto-optical tunable filtering

We describe the construction and performance of an acousto- optic tunable filter (AOTF) camera system for obtaining images in the range of 450 to 1000 nm. A combination of a 10-bit digital video camera and a high speed frame grabber board allows continuous display of high-resolution, filtered images on a computer monitor at 30 frames per second. Ability for target recognition is significantly enhanced by processing filtered images. A typical speed in a basic operation that requires two frame grabs at two different filter settings and image processing is currently limited to 6 frames per second. The pre-processing of the target image by the AOTF simplifies subsequent image processing and is nearly real time.

Spectropolarimetric imaging using a field-portable imager

The Army Research Laboratory has a program to develop and characterize compact field-portable hyperspectral and polarization imagers using electronically tunable spectral filters-acousto-optic tunable filters (AOTFs)-that are polarization sensitive. A spectropolarimetric imager has been designed that combines a liquid-crystal retardation plate with an AOTF and an off-the-shelf charge coupled device (CCD) camera. The imager uses a tellurium dioxide (TeO2) AOTF that operates from the visible to the near-infrared region. The imager is relatively compact, lightweight, and programmable. We used this imager to collect spectral and polarization data from various objects and backgrounds, both in the laboratory and in field tests. The spectral images were collected from 450 to 1000 nm at 10- or 20-nm intervals, at two or four polarization settings for each spectral interval. We analyzed a portion of these data to assess the effectiveness of this system for target detection and identification. Here we present and discuss our measurements and analysis results.