Joel H. Blatt

Monolithic Fourier-transform imaging spectrometer

The design of a monolithic imaging Fourier-transform spectrometer based on a Sagnac interferometer is discussed.

Hyperspectral imaging Fourier transform spectrometers for astronomical and remote sensing observations

The Florida Institue of Technology and the Phillips Laboratory have developed several advanced visible (0.4-0.8 micrometers ) imaging fourier transform spectrometer (IFTS) brassboards, which simultaneously acquire one spatial and one spectral dimension of the hyperspectral image cube. The initial versions of these instruments may be scanned across a scene or configured with a scan mirror to pick up the second spatial dimension of the image cube. The current visible hyperspectral imagers possess a combination of features, including (1) low to moderate spectral resolution for hundreds/thousands of spectral channels, (2) robust design, with no moving parts, (3) detector limited free spectral range, (4) detector-limited spatial and spectral resolution, and (5) field widened operation. The utility of this type of instrument reaches its logical conclusion however, with an instrument designed to acquire all three dimensions of the hyperspectral image cube (both spatial and one spectral) simultaneously. In this paper we present the (1) detailed optical system designs for the brassboard instruments, (2) the current data acquisition system, (3) data reduction and analysis techniques unique to hyperspectral sensor systems which operate with photometric accuracy, and (4) several methods to modify the basic instrument design to allow simultaneous acquistion of the entire hyperspectral image cube. The hyperspectral sensor systems which are being developed and whose utility is being pioneered by Florida Tech and the Phillips Laboratory are applicable to numerous DoD and civil applications including (1) space object identification, (2) radiometrically correct satellite image and spectral signature database observations, (3) simultaneous spactial/spectral observations of booster plumes for strategic and surrogate tactical missile signature identification, and (4) spatial/spectral visible and IR astronomical observations with photometric accuracy.

Radiative transfer modeling and analysis of spatially variant and coherent illumination for undersea object detection

Increasing the optical range of target detection and recognition continues to be an area of great interest in the ocean environment. Light attenuation limits radiative and information transfer for image formation in water. In this paper, the authors briefly review current methods of imaging and then describe a variation of the spatial interferometric technique that relies upon projected spatial gratings with subsequent detection against a coherent return signal for the purpose of noise reduction and image enhancement. A model is developed that simulates the projected structured illumination through turbid water to a target and its return to a detector. The model shows an unstructured backscatter superimposed upon a structured return signal. The model has been extended to predict what a camera would actually see, so that various noise-reduction schemes can be modeled. Finally, some water-tank tests are presented, validating original hypothesis and model predictions. The method is advantageous in not requiring temporal synchronization between reference and signal beams and may use a continuous illumination source. Spatial coherency of the beam allows for the detection of the direct return, while scattered light appears as a noncoherent noise term.

Enhanced three-dimensional reconstruction of surfaces using multicolor gratings

Most optical topolography systems use a single-wavelength laser for projection, using a swept spot, a moving line, or a projected grating. In a typical projected grating system, the gratings are shifted and a series of images is used to recover the 3-D shape of the target. When the series of images is analyzed in the normal phase shift manner, the resulting 2-D phase map typically has phase unwrapping problems due to noise and Nyquist limits. Surfaces with large vertical discontinuities present a problem for phase unwrapping 3-D shape recovery. We look at simultaneously projecting multiple wavelengths onto a surface to help avoid problems in unwrapping the 2-D phase map. Multicolor projection and shape recovery are demonstrated with a white-light Michelson interferometer and with a two-color Mach-Zehnder interferometer. Using multiple wavelengths, it is unnecessary to rotate the interferometer mirror to change the grating pitch and some operations can be done in parallel, which reduces scanning time. Limitations and improvements in the current system are discussed.

Real-time optically processed target recognition system based on arbitrary moire contours

Experiments using Liquid Crystal Televisions (LCTVs) as spatial light modulators for optical correlators, and optical input devices, have been reported upon widely. Moreover, applications of these devices for target recognition and automatic inspection systems are well documented. These systems often require the implementation of computer pre- and post- processing for image filtering and target recognition which handicaps real-time optical processing applications. It is possible to construct custom reference gratings that form a desired moire pattern when mixed with images of structurally illuminated targets. The moire patterns can be in any form, from equal depth contours, to error maps, to any arbitrary pattern desired. We have demonstrated video methods to generate such error maps in real-time. Furthermore, we have removed restrictions on the shape of the output moire contours, thus, developing a real-time automated inspection system based on the optical processing of arbitrary moire contours. We chose the moire pattern to be in the form of a Fresnel zone plate which is sent to an LCTV. Illumination of this zone plate with parallel coherent light results in a diffracted beam which produces a focused line on a detector. The result is a mixed video- optical processing system that could be used for real-time quality level sorting or other automated inspection requirements.

Adaptation of video moiré techniques to undersea mapping and surface shape determination

Abstract Fixed and variable resolution video moire techniques have been used to project structured illumination in a model undersea environment and a prototype system has been developed which generates equal depth contours of undersea objects and has applications in sizing, orientation and ranging. An advantage of this system is that the entire field is continously illuminated, and the moire contours and images are formed at video rates. The spatial frequency of the structured illumination can be continously varied, providing optimal contours for a variety of object sizes. The data can be easily interpreted by eye or processed by computer to obtain surface shape, range and orientation of a known structure.

Generation of surface shape from variable-resolution video moire contours

Several methods for generation of three dimensional surface shapes from variable resolution video moire contours are described. In a classical moire system, a physical grating is projected on a target and also used to view the target. The moire contours are generated in the plane of the viewing grating. An unambiguous surface shape can then be computed by processing a set of moire images where the grating, target, or both are moved. By using an interferometer to generate and project variable pitch gratings and video technology to generate the moire contours, a 3-D surface can be scanned at different resolutions and used on a wide range of object sizes. The elimination of the physical grating also leads to surface generation techniques that do not use moving parts, increasing reliability. From these video moire contours, it is possible to uniquely reconstruct the 3-D surface, making the distinction between concave and convex surfaces. In one technique, a computer is used to mix digitized images of distorted gratings projected on the object with computer generated gratings, creating the moire patterns. By shifting one grating, it is possible to reconstruct the surface without having to move the object being scanned.

Video Applications to Moiré Metrology

Video technology is applied to the problem of moire metrology. In the past, moire metrology seemed a promising yet limited method in the measurement and comparison of surface shape. The use of video technology has widened the area of application of moire metrology by reducing the complexity of the optical set up and providing real time information on surface shape and deformation. A continuously variable grating projector and analog video circuitry are used to generate real time additive (bright line) and transmissive (dark line) moire patterns. These patterns are used to compare a test object against a “perfect” reference object. This is done in both real time and through the use of computer image processing. Depth resolutions on the order of 0.3 mm are obtained on a cone 25.4 mm high and 50.8 mm wide. The projection system allows easy expansion to large objects. Because of the use of video technology moire metrology can now be more readily applied to robotic vision and factory assembly line quality cont...

Variable-resolution video moire error map system for inspection of continuously manufactured objects

Moire techniques can be a powerful tool to determine deviation of a manufactured shape from a desired shape. In a traditional moire system, distorted gratings on an object are viewed through an undistorted grating. The moire contours that result represent equal depth contours over the entire viewed surface. By generating the moire patterns in video, it is possible to view the distorted gratings on a test object through a set of gratings that has been distorted by a similar but perfect object. The output is then a set of moire contours that corresponds to the differences between the two surfaces. This difference or error map eliminates much of the unnecessary information generated in traditional moire inspection and thus becomes a valuable tool for comparisons between an imperfect test object and a manufacturing standard. We have developed a variable resolution video system for creating this error map using a Michelson interferometer to generate the gratings. We have successfully applied this system to damage detection on a long, continuous lengths of pipe by having two side-by-side cameras looking at different sections of pipe and also by having one cameras view filtered with a video-taped recording of an undamaged section of the pipe.

Application of acousto-optic cells and video processing to achieve signal-to-noise improvements in variable resolution moire profilometry

Moire techniques can be a powerful tool to determine surface shape or deviation of a shape in progress from a final or desired shape. The presence of the high-contrast viewing grating and the distorted grating in the final image plane makes the moire pattern hard to see. Moving grating techniques have been developed to improve the visibility of the moire pattern, but at the expense of complex moving parts. Several variable resolution projection moire techniques have been developed that either move the grating or eliminate its presence electronically, and have neither mechanical moving parts nor any physical gratings. One system uses an acousto-optics cell to generate, project, and move the gratings, while the moire is viewed through a second synchronized A-O cell. The second system uses an interferometer to generate and project variable spacing gratings that are made to move across the target and across a reference surface by an A-O beam deflector. Video processing of the reference image generates the transmissive filter that produces the moire pattern. A third system removes the grating presence electronically but retains high-contrast moire contours. Noise reduction is shown in a series of moire images of targets.