Edward R. Dowski

Wavefront coding: jointly optimized optical and digital imaging systems

Many of the limitations of traditional optical-only imaging systems can be eliminated with jointly optimized optical and digital imaging systems. Jointly optimized optical and digital imaging systems exploit the complementary aspects of optics and digital signal processing to form systems with characteristics not possible with traditional optics-only systems. For example, in traditional imaging systems light gathering and large depth of field are competing goals and are inversely related. On the other hand, in optimized optical/digital imaging systems light gathering and large depth of field can be independent parameters. Instead of requiring a small aperture to produce a large depth of field, a large aperture and a large depth of field are both possible and practical. We can jointly optimized optical and digital imaging systems Wavefront Coded imaging systems. Concepts of Wavefront Coding are illustrated below through an athermalized, refractive, silicon/germanium IR imaging system with aluminum optical mounts subject to an ambient temperature range of -20 degree(s)C to +70 degree(s)C.

Design and optimization of aberration and error invariant space telescope systems

Telescope performance is often limited by aberrations, and/or fabrication and alignment errors. Additionally, image formation in large space-based systems is sensitive to changes in physical form parameters such as temperature-related deformations, mirror structure, piston position and detector alignment. Changes in these parameters significantly degrade image quality and often limit the performance of the system. A fundamental new technology called Wavefront Coding has been successfully demonstrated via simulations for large space-based imaging systems that promise to surpass the performance attained by traditional optical designs. Wavefront Coding uses specialized aspheric optics and signal processing of the detected image to correct defocus-like aberrations thereby enabling a new paradigm in aberration balancing for telescope systems. Wavefront Coding can provide dramatic new mission capabilities by allowing space-based imaging systems that are simpler, lighter, and cheaper, while also providing high quality imagery in dynamic environments that are difficult or impossible to image in with traditional imaging systems. As an example two systems are presented that allow the telescope to repoint the boresight through the actuation of the primary segments or through the use of a scan mirror. Traditional systems with the same goal of repointing the boresight historically have not been feasible due to either the increased error space or due to constraints on system cost and complexity.

Modeling of Wavefront Coded Imaging Systems

Wavefront Coded imaging systems are jointly optimized optical and digital imaging systems that can increase the performance and/or reduce the cost of modern imaging systems by reducing the effects of aberrations. Aberrations that can be controlled through Wavefront Coding include misfocus, astigmatism, field curvature, chromatic aberration, temperature related misfocus, and assembly related misfocus. The design and simulation of these systems are based on a model that describes all of the important aspects of the optics, detector, and digital processing being used. These models allow theoretical calculation of ideal MTFs and signal processing related parameters for new systems. These parameters are explored for extended depth of field, field curvature, and temperature related misfocus effects.

Reducing size, weight, and cost in a LWIR imaging system with wavefront coding

By jointly optimizing the design of optics, mechanics, and electronics systems with reduced size, weight, and cost can be realized. This joint optimization acts to increase the system trade-space compared to systems that optimize each component separately. Increasing the size of the system trade-space allows highly customized system design. An example of joint optimization is given for a LWIR imaging system with a conformal first surface. This example demonstrates an approximately 50% reduction in size, weight, and cost compared to acceptable traditional system solutions.

Increasing the depth of field in an LWIR system for improved object identification

In a long wave infrared (LWIR) system there is the need to capture the maximum amount of information of objects over a broad volume for the identification and classification by the human or machine observer. In a traditional imaging system the optics limit the capture of this information to a narrow object volume. This limitation can hinder the observers ability to navigate and/or identify friend or foe in combat or civilian operations. By giving the observer a larger volume of clear imagery their ability to perform will drastically improve. The system presented allows the efficient capture of object information over a broad volume and is enabled by a technology called Wavefront Coding. A Wavefront Coded system employs the joint optimization of the optics, detection and signal processing. Through a specialized design of the system’s optical phase, the system becomes invariant to the aberrations that traditionally limit the effective volume of clear imagery. In the process of becoming invariant, the specialized phase creates a uniform blur across the detected image. Signal processing is applied to remove the blur, resulting in a high quality image. A device specific noise model is presented that was developed for the optimization and accurate simulation of the system. Additionally, still images taken from a video feed from the as-built system are shown, allowing the side by side comparison of a Wavefront Coded and traditional imaging system.

IR imaging system with decreased hyperfocal distance

A long wave infrared (LWIR) computational imaging system has been designed and fabricated that has a decreased hyperfocal distance compared to traditional optics. Through the combination of aspheric optics and signal processing the near point with clear imagery has been reduced from 50m to less than 10m. Both systems deliver high quality imaging when the object is at infinity. The decrease in the hyperfocal distance was realized though the use of Wavefront Coding, a technology where all system components are jointly optimized. The system components include the optics, detector and signal processing. System optimization is used with optical/digital constraints such as manufacturability, cost, signal processing architecture, FPA characteristics, etc. Through a special design of the system’s optical phase, the system becomes invariant to the aberrations that traditionally limit the effective operational range. In the process of becoming invariant, the specialized phase creates a uniform blur across the detected image. Signal processing is applied to remove the blur, resulting in a high quality image. In this paper imagery from the Wavefront Coded system is described and compared to traditional imagery.

Modern wavefront-based optical antialiasing filter

We have developed a completely new type of optical antialiasing filter that offers both high performance and low cost. This type of filter can sufficiently attenuate all spatial frequencies beyond the CCD detector bandlimit. Antialiasing filters currently in use typically just null out very narrow bands of spatial frequencies.