Afocal Optics Research Articles

Focal surface occlusion.

This paper proposes focal surface occlusion to provide focal cues of occlusion masks for multiple virtual objects at continuous depths in an occlusion-capable optical see-through head-mounted display. A phase-only spatial light modulator (PSLM) that acts as a dynamic free-form lens is used to conform the focal surface of an occlusion mask to the geometry of the virtual scene. To reproduce multiple and continuous focal blurs while reducing the distortion of the see-through view, an optical design based on afocal optics and edge-based optimization to exploit a property of the occlusion mask is established. The prototype with the PSLM and transmissive liquid crystal display can reproduce the focus blur of occluded objects at multiple and continuous depths with a field of view of 14.6°.

6DOF optical tracking system using afocal optics for image guided surgery

Image guided surgery using medical robots is becoming popular these days. For image guided surgery, a tracking system is required to provide 6DOF information of patient coordinate, surgical instruments and medical robots used in surgery. To provide 6DOF information, a marker has to be attached to the target. However, it is hard to use many markers all together because the markers will take too much space in the surgical area. The tracking system proposed in this study utilizes down sized markers compared to traditional markers by using micro-engraved data- coded pattern with a lens instead of using geometrically specified marker spheres as a tracking target. A tracking system is developed that has a measurement area of 1m to 2.5m from the tracking system. Experiment has been done for surgical navigation using the proposed tracking system and a medical robot.

Optical coordinate tracking system using afocal optics for image-guided surgery.

Image-guided surgery using medical robots supports surgeons by providing critical real-time feedback information, such as surgical instrument tracking, patient-specific models, and the use of surgery robots. An image-guided surgery system based on afocal optics was developed to overcome the problems associated with conventional optical tracking systems. An optical tracking system was developed that utilizes afocal optics. Instead of using geometrically specified marker spheres as tracking targets, the proposed system uses a marker with a lens and a micro-engraved data-coded pattern. A position and orientation-tracking algorithm was developed to utilize the observed afocal images of the marker patterns. The marker used in this tracking system can be manufactured in a smaller size than traditional optical tracker markers, and the accuracy of the proposed tracking system has significant potential for improvement due to its focused and highly magnified image. The system was tested in vitro on an optical bench with position and orientation measurement experiments using a commercial optical tracker, Polaris Vicra (NDI Corp.) for comparison. The afocal optical system provided accuracy in position and orientation that was equal or better than a commercial optical tracker system, and provided a high degree of consistency during in vitro testing. The position error was 21μ m, and the orientation error was 0.093°. An afocal optical tracker is feasible and potentially advantageous for surgical navigation, as it is expected to have fewer occlusions and provide greater efficiency for coordinate matching and tracking of patient-specific models, surgical instruments, and surgery robots. This promising new system requires in vivo testing.

Psychometrically appropriate assessment of afocal optics by measurement of the Strehl intensity ratio

Several different performance criteria have been proposed for assessing the quality of visual afocal sights. Earlier research by one of the authors (Haig) has shown that a high degree of correlation exists between a subjective assessment of performance and the Strehl intensity ratio of the optical system. We discuss some of the problems in choosing an objective performance criterion for visual sights and describe equipment that has been developed for measuring the line Strehl ratio of binoculars, both on and off axes. The equipment can be modified for testing other types of visual sight. It can also be used for measuring several additional performance parameters such as the modulation transfer function, transmission, and field curvature.

Wave-front control using liquid-crystal devices

We report on complete wave-front control with amplitude- and phase-modulation liquid-crystal devices (LCD's). A twisted nematic device is used for amplitude modulation, and an electrically controlled birefringent device is used for phase modulation. Because the LCD's are optically coupled with afocal optics and are driven by individual LCD driver circuits, the amplitude and the phase can be controlled two dimensionally and independently. Complex amplitude data are calculated, and on-axis computergenerated holograms are directly recorded. Furthermore, we discuss LCD performance requirements for high-quality reconstruction.

Comparison of focal and afocal optics in insect eyes

Recent experiments suggest that an afocal lens system may occur in insect apposition eyes which are traditionally believed to contain focal systems. We present a theoretical analysis of lens-photoreceptor systems with focal and afocal optics. A simple parameterization of the systems is obtained and formulas are given for calculating receptor response to on-axis incident light and angular sensitivity. The simple parameterization and formulas allow the different lens photoreceptor systems to be compared: both systems may be similarily optimized for a given wavelength, but the focal lens may be marginally superior over a range of wavelengths. Possible refinements to the theory are discussed.