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Development of a single lens dual-aperture stereo imaging for an application in stereo endoscope

  • Author(s): Bae, Youngsam
  • Advisor(s): Monbouquette, Harold
  • et al.

Stereopsis is the impression of depth perceived from seeing two disparate images with our two eyes. It is the most potent cue in personal space where our motor functions are often involved in dealing with an object. Thus, stereo imaging makes the most profound impact on depth perception when it feeds a human with a view of personal space for remote operations such as minimally invasive surgery (MIS). For successful implementation of stereo imaging in MIS, an imaging device or stereo endoscope requires two factors: (1) The two viewpoints be contained in an area corresponding to a diameter less than a few millimeters, which minimizes the surgical opening size necessary; and (2) high definition imaging for adequate visualization of the operation space.

In this dissertation, a single lens having a dual aperture is demonstrated to fulfill the requirements of stereo endoscopy for MIS better than a typical dual lens system. However, the left and right perspective images generated by the dual aperture overlap on the image plane unless a mechanism is incorporated to switch between the two apertures. A novel method based on complementary multi-band bandpass filters (CMBFs) is presented here to address this key issue for the miniaturization of a dual-aperture imaging device.

The goal of this dissertation is to investigate if a single lens/dual aperture system employing CMBFs can be scaled down for use in a stereo endoscope suitable for MIS. This goal is addressed with experimental studies as well as construction of prototype devices.

The motivation for using the dual aperture scheme and CMBFs is presented. The key role of stereopsis in human visuomotor coordination is discussed as well as how stereo imaging can improve visuomotor tasks for successful MISs. The requirements of a stereo endoscope for MIS are reviewed in the context of past work, and an argument is made that a prototype based on the dual aperture and CMBF can best meet these requirements.

The development of a prototype based on the dual aperture and CMBFs is described and an experiment to measure the stereo depth effect (SDE) of the prototype is presented. A calculation predicted and an experiment confirmed that the SDE of the 3-mm diameter prototype is 4/7 of that for a hypothetical dual lens system of the same diameter. Even at this reduced SDE, the prototype provides a SDE suitable for skull-base MIS, which is characterized by a viewing range of 1 to 2 cm. Additional simulation results of the dual aperture prototype including the resolution, the F#, the magnification, and the focal distance also are presented. It is made evident that a greater SDE could be enabled through a custom lens design.

The concept of CMBFs is described in detail. The transmission bands of each filter has the shape of a comb, which enables the two bandpass filters of a CMBF pair to be interwoven and which, when a filter is illuminated with matching spectral light bands, makes that filter open to that light while making the other filter closed to the same light. The experiment shows the contrast between the open/close states of a CMBF pair as high as 28000:1. In addition, because each CMBF in a pair passes a different half of the visible spectrum, another experiment is conducted to show capacity to render an RGB color image from a CMBF/dual aperture system. However, these image pairs are composed of different color spectra due to the complementary nature of the CMBFs. This is a concern because the difference can ultimately lead to color rivalry, in which the two different color images of a pair cannot be fused by the human visual perception system. A simulation is built to show that each of the CMBFs in a pair should have 4 or greater transmission bands to yield an indiscernible color difference.

The following changes to the prototype are recommended to improve the SDE and resolution of the CMBF/dual aperture system and to decrease the color difference of image pairs: (1) Design a custom lens system and detector array, (2) Develop complementary quadruple bandpass CMBFs, and (3) Consider using a dichroic mirror to avoid the need to match the light source to the CMBF.

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