Foveated endoscopic lens

Abstract

We present a foveated miniature endoscopic lens implemented by amplifying the optical distortion of the lens. The resulting system provides a high-resolution region in the central field of view and low resolution in the outer fields, such that a standard imaging fiber bundle can provide both the high resolution needed to determine tissue health and the wide field of view needed to determine the location within the inspected organ. Our proof of concept device achieves 7 ≈ 8 μm resolution in the fovea and an overall field of view of 4.6 mm. Example images and videos show the foveated lens' capabilities.

Fig. 1

(a) The optical layout of the foveated lens, (b) spotsize diagrams, and (c) MTF curves. The spotsize diagrams include a circle showing the 2.6 μm radius Airy disk for the on-axis NA. The MTF plot uses a maximum spatial frequency corresponding to the optical cut-off frequency of the on-axis NA.

Fig. 2

(a) Field curvature and (b) distortion in the design. The two diagrams in the bottom row show a ray tracing simulation of an image acquired with the foveated objective: (c) the object in view, (d) its image. (Note that the image has been inverted for easy comparison with the object.)

Fig. 3

The experimental setup for testing the prototype foveated lens. Shown at left an Olympus UMPlanFl $5\times$ microscope objective lens used for relaying and magnifying the foveated lens’ image onto a detector array. At right are the (white) lens disc mounts, with the optical surfaces themselves comprising only the central 3 mm of each disc, which have been painted blue to prevent stray light.

Fig. 4

(a) The ideal calibration target, (b) its image through the prototype lens, and (c) the distortion-corrected image. The target consists of a Cartesian grid of dots spaced 0.628 mm apart and designed to overfill the field of view. Note that the bottom image contains about 2.4 times the number of pixels ($1181\times 1166$) as the measured image ($752\times 752$).

Fig. 5

A closeup of a 1951 USAF resolution target imaged through the prototype lens, showing a closeup of (a) groups 4 to 7, and (b) groups 6 and 7 on the target (shown by the red box outline in the full image), indicating that either element 1 or 2 of group 7 is just resolved (i.e. a resolution of $7\sim 8\mathrm{\mu m}$). The fields of view of the two images are 1.19 mm and 238 μm respectively.

Fig. 6

An image sequence showing the resolution target as it translates across the field of view, in raw data (left) and after distortion correction (right). (Video 1, 5.4MB MPG) (URL: http://dx.doi.org/10.1117/1.JBO.17.2.021104.1)

Fig. 7

(Top) An image of a hematoxylin and eosin (H&E) stained slice of mouse esophagus, with diameter at the object of 4.6 mm. In the foveal image (bottom, 1.1 mm diameter at the object) one can see individual nuclei (dark spots). The annotated tissue regions are: (1) epithelium, (2) muscle/cartilage, (3) thyroid, and (4) salivary gland. Note that the 660 nm illumination results in poor contrast for the tissue’s eosinophilic structures.

Fig. 8

An illustration of the long depth of field of the lens. While a hand is waved about 350 mm in front of the foveated lens’ nominal object plane, the raw image data (see the computer screen shown in the image) shows the shape of the hand is resolved except in the high-resolution central field. (Video 2, 1.1 MB MPG) (URL: http://dx.doi.org/10.1117/1.JBO.17.2.021104.2)