|Angular control over illumination using our prototype light field illumination system. The specimen is a blond human hair. Scale bar is 100u. At top are the patterns displayed on our video projector. After passing through a microlens array and the microscope objective Ob, we obtain the illumination shown in the middle row. At bottom are photographs captured by a color camera (Canon 5D) using a 10x/0.45NA objective and no imaging microlens array. (a) Brightfield. (b) Quasi-darkfield. Note the enhanced visibility of scattering inside the hair fiber. (c) Headlamp, i.e. brightfield with a reduced aperture. This produces a specular highlight, whose undulations arise from scales on the fiber surface. (d) Oblique, produced by shifting (c) to the edge of the aperture. In this case some of the light reflects from the top surface of the hair fiber, producing a specular highlight, but some of it continues through the fiber, taking on a yellow cast due to selective absorption by the hair pigment. Eventually this light reflects from the back inside surface of the fiber, producing a second highlight (white arrow), which is colored and at a different angle than the first. This accounts for the double highlight characteristic of blond-haired people.
Journal of Microscopy, Volume 235, Part 2, 2009, pp. 144-162.
By inserting a microlens array at the intermediate image plane of an optical microscope, one can record 4D light fields of biological specimens in a single snapshot. Unlike a conventional photograph, light fields permit manipulation of viewpoint and focus after the snapshot has been taken, subject to the resolution of the camera and the diffraction limit of the optical system. By inserting a second microlens array and video projector into the microscope's illumination path, one can control the incident light field falling on the specimen in a similar way. In this paper we describe a prototype system we have built that implements these ideas, and we demonstrate two applications for it: simulating exotic microscope illumination modalities and correcting for optical aberrations digitally.
2-minute video (.mov file, H.264, 16 MB) showing digital refocusing of illumination and observation in our microscope.
This video corresponds to figure 11 in the paper.
2-minute video (.mov file, H.264, 14 MB) showing focused illumination passing through a fluorescein-filled chamber.
This video corresponds to figure 19 in the paper.
The analysis of double highlights in blond-haired people, mentioned in the figure caption above and in figure 8 of our PDF file, is due to a paper by Steve Marschner . Figure 1 in Marschner's paper, which is a longitudinal cross-section of a hair fiber, shows a second (yellow) highlight that is displaced from the first (white) highlight axially, i.e. along the axis of the hair fiber. Figure 9 in his paper, which is an axial cross-section of a hair fiber, also shows a second highlight, in this case, displaced from the first highlight laterally, i.e. perpendicular to the axis of the fiber.
In the figure above, we point out a highlight that is laterally displaced. However, only a highlight that is axially displaced would lead to a visible double highlight on the head of a blond person. In particular, in an area where the hair fibers are predominately vertical with respect to the ground, i.e. on the side of a person's head, and the lighting is overhead, this axial displacment will produce two horizontal bands of reflected light, a white one below and a yellow one above (assuming white illumination). The lateral displacement of highlights pointed out in our figure, although arising from a similar geometry to the axial displacement, would have no noticeable visual effect. Thus, the caption of our figure is strictly correct but a bit misleading.