Human stereopsis lets observers perceive either of the two virtual images. In figure 5 an observer's eyes are positioned at O1 and O2, and they observe the ellipse virtual image described in figure 4. However, the observer is able to see only a tiny segment of this ellipse by looking through the curved scatterer, as if the scatterer behaves as a slit-aperture. This slit masks the vertical extent of the ellipse virtual image locus, so the observer's eyes at O1 and O2 perceive only a single bright point P positioned somewhere behind the curved scatterer. A second point lying on the ellipse-image in the positive half of the Z plane is not seen. This second point can be observed only if the eyes are moved to the opposite side of the XY plane. Or in other words, the line-scatterer produces both a reflection-mode image sent to one side of the XY plane, and a transmission-mode image sent to the other. This resembles conventional hologram optics.
The source of illumination in fig. 5 is required to have very limited extension, otherwise the observed image point becomes a horizontal line segment and the image suffers an astigmatic blur effect. This blur is similar to that observed in Rainbow holograms [4]. The blur is reduced when radius r is made shorter (less blur when the virtual image P is very close to the film plane.) Thus we see another similarity to conventional holography.
Note that the curved scratch scatters light in the vertical Y dimension as well, so if human eyes are positioned at points O3 and O4 in figure 5, stereopsis would force them to perceive the light as coming not from point P, but instead from the vertices of the cones of rays located within the curved scatterer itself. For this reason the image produced by the curved scatterer can have horizontal parallax only. If viewed with eyes turned 90 degrees, the image lacks depth: it appears at the location of the scratches on the surface of the plate. Obviously this is a major similarity to Rainbow holography.
What is the depth (focal length f) of point P in figure 5? Knowing that P lies on the ellipse-shaped locus in figure 4, the value for f must vary with the position of the observer and with the angle of the rays from the distant point source. In figure 5 we have an observer positioned broadside to the curved scatterer, with eyes at O1 and O2. Inspecting the diagram from a position above it, we find that certain rays from each cone described in fig 4 can be extended through the cone vertex to converge on the virtual image point P in figure 5. Angle b is equal to angle a since the lines forming them are both part of the same cone of rays. Since the side of the triangle opposite to angle b is also shared by the side of a second triangle opposite to angle a, we have identical triangles, therefore focal length f simply equals radius of curvature r of the scatterer. This is only true for vertically-illuminated "broadside" views of the scatterer under vertical illumination and for small angles a (i.e. for observer distance >>interocular spacing.) . For example, if the light from the scratch was viewed from above (i.e. looking down from the location of point source PS), the curved scatterer would act instead like a spherical mirror, and the focal length measured from the scatterer to the virtual image point would then lie between the scatterer and point C. Focal length f would then have the usual value of 0.5r. On the other hand, if the observer's eyes O1, O2 are moved downward in the negative Y direction, the image point P migrates upwards along the ellipse, and the image depth increases (it eventually becomes nearly the same as the distance to point source PS.) But for viewing angles close to those shown in figure 5, the virtual depth remains close to the value for r.
Virtual depth of a point approximates the scratch radius r. This is a useful result. Suppose we were to employ a double-pointed compass (a dividers) to scribe a curved scratch onto a plastic plate. Make the scratch resemble a circular arc as shown in figure 5. We could hang this plate on a wall, illuminate it with a distant point source placed vertically above the plate, then observe the plate with two eyes oriented horizontally. Wed see a glowing spark of light shining from within the curved scratch. The virtual depth of the glowing spot would be the same as the radius of the scratch. Now suppose we lay down several hundred similar scratches, each with a different XY position, plus a Z position as set by the spacing of the dividers. Could we not draw arbitrary objects or scenes in 3D as sets of glowing points? This actually works very well.
Human stereopsis lets observers perceive either of the two virtual images. In figure 5 an observer's eyes are positioned at O1 and O2, and they observe the ellipse virtual image described in figure 4. However, the observer is able to see only a tiny segment of this ellipse by looking through the curved scatterer, as if the scatterer behaves as a slit-aperture. This slit masks the vertical extent of the ellipse virtual image locus, so the observer's eyes at O1 and O2 perceive only a single bright point P positioned somewhere behind the curved scatterer. A second point lying on the ellipse-image in the positive half of the Z plane is not seen. This second point can be observed only if the eyes are moved to the opposite side of the XY plane. Or in other words, the line-scatterer produces both a reflection-mode image sent to one side of the XY plane, and a transmission-mode image sent to the other. This resembles conventional hologram optics.
The source of illumination in fig. 5 is required to have very limited extension, otherwise the observed image point becomes a horizontal line segment and the image suffers an astigmatic blur effect. This blur is similar to that observed in Rainbow holograms [4]. The blur is reduced when radius r is made shorter (less blur when the virtual image P is very close to the film plane.) Thus we see another similarity to conventional holography.
Note that the curved scratch scatters light in the vertical Y dimension as well, so if human eyes are positioned at points O3 and O4 in figure 5, stereopsis would force them to perceive the light as coming not from point P, but instead from the vertices of the cones of rays located within the curved scatterer itself. For this reason the image produced by the curved scatterer can have horizontal parallax only. If viewed with eyes turned 90 degrees, the image lacks depth: it appears at the location of the scratches on the surface of the plate. Obviously this is a major similarity to Rainbow holography.
What is the depth (focal length f) of point P in figure 5? Knowing that P lies on the ellipse-shaped locus in figure 4, the value for f must vary with the position of the observer and with the angle of the rays from the distant point source. In figure 5 we have an observer positioned broadside to the curved scatterer, with eyes at O1 and O2. Inspecting the diagram from a position above it, we find that certain rays from each cone described in fig 4 can be extended through the cone vertex to converge on the virtual image point P in figure 5. Angle b is equal to angle a since the lines forming them are both part of the same cone of rays. Since the side of the triangle opposite to angle b is also shared by the side of a second triangle opposite to angle a, we have identical triangles, therefore focal length f simply equals radius of curvature r of the scatterer. This is only true for vertically-illuminated "broadside" views of the scatterer under vertical illumination and for small angles a (i.e. for observer distance >>interocular spacing.) . For example, if the light from the scratch was viewed from above (i.e. looking down from the location of point source PS), the curved scatterer would act instead like a spherical mirror, and the focal length measured from the scatterer to the virtual image point would then lie between the scatterer and point C. Focal length f would then have the usual value of 0.5r. On the other hand, if the observer's eyes O1, O2 are moved downward in the negative Y direction, the image point P migrates upwards along the ellipse, and the image depth increases (it eventually becomes nearly the same as the distance to point source PS.) But for viewing angles close to those shown in figure 5, the virtual depth remains close to the value for r.
Virtual depth of a point approximates the scratch radius r. This is a useful result. Suppose we were to employ a double-pointed compass (a dividers) to scribe a curved scratch onto a plastic plate. Make the scratch resemble a circular arc as shown in figure 5. We could hang this plate on a wall, illuminate it with a distant point source placed vertically above the plate, then observe the plate with two eyes oriented horizontally. Wed see a glowing spark of light shining from within the curved scratch. The virtual depth of the glowing spot would be the same as the radius of the scratch. Now suppose we lay down several hundred similar scratches, each with a different XY position, plus a Z position as set by the spacing of the dividers. Could we not draw arbitrary objects or scenes in 3D as sets of glowing points? This actually works very well.
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