Specular holography
Encyclopedia
Specular holography is a technique for making three dimensional imagery by controlling the motion of specularities on a two-dimensional surface. The image is made of many specularities and has the appearance of a 3D
surface-stippling
made of dots of light. Unlike conventional wavefront holograms, specular holograms do not depend on wave optics, photographic media, or lasers.
The principle of operation is purely one of geometric optics: A point light source produces a glint on a curved specular (shiny) surface; this glint appears to travel on the surface as the eye or light source moves. If that motion is projectively consistent with binocular disparity
, the viewer will perceive — via stereopsis
— the illusion that the glint occurs at a different depth than the surface that produces it. A specular hologram contains many such curved surfaces, all embedded in a host surface. Each produces a glint and the brain integrates the many 3D cues to perceive a 3D shape.
. It is used most often in art and optics demonstrations. Historically, it was not very successful because it produced images with severe distortion. It was not until 2008 that the correct geometry for distortion-free images was demonstrated by Matt Brand.
The earliest conception of specular holography appears to be a 1934 United Kingdom patent by Hans Weil. The patent noted that scratches in a shiny surface produce glints that are only visible to certain viewpoints, depending on the scratch orientation; this anisotropy could be exploited to produce different images for different viewers. Weil appreciated that this might be used to produce 3D imagery, but it not clear whether he knew how to do so, especially considering that modern techniques are heavily computational.
In the 1970s, Gabriel Liebermann discovered that a scratch in the shape of a circular arc produces glints whose motion is approximately consistent with binocular disparity. His 1980 artwork World Brain is made of CNC-machined semi-circular arcs that produce a holographic effect. The phenomenon was independently discovered in the 1990s by William Beatty who popularized a method of making hand-drawn holograms using a compass (drafting)
. This has come to be known as scratch holography.
Beatty established a connection between scratch holography and conventional wavefront holography by pointing out that a circular arc approximates a scaled-up Benton rainbow hologram of a single point. This explains why scratch hologram images are subject to distracting distortions and collapse of the depth image outside of a very narrow field of view --- circular arcs are a fairly poor approximation to rainbow hologram fringes.
Beatty also pointed out that the rainbow hologram of a single point is a rectangular section of nested parabolics. If one were to view that geometry as a 3D reflective surface under collimated light, one would observe glint motion that is consistent with horizontal parallax. An everyday example is the parabolic Fresnel mirror used in many solar cookers. On cookers with fine Fresnel patterns, the holographic image of depth-varying bar of light is readily apparent.
In 2008, Matt Brand demonstrated a distortion-free form of specular holography. Instead of scratches, it employs very fine curved mirrors or refractors, each computationally designed to produce distortion-free parallax over a wide field of view. Brand's method considers the bundle of light rays that must be delivered to the viewer as the viewer, light source, hologram, and holographic image move relative to each other. Through the law of reflection
or Snell's law
, this determines a set of differential or integral equations that relate the position and normal of each point on an optical surface. The equations specify a foliation
of possible optical surfaces; the hologram is an intersection of this foliation and a thin shell that conforms to the host surface. Solar cookers represent one such foliation; scratch holograms do not, hence their distortion. One interesting property of the foliation approach is that it yields solutions for non-flat holographic surfaces and for unconventional viewing geometries. Brand has exhibited holograms with 3D scenes, animation, and ultra-wide field of view.
Stereoscopy
Stereoscopy refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth...
surface-stippling
Stippling
Stippling is the creation of a pattern simulating varying degrees of solidity or shading by using small dots. Such a pattern may occur in nature and these effects are frequently emulated by artists.-Art:...
made of dots of light. Unlike conventional wavefront holograms, specular holograms do not depend on wave optics, photographic media, or lasers.
The principle of operation is purely one of geometric optics: A point light source produces a glint on a curved specular (shiny) surface; this glint appears to travel on the surface as the eye or light source moves. If that motion is projectively consistent with binocular disparity
Binocular disparity
Binocular disparity refers to the difference in image location of an object seen by the left and right eyes, resulting from the eyes' horizontal separation. The brain uses binocular disparity to extract depth information from the two-dimensional retinal images in stereopsis...
, the viewer will perceive — via stereopsis
Stereopsis
Stereopsis refers to impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due the the eyes' different positions on the head...
— the illusion that the glint occurs at a different depth than the surface that produces it. A specular hologram contains many such curved surfaces, all embedded in a host surface. Each produces a glint and the brain integrates the many 3D cues to perceive a 3D shape.
Overview and history
Specular holography dates back to Hans Weil's attempts in the 1930s and thus has a longer history than conventional wavefront holographyHolography
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...
. It is used most often in art and optics demonstrations. Historically, it was not very successful because it produced images with severe distortion. It was not until 2008 that the correct geometry for distortion-free images was demonstrated by Matt Brand.
The earliest conception of specular holography appears to be a 1934 United Kingdom patent by Hans Weil. The patent noted that scratches in a shiny surface produce glints that are only visible to certain viewpoints, depending on the scratch orientation; this anisotropy could be exploited to produce different images for different viewers. Weil appreciated that this might be used to produce 3D imagery, but it not clear whether he knew how to do so, especially considering that modern techniques are heavily computational.
In the 1970s, Gabriel Liebermann discovered that a scratch in the shape of a circular arc produces glints whose motion is approximately consistent with binocular disparity. His 1980 artwork World Brain is made of CNC-machined semi-circular arcs that produce a holographic effect. The phenomenon was independently discovered in the 1990s by William Beatty who popularized a method of making hand-drawn holograms using a compass (drafting)
Compass (drafting)
A compass or pair of compasses is a technical drawing instrument that can be used for inscribing circles or arcs. As dividers, they can also be used as a tool to measure distances, in particular on maps...
. This has come to be known as scratch holography.
Beatty established a connection between scratch holography and conventional wavefront holography by pointing out that a circular arc approximates a scaled-up Benton rainbow hologram of a single point. This explains why scratch hologram images are subject to distracting distortions and collapse of the depth image outside of a very narrow field of view --- circular arcs are a fairly poor approximation to rainbow hologram fringes.
Beatty also pointed out that the rainbow hologram of a single point is a rectangular section of nested parabolics. If one were to view that geometry as a 3D reflective surface under collimated light, one would observe glint motion that is consistent with horizontal parallax. An everyday example is the parabolic Fresnel mirror used in many solar cookers. On cookers with fine Fresnel patterns, the holographic image of depth-varying bar of light is readily apparent.
In 2008, Matt Brand demonstrated a distortion-free form of specular holography. Instead of scratches, it employs very fine curved mirrors or refractors, each computationally designed to produce distortion-free parallax over a wide field of view. Brand's method considers the bundle of light rays that must be delivered to the viewer as the viewer, light source, hologram, and holographic image move relative to each other. Through the law of reflection
Reflection
Reflection or reflection may refer to:-Science:* Mirror image, a reflection in a mirror or in water* Reflection nebula, a nebula that is extended and has no boundaries* Reflection , a common wave phenomenon...
or Snell's law
Snell's law
In optics and physics, Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass...
, this determines a set of differential or integral equations that relate the position and normal of each point on an optical surface. The equations specify a foliation
Foliation
In mathematics, a foliation is a geometric device used to study manifolds, consisting of an integrable subbundle of the tangent bundle. A foliation looks locally like a decomposition of the manifold as a union of parallel submanifolds of smaller dimension....
of possible optical surfaces; the hologram is an intersection of this foliation and a thin shell that conforms to the host surface. Solar cookers represent one such foliation; scratch holograms do not, hence their distortion. One interesting property of the foliation approach is that it yields solutions for non-flat holographic surfaces and for unconventional viewing geometries. Brand has exhibited holograms with 3D scenes, animation, and ultra-wide field of view.