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from 5/22/2012

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A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is greater than zero. This is unlike a planar mirror, which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence.

A set of three mutually perpendicular reflective surface, placed to form the corner of a cube, work as a retroreflector. The three corresponding normal vectors of the corner's sides form a basis (x, y, z) in which to represent the direction of an arbitrary incoming ray, [a, b, c]. When the ray reflects from the first side, say x, the ray's x component, a, is reversed to -a while the y and z components are unchanged. Therefore as the ray reflects first from side x then side y and finally from side z the ray direction goes from [a,b,c] to [-a,b,c] to [-a,-b,c] to [-a,-b,-c] and it leaves the corner with all three components of motions exactly reversed.

Corner reflectors occur in two varieties. In the more common form, the corner is literally the truncated corner of a cube of transparent material such as conventional optical glass. In this structure, the reflection is achieved either by total internal reflection or silvering of the outer cube surfaces. The second form uses mutually perpendicular flat mirrors bracketing an air space. These two types have similar optical properties.

A large relatively thin retroreflector can be formed by combining many small corner reflectors, using the standard optimal packing of the plane with congruent triangles.

The term cat's eye derives from the resemblance of the cat's eye retroreflector to the optical system that produces the well-known phenomenon of "glowing eyes" or eyeshine in cats and other vertebrates (which are only reflecting light, rather than actually glowing). The combination of the eye's lens and the aqueous humor form the refractive converging system, while the tapetum lucidum behind the retina forms the spherical concave mirror. Because the function of the eye is to form an image on the retina, an eye focused on a distant object has a focal surface that approximately follows the reflective tapetum lucidum structure, which is the condition required to form a good retroreflection.

 This type of retroreflector can consist of many small versions of these structures incorporated in a thin sheet or in paint. In the case of paint containing glass beads, the paint glues the beads to the surface where retroreflection is required and the beads protrude, their diameter being about twice the thickness of the paint.

(Wikipedia, Retroreflector, 5/22/2012)


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Roger D. Corneliussen

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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 5/22/2012.