> > > >

1947-48 Theatre Catalog, 6th Edition, Page 266 (252)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 266
Page 266

1947-48 Theatre Catalog, 6th Edition, Page 266

Piping Light with the Acrylic Plastics

"Painting with Light" and "Radiant Walls"

Are Effective Ideas for Novel Decoration

It has long been known that colorless transparent materials such as optical glass or fused quartz will carry light from one point to another-even around corners and curves-but until the development of the acrylic plastics, applications for this light-piping effect were strictly limited. Residual color in ordinary glass absorbs much of the light if the path of travel exceeds a few inches, and the special types of glass and other colorless materials are restricted by cost or other factors to a few special applications.

These restrictions to light-piping are removed by the acrylic plastics. Being colorless and crystal-clear, they show almost no absorption of the light passing through them; since they are thermoplastic, they can be shaped readily into any desired form, and they are, in addition, easy to machine. Finally, they are reasonable in price.

By means of an acrylic bar or rod, light may be piped to illuminate an area at a considerable distance from the source of light. Acrylic dials may be made visible by light piped from concealed bulbs; signs are spotlighted by mysterious light pouring out of the end of a transparent rod; photo-electric cells to perform all sorts of operations may be activated by light piped in a similar manner. Medical instruments of many special types use light piped through acrylic rods to give illumination, at the exact spot where it is needed, free from the heat of the bulb.

A special application of light-piping is the effect known as edge-lighting. The distinguishing characteristic of edgelighting is that the light introduced into the edge of the sheet is caused to leave the sheet at spots along the surface as desired by the designer. The sheet at these points, therefore, appears to glow. The effect can be accomplished in a number of ways, but the fundamental requirement is that the high polish on the surface of the sheet be removed or coated over. It may be sanded, engraved, scribed, etched, painted or treated in any other way that serves to disturb or cover the surface luster. A film of oil or grease-*even a layer of water on the sheet surface will change the behavior of the light rays at the surface of the plastic and allow seine to escape.

FIGURE Self the sides of the piece of plastic are parallel. reflected light rebounds from one surface to another, advancing lengthwise through the medium but remaining entirely within it.

42.2'\/ \/42.2' / \

' \


HENRY PEARSON Rohm and Haas Company

Path of Light A

PLEXIGLAS critical angle = 42.20

FIGURE l-Light, passing irom one material to another, at other than a right angle, is retracted (AOB). As the angle of incident ray increases, that of refraction also correspondingly increases.

Bl \



AIR \ o PLEXIGLAS 7 lg: / \ \X \ . / l, \rA

A X , . crltical angle

FIGURE Z-Light ray in the plastic that hits the outer surface at an angle of more than 42.4 degrees (AOA') cannot escape. Because the angle of in cidence is less than 42.2 degrees, ray 308' does.

PRINCIPLES OF LIGHT BEHAVIOR Many of the new and unusual applie cations for the acrylic plastic, Plexiglas, as a means of transmitting light were worked out in the Product Design laboratory of our company. An adequate explanation of the reasons for these effects is necessarily somewhat complicated, but the designer or fabricator wishing to apply the techniques will find an understanding of the basic principles of light behavior helpful. Fundamentally, the phenomenon is simpleelight enters the plastic, is refiected efficiently back and forth by the polished surfaces, and remains in the sheet until it reaches the opposite edge, or until it strikes a point where the surface has been disturbed. In practice, however, there are many complicating factors involved which may lead the designer into difficulty unless he understands the primary principles of light behavior.

The internal refiections which regulate the passage of light through a transparent medium are, dependent on the so-called critical angle of the material. Light passing from one material into another at any angle other than a right angle with the surface is bent or refracted. (Figure .1.) As the angle of the incident ray increases, the angle

of the refracted ray also increases. When. a light ray in air strikes an acrylic plastic at the greatest possible angle with the perpendicular, that is, at an angle practically parallel to the surface of the plastic, it will enter the plastic at an angle of 42.2 degrees from the perpendicular. The complement of this critical angle, the angle between the ray and the surface is, of course, 47.8 degrees.

Conversely, if the ray of light in the plastic hits an outer surface in contact with air at an angle more than 42.2 degrees with the perpendicular, it cannot escape, but is refiected from the surface internally and rebounds, at an. equal and opposite angle, within the plastic as indicated by lines AOA' (Figure 2). If the two surfaces oflthe plastic are parallel, the internally reflected rays rebound from one surface to the other, advancing lengthwise through the material but remaining entirely within it, until the edge opposite the point of entry is reached (Figure 3.) If the plastic is curved the minimum outer radius of curvature that will carry light without serious leakage can be determined by mathematical derivation but, loosely, it is three times the diameter (or thickness) of the plastic. For example, 1inch rods can be bent to a 3-inch radius without allowing important light leakage (Figure 4).

As explained above, critical angle, technically, describes the angle between the light ray and an imaginary line perpendicular to the surface (Figure 5). Experience, however, has shown that fabricators and designers find it more practical to deal with the complement of this angle. For this reason, in the disw cussion that follows, the 47.8 degree value has been chosen.


The more spectacular effects of lightpipingesuch applications as cold-light

FIGURE 441! the plastic is curved. the minimum ,

outer radius of curvature that will carry light without serious leakage is, roughly lspeaklng, three times the thickness of the acrylic plastic.


R= ST or more

1947-48 Theatre Catalog, 6th Edition, Page 266