Kinetic Art: A Mural of Variably Stressed Photoelastic Material With Light Polarizers

Leonardo,Vol. 6, pp. 325-327. Pergamon Press 1973. Printed in Great Britain KINETIC ART: A MURAL OF STRESSED PHOTOELASTIC WITH LIGHT POLARIZERS* Joseph A. Burns** and Judith Klein Burns*** VARIABLY MATERIAL A prototype mural, whose construction will be of interest to artists and architects, was made in 1972 at Cornell University from materials commonly used to analyze stresses in models of mechanical parts and structures by means of polarized light [l]. The mural, in the form of a +in. thick, 12 x 7.5ft. panel is shown in Fig. 1 as installed in the lobby of the Cornell Engineering Library about two feet in front of a large window. Sunlight illuminates it for viewers in the lobby during the day and interior electric light, for viewers outside the building at night. A cross section of the construction of the mural is shown in Fig. 2. The mural is suspended from the ceiling of a room, so that it hangs freely within a few inches from the floor. Two angle irons are attached at the lower edge and a false floor is laid between the angle irons and parallel supports eight feet away. (The installation shown in Fig. I has the false floor only on the lobby side of the mural.) The weight of a viewer on the false floor causes the mural to be stretched and, as he walks along, the local stresses in the mural to change, giving rise to a composition in which patterns and colors vary, that is, one has a kinetic picture (Fig. 3, cf. color plate). The mural is made of a sandwich construction consisting of outer layers of 0.03 in. thick smokecolored circular polarizing plastic sheet having the designation Polaroid HNCP 37. It can be obtained from the Polaroid Corp., Cambridge, Mass. 02139, U.S.A. This material will be referred to as a polarizer. This polarizer has the property of allowing only that fraction of incident light to pass through it that has its waves in a particular plane, called the optical axis of the polarizer. When two polarizers are superposed so that the optical axes of both are parallel to each other, all the light ~ * The mural described in this article was a project supported by a grant from Collaborations in Art, Science and Technology in conjunction with the New York State Council on the Arts. ** Assistant Professor, Department of Theoretical and Applied Mechanics, Cornell University, Ithaca, N.Y. 14850,U.S.A. *** Artist living at 1089 Taughannock Boulevard, Ithaca, N.Y., 14850, U.S.A. (Received 10November, 1972.) passing through the first polarizer, neglecting absorption and scattering, will also pass through the second one. If the second polarizer is slowly rotated, then the amount or intensity of light it transmits will decrease until all light transmission is stopped when its optical axis is at 90" to the optical axis of the first one. The inner layer of the sandwich is made of 4in. thick birefringent material of the type Hysol4485, a polyurethane rubber of high sensitivity, and is manufactured by Hysol Corporation, Olean, N.Y., U.S.A. Hysol4485 can be either mixed and cured by the user himself, or for considerably more money, purchased in small pre-cast sheets from the manufacturer. This material has the property of becoming birefringent under stress and is called a photoelastic material, hereafter referred to asPEmaterial [l]. A material is called birefringent because it has the property of splitting a wave of incident light into two waves that propagate through the material at right angles and at different velocities, thus giving rise to light interference effects when recombined. The transmission of each of the two waves and, therefore, the interference effects between them, are affected by the amount of stress imposed on the material at any point. Consider, for example, sunlight, that is, light containing essentially all the waves of the color spectrum, incident upon a sandwich made up of two polarizers and a PE material. If the PE material is unstressed, then it does not affect the behavior of light passing through the polarizers. When a local stress is imposed on the PE material, the birefringent effect comes into action; the...