POLARISED LIGHT

Evening Post, Volume CXXI, Issue 84, 8 April 1936, Page 18

POLARISED LIGHT

AND THREE-DIMENSION

FILMS

A PHYSICIST'S WORK

Since 1933 patent after patent has been issued to Edwin H. Land, a young physicist of Boston, Massachusetts, for apparently every imaginable use of polarised light—removing glare from automobile headlights, producing stereoscopic motion pictures, giving beauty specialists a new method of viewing the skin, providing engineers with the means of detecting defects in steel and other metal structures, says the "New York Times." Land's use of polarised light for removing headlight glare has been sufficiently dwelt upon. Here we confine ourselves largely to the production of three-dimensional motion pictures.

Everybody knows that light is a wave motion in the ether —a motion transverse to the direction of the beam. There are any number of plans in which the waves can travel In this fashion. The waves of polarised light are limited to one.

There are various ways ,of. polarising light. The most familiar of all is to pass it through suitable crystals. Thus a crystal of Iceland spar or of tourmaline combs out the tangle of light ■ waves —polarises them. If the beam is then passed through another crystal the axis of which is crossed relatively to the first, it cannot pass through. The eye sees nothing. It is as if a: black curtain intervened.

To understand how this principle is applied to three-dimensional photography we must explain vision. Our eyes' are separated. Each sees something that the other does not. Each looks. a' little; around an object, as it were. The brain combines the' two images. Partly because they are different and partly because of our experience with.the world around us we behold trees, houses, and people as three-dimensional bodies. SEPARATED LENSES. Stereoscopic photographs are made with cameras that have two lenses separated just as the ■ eyes are separated. The two pictures thus produced, when mounted on a card and separated by just the right amount, will appear as a single three-dimensional image when viewed through a stereoscope. ■ Whether we look at still or moving stereoscopic pictures, we must always apply this principle of making two pictures —one for each eye to look at—and then combining them by some sort of two-lens-viewing device. It takes no great amount of thought to see that with polarised light stereoscopic movies can be presented which are as brilliant as those.to which we are accustomed. All we need to do is to project separate pictures through polarisers with axes at right angles to each other. Put on spectacles with the right polarising glasses, so that each eye sees only the picture intended for it, and the result is a true stereoscopic picture-^-a vivid scene in three dimensions with life and lustre and texture. Land is the first to tell you that he is not the pioneer inventor of polarised movies. His originality lies in his manner of polarising light—in a system. Iceland spar, tourmalne, and other polarising crystals are expensive and small. Land can polarise a beam of any diameter with his polariser. He takes a film of the kind on which photographs are usually made—cellulose acetate. He embeds in it huge numbers of little polarising crystals— for instance, needles of herapathite, which is a sulphate of iodoquinine. Each crystal can polarise light. But since the axes lie helter-skelter they are useless for Land's purpose. They must be straightened out- -made to lie with their axes parallel. DIFFERENT WAYS. There * are various ways of thus orienting the needles. Land prefers to stretch the embedding film while it is still plastic, just as if it were rubber. Stretching arranges the crystals neatly with their axes parallel—just as Land wants them. Hold up such a stretched sheet and the eye sees polarised light. It'is just as if you looked through a huge single crystal. The film is only a thousandth of an inch thick, but in each square inch about a-thousand billion crystals are embedded. . If you want a more scientific explanation, listen to Land:— Nearly all natural crystals are dichroic. This means that they form two pjane-polarised beams moving at different velocities. Dichroic crystals— herapathite is only one—absorbs one of these two beams internally more markedly than the other. Herapathite absorbs virtually all the visible light in one of its plane-polarised components and transmits virtually all of the other components. The -rest is obvious. Take your stereoscopic movies with two lenses. Project the pictures ' through Land's polariser, crossed at right angles. Put on spectacles which have windows made of Land's transparent polarising film, and you behold three-dimensional movies. With Land's new polarising material it also becomes possible to test anything from a steel girder to a milk bottle for strain, and hence for safety. Subject any material to strain and look at it by polarised, light and you see brilliant colours. From the colours a physicist can tell much about the strains—how dangerous they are. The principle also has its advertising and stage uses. Crumple up a piece of cellophane, put it between two pieces of polarising material, and brilliant colours appear. Yet there is no colour in the cellophane at all.

Glare from the sea or the sands is also removed by polarised light. _ Indeed, reflected light is usually polarised. With sun glasses so turned as to transmit only the unreflected rays, the eyes are relieved of all glare. Only the natural unpolarised light of the sun is seen. Since we can't look down into water because of the glare, we have only to put on a pair of Land's glasses to see fish swimming. Iceland fishermen have known of the trick for generations. ' They use crystals of Iceland spar. ■ • So with the human face. Surface reflections can be cut off. • With the right spectacles, it becomes possible to look a little way into the skin, so to speak. At any rate, faces are not the same.