Polaroid and Its Technical Applications

A. Pollard1

In 1828 the optical anisotropy of Iceland spar was used by William Nicol as an effective means of obtaining a linearly polarized beam of light. However, the aperture of the Nicol prism is limited by the dimensions of optically perfect crystals of spar, and the largest prism actually made has an aperture of approximately 10 cm. Ordinarily a Nicol is much smaller than this, and even optically perfect crystals of spar of moderate size are becoming scarce.

Beginning with Foucault, in 1857 a whole series of investigators—Dove, Hartnack, Prazmowski, Jamin, Glan, Feussner, Zenker, Abbe, Ahrens, S. Thompson, and others—designed various prisms with devices that increased the aperture and economized the precious spar.

Until now nothing equal to the Nicol had yet been invented with respect to transparency and completeness of polarization of the transmitted light; but when large apertures are required, only one course remains—to use reflecting polarizers, which form a partially polarized beam with a large loss of light. A recent work by E. Land, using the dichroic properties of certain substances, provides a simple device for filtering from natural light a component with a high percentage of linear polarization, and at the same time with a beam cross-section of any desired size.

In order fully to understand Land’s invention, it is necessary to recall that optical anisotropy can manifest itself in the form of double refraction, in which the ordinary and extraordinary rays pass with different absorption, and the difference of the absorption coefficients may be called true dichroism.

The decrease in the intensity of light when passing through an optically anisotropic plate may occur partly as a result of scattering and partly as a result of absorption. The difference of the scattering coefficients is called Tyndallism and, together with true dichroism, constitutes complete dichroism. In most crystalline substances Tyndallism is insignificant, but in colloidal anisotropic systems it may be substantial.

Absorption depends not only on the thickness of the plate, but also on the wavelength, and therefore in almost all dichroic crystals the transmitted light becomes colored. An outstanding and historically important example of a dichroic uniaxial crystal, in which the transmitted light is sharply polarized, is tourmaline. In some tourmalines the ordinary ray is completely absorbed, while the extraordinary ray, vibrating in a plane parallel to the trigonal axis, passes through, unfortunately, with such great absorption that the tourmalines, though polarizing in the best manner, become useless under weak illumination.

In 1851 Dr. William Bird Herapath¹ discovered a remarkable compound of iodine and quinine sulfate—iodosulfate of quinine,
$4Qu \cdot 3H_2SO_4 \cdot 2HJ \cdot J_4 \cdot 6H_2O$, which at the time aroused great interest and was subsequently named herapathite by Haidinger. Later he found a similar dichroic compound of strychnine.

The birefringent herapathite can be crystallized in the form of very small hexagonal plates, each plate 0.01 cm thick completely absorbing one of the rays and allowing the other to pass with barely noticeable absorption. Large crystals of herapathite cannot be obtained; nevertheless Dr. Herapath obtained some of them so large that they could be used by Beale as polarizers in a microscope². These crystals were used for the microscope very soon after their discovery, but their permanence cannot be relied upon³. If the crystal is exposed to air or placed in Canada balsam dissolved in xylene, the iodine disappears and the crystal loses its polarizing properties. They can, however, be preserved in Canada balsam dissolved in ether.

I succeeded in obtaining such crystals 2–3 mm in diameter, but they were not flat and at the same time were too fragile for flat plates to be made from them.

Herapath’s discovery seemed to have been forgotten until Land succeeded in obtaining thin sheets of nitrocellulose containing ultramicroscopic crystals of herapathite whose optical axes are parallel to one another. Each sheet behaves like a single elongated crystal, with the difference that, owing to the overlapping of the crystalline particles one another in the direction of the thickness of the film, the transmitted ray is absorbed more than it would be in a single crystal, and the sheet acquires a smoky-brown coloration.

In his patent application⁴ Land describes the process by which these sheets can be obtained. One method is the preparation of a jelly-like mass containing herapathite, well mixed with viscous nitrocellulose or with a solution of cellulose acetate. Since the particles are asymmetric, they will orient themselves in one definite direction when the viscous mass is forced through a slit-like grating. But the retarded flow of the outer parts of the material during passage—

passing through the grid will cause a surface shear that completely changes the regular arrangement of the crystals. To avoid this difficulty, before passing through the grid the layer of charged mass is placed between two layers of uncharged mass. Then the middle charged layer flows through the cross-section with a practically uniform velocity, and the particles are distributed uniformly parallel to one another.

Another method consists in placing a viscous polarizing substance on a backing of a viscous nonpolarizing substance, such as liquid celluloid or another colorless oily ether, spread on a glass plate in such a way as to stretch the polarizing medium. The orientation of the particles can thus be effected by subjecting the diluted colloidal suspension to the action of an electric or magnetic field.

It was precisely by similar methods, described by Land, that the Polaroid Corporation of Boston, USA, succeeded in making “Polaroid”—sheets of polarizing material that have now appeared on the market. Other firms have secured the right to use this material in special instruments. Thus, one firm supplies Polaroid analyzers and polarizers for microscopes, projection lanterns, and other apparatus. Another firm uses Polaroid for ophthalmological instruments. The Eastman Kodak Company uses this material for photographic purposes under the trade name “Pola Screen.”

At present there are two types of “Pola Screen”—type I and type II. Type I consists of sheets of this material cemented between glass plates of grade A or B, and is intended for use in front of a camera lens. The grade A glass plates are optically flat, with polishing of the highest quality; the B plates are likewise optical glasses of good quality, not altering the degree of sharpness. The plates are set into a round frame of light metal with an opening up to 12 cm. Similar screens with Bakelite frames are supplied in Great Britain by Polarizers Ltd. Type II is intended for use with light sources and consists of a Polaroid sheet glued to one glass plate. In this form the polarizer slightly scatters the incident light, but it can be manufactured in sizes up to 75 cm². The degree of polarization by one Polaroid plate and the transparency for ordinary light of two parallel or crossed plates, as well as through two parallel or crossed films, were determined by Ingersoll, Winans, and Krause^5 for wavelengths from 40,000 to 20,000 Å and by Strong^5—for wavelengths from 3000 to 11,000 Å.

Since the behavior of this new material cannot be evaluated without knowing the results of similar measurements, the following table gives average values taken from the curves obtained by these authors. They apparently differ for different specimens.

The dichroic properties of the material can be clearly visible at a small percentage of polarized light at the violet and red ends of the spectrum, in contrast to the middle part of the spectrum, where the absorption of a ray with one direction of polarization is almost complete. Therefore, when a bright color is observed through two crossed plates, it is colored dark red.

TABLE 1

The transparency of films and plates for ordinary light is not uninteresting. The material is opaque to ultraviolet rays, but very transparent to infrared rays both with parallel and with crossed plates; thus a pair of crossed films serves as an excellent infrared filter and does not polarize the transmitted radiation. The transparency of films for the visible spectrum is not as great as the transparency of plates, which is evidently caused by greater scattering, but, of course, the glass of the plates absorbs a larger percentage of infrared rays.

In a subsequent patent application6 Land describes a method of using and preparing films of nitrocellulose containing polarizing ultramicroscopic particles of inorganic structures, especially purple cobalt chloridosulfate. He established that films prepared with particles of this substance, suitably oriented, give complete polarization with uncolored transmitted light and especially small losses due to absorption. This material may have great advantages in comparison with herapathite.

The scientific and technical application of polaroid is almost unlimited. The most obvious and most important application, the first that comes to mind and specifically mentioned by Land in

his first patent application, concerns the much-discussed problem of the dazzling action of automobile headlights. When the apertures of the front headlights are covered with a Polaroid having a plane of polarization arranged in a definite way, for example parallel to the vertical plane, an observer looking through a Polaroid screen with its plane of polarization parallel to the horizontal plane will see the headlight itself merely as a faint dark-red source; but all objects illuminated by the headlight will be visible almost as clearly as without the screen. The light of the lamp is polarized, with its intensity reduced by somewhat more than 50%, but this polarized light is depolarized into ordinary light when scattered from the surfaces of objects, the visibility of which is therefore not diminished when observed through crossed screens.

In order to achieve results in the matter of combating the blinding action of headlights, it is necessary to insist on the enactment of a law concerning the method of use and manufacture of this material.

Since ordinary light, upon specular reflection from nonmetallic surfaces at an angle of about 32–37° to the surface, is strongly polarized in the plane of incidence, i.e., the vibrations are parallel to the surface, it will be absorbed when passing through a Polaroid film with its plane of polarization parallel to the surface. Consequently, the details of objects reflecting light may be seen more clearly through Polaroid films, which is of great importance in photographic work. Pola screens placed in front of the camera lens make possible the otherwise impracticable photography obliquely through glass or water. Reflections that conceal surface details or interfere with a good image can be softened.

A single screen, used in the form of spectacles, gives the observer all the advantages mentioned above when viewing glazed paintings in art galleries with respect to the elimination of reflections. But such spectacles do not provide sufficient protection against surface glare in the streets at sunset—glare that so interferes with driving an automobile—because this oblique reflection is only slightly polarized.

If, however, a second screen is used in the spectacles, one that can be rotated, then special “canned” spectacles can be constructed, as described in Land’s patent application.^7 Screens arranged in this way can be used as a practically neutral device with variable transmittance—a very simple and useful one in many apparatuses. Light scattered from the blue sky is, approximately, in a plane perpendicular to the sun’s rays, strongly polarized in the plane containing the sun and the ray reflected from the sky. Consequently, when photographing in a direction perpendicular to the sun’s rays, the tone of a clear blue sky can be changed from light to very dark by rotating a Pola screen placed in front of the lens. Surrounding objects may thus be represented in the photograph in the most unusual way.

If the illumination is plane-polarized by placing a Pola screen of type II in front of the light source, then various useful effects can be obtained when observing or photographing through a second screen. Although polarized light becomes natural upon diffuse reflection, it retains its polarization upon specular reflection from nonmetallic surfaces, and the troublesome reflection can be eliminated by a second screen.

Reflection from metallic surfaces, however, is more complicated. Natural light is only partially polarized upon reflection from a metallic surface, and linearly polarized light is reflected as such only when the plane of polarization lies in the plane of incidence or is perpendicular to it. At all other azimuths, linearly polarized light, upon metallic reflection, is transformed into elliptically polarized light, and thus the second screen cannot completely eliminate the reflection.

The advantages of polarized illumination depend on the fact that specular reflection is extinguished by the second screen, whereas depolarized reflection passes through it. When a translucent object is examined or photographed in polarized light, the surface specular reflection can be suppressed, but the light scattered from deeper layers passes through the screen, and such an object takes on a new appearance. Thus, the true structure of the skin becomes clearer when the illumination is intense.

It is known that some time ago in New York the inventor of Polaroid demonstrated stereoscopic motion-picture projection. For this purpose two stereoscopic motion-picture films were projected onto one and the same screen, one over the other, through polaroid plates whose planes of polarization were perpendicular to one another. The audience was provided with polaroid spectacles, in which the plate for the right eye extinguished the picture for the left eye, while the plate for the left eye extinguished the picture for the right. The usual interference color effect obtained by introducing a plate of a uniaxial crystal, or a substance such as cellophane, which optically behaves like a uniaxial crystal, between two crossed polaroid plates, can be used for projecting onto a stage a colored background of various tones.

The polaroid screen will thus find application, especially if several arbitrary prices now in force are substantially reduced when mass production begins. It cannot replace a Nicol prism in measuring instruments where complete polarization is required, but, evidently, it can be used with great success in apparatus for the study of photoelasticity, in projection lanterns, microscopes, and ocular instruments in which the Nicol limits the aperture or serves as an obstacle to the sharpness of the optical image.

Now that Land has so successfully demonstrated the possibility of producing polarizing screens, attention will undoubtedly be concentrated on producing a transparent material that transmits a higher percentage of polarized light uniformly throughout the entire visible spectrum. It may be supposed that, when the remarkable phenomena of photoanisotropy observed by Weigert, Zocher, and Coper are fully understood, means will be found by which we shall be able completely to control the production of plates polarizing light circularly or elliptically.

Weigert found that linearly polarized light can transform an isotropic solid colloidal system into an anisotropic one with double refraction and dichroism. When a dry layer of silver chloride suspended in gelatin is exposed to white light, bluish-red photochloride is formed. If this photochloride is then subjected to the action of intense polarized red light, the photochloride acquires double refraction and dichroic properties with a plane of polarization parallel to the plane of polarization of the exciting light. Violet or ultraviolet light does not produce such an effect, just as anisotropy never develops at very low temperatures. Werner and Kuhn also found that photoanisotropy appears in an aqueous gel of a yellow cotton dye under the action of linearly polarized light.

But Zocher and Coper showed that circularly polarized light transforms thin chloride layers, prepared by chlorinating silver mirrors, into layers giving circular dichroism and circular double refraction. The circular dichroism obtained has the same characteristics as the dichroism obtained with linear polarization. Thus it may be said that a photoanisotropic layer is more transparent to circularly polarized light of the opposite direction.

Circular double refraction, in which a circularly polarized vibration in one direction is retarded relative to a vibration of the opposite direction of rotation, is simple optical activity, and this is the first case in which optical activity is caused by light itself.

For those insufficiently familiar with these interesting and important studies on anisotropy, it may be noted that a brief and clear description of them, with an extensive bibliography, has been given by Prof. Freundlich 8.

References

1. W. B. Herapath, Phil. Mag., 3, 161, 1852; 6, 346, 1853; 7, 352, 1854; 9, 366, 1855.
2. Beale, “How to Work with the Microscope,” 5th edn., 1880, p. 23.
3. B. Carpenter, The Microscope and its Revelations, 2nd edn., 1887, pp. 127, 128.
4. Land, British Patent No. 412,179, Dec. 16, 1932.
5. L. Ingersoll, J. Winans and E. Krause; J. Strong, J. Opt. Soc. Am. 26, p. 233 and p. 256, 1936.
6. Land, British Patent No. 433,455, Jan. 15, 1934.
7. Land, British Patent No. 442,825, June 17, 1935.
8. H. Freundlich, Photographic J., 76, 395, 1936.

1. Nature, Aug. 22, 1936, p. 311. ↩