TRANSPARENCY AND POLARIZING ABILITY OF POLYVINYL POLAROIDS IN THE ULTRAVIOLET

Dichroic films (polaroids) have come into use in physical experiments comparatively recently, but they have already to a considerable extent displaced polarizing prisms. In addition to their convenience in handling and their relatively lower cost, they have made possible the creation of polarizing devices of practically unlimited area, suitable for wide beams of light, which has opened up entirely new fields of application for such devices. At the same time, polaroids also have major shortcomings—first of all incomplete polarization, coloration, a comparatively low transmission coefficient, and, finally, the limited spectral region of their usefulness. The data available in the literature indicate that the region of applicability of polaroids, bounded on the short-wavelength side by a sharp increase in absorption and on the long-wavelength side by a decrease in the degree of polarization, extends over almost the entire visible spectrum, but does not go beyond its limits. The author of the paper under review1 has shown in a series of experiments that, in the case of polyvinyl polaroids (the so-called H-filters), strong absorption in the ultraviolet region of the spectrum is caused not by the dichroic film itself, but by the glass or celluloid backing on which it is usually deposited.

The author had at his disposal the latest factory samples of dyed and stretched films of polyvinyl alcohol, which possessed strong dichroism and good polarizing action in the visible region. Two crossed films almost completely extinguished the light from the most intense sources. The slight residual light had a pale bluish-gray tint, with no signs of the red color usual for earlier samples. The results of quantitative spectrophotometric measurements are shown in the figure. Curve A corresponds to a single layer of dichroic film. Curve B also refers to a single layer, but

the technology of dyeing the film was somewhat different. The transparency of two crossed films of type A is represented by curve B. Thus, the transparency of a single layer for unpolarized incident light varies approximately from 5 to 30%, nowhere reaching 50%, which would correspond to the absence of absorption and to complete polarization. In general, the transmission is somewhat lower than in the visible region, but rather large. In the absorption spectra of an undyed, unstretched polyvinyl film 0.13 mm thick, a weak absorption band is observed in the region of about 270–280 mμ. The author rightly points out that

Transparency of single and crossed polyvinyl-polaroid films in the ultraviolet (as percentages of the intensity of unpolarized light incident on the film).

this band is too weak to appear noticeably in the case of undyed stretched films, whose thickness is usually less than 0.06 mm. Thus, the absorption is connected entirely with the dyeing of the film and, as is evident from the figure, can be changed by changing the dyeing technology. The transparency region of the film extends approximately to 230 mμ.

The degree of polarization in the ultraviolet, as in the visible region, is very high. The maximum of polarization is located at about 280–300 mμ and, as the author indicates, can be shifted by changing the technology of film manufacture. Thus, a film was obtained with a maximum polarization between 240 and 300 mμ.

For films of type A the degree of polarization in the wavelength region below approximately 270 mμ drops sharply, and in the wavelength interval 248–270 mμ the crossed films have a transmission band. The author indicates that by changing the technology of film manufacture it is possible to increase the transparency of crossed films in the region λ ∼ 260 mμ to 40%. Thus, crossed polaroid films can serve as a narrow light filter, for example, for isolating the resonance line of mercury (λ = 254 mμ).

Measurements in the infrared region showed that crossed films possess high transparency (55% at λ = 1 μ, 76% at λ = 2 μ, and 68% at λ = 3 μ) and can serve as a light filter for isolating the near infrared region, including the region of sensitivity of photographic plates and photoelements.

In conclusion, it should be noted that the possibility of using polaroids in the ultraviolet region of the spectrum considerably broadens the range of applicability, in the ultraviolet, of polarization-interference light filters, substantially increasing their aperture. However, the experimental data obtained in this direction by the author are not of serious interest.

It is quite obvious that the work reviewed is only the beginning of investigations in the field of creating polaroids for invisible rays. There can hardly be any doubt that the further development of research will make it possible here to create polaroids that not only successfully compete with expensive crystalline prisms, but also make it possible to solve problems inaccessible to crystal optics.

V. Yur’ev

References

1. See, for example, Handbook of Military Optics, Gostekhizdat, 1945.
2. R. Bager, J. Sci. Instr. 26, 325 (1949).

1. J. K. Orr, Phys. Rev. 76, No. 1, 155 (1949). ↩