Optical constants of far infrared materials. 3: plastics Donald R. Smith and Ernest V. Loewenstein Room temperature optical constants of plastic materials have been measured over the 50-350-cm'1 spec- tral range. The materials reported include high density polyethylene, TPX, Aclar, Kapton, Surlyn, and Mylar. All except TPX are available in sheet form and exhibit birefringence as a consequence of stretch- ing during the manufacturing process. Only the average of the two sets of optical constants is reported for each material. The refractive index was calculated from the channeled spectrum as observed in reflection from the sample, while the absorption coefficient was determined, in all cases but polyethylene, from a transmission measurement. 1. Introduction In two previous papers"12 we have discussed a method for determining far ir optical constants of solids from channeled spectra and presented data on certain crystalline materials. In this paper we report optical constants of a number of useful and readily available plastic materials. Some of these materials have been used for window, beam splitter, substrate, or lens material, and those that have nbt been so used may well find application. All 'the materials are available as films except for TPX, which comes as sheet, rod, or pellets for molding. The plastics most widely used for far ir optical applications are polyeth- ylene, Mylar (polyethylene terephthalate), and TPX (4-methyl pentene-1). The latter is valuable as lens material because of its unique property of being high- ly transparent in both the visible and far ir region with nearly the same refractive index in both spectral regions.3 The optical constants of Mylar are tabulat- ed in this paper, although they were already reported graphically in Ref. 1. In addition to the above, this publication reports the optical constants of polyeth- ylene, Aclar, Kapton, and Surlyn. A. Experimental Procedure The procedure used in the measurements reported here is the same as that reported in Ref. 1. All the measurements were made in reflection because, as was shown there, the contrast of the channeled spec- trum fringes from low index materials is substantially greater when viewed by reflection than by transmis- sion. A special time shared double beam system was The authors are with the Optical Physics Laboratory, Air Force Cambridge Research Laboratories, Bedford, Massachusetts 01731. Received 21 January 1975. set up to measure a reflection and a reference (alumi- num mirror) interferogram simultaneously. A mir- rored chopper with an open segment in it was placed near a focal point in the optical system and the sam- ple mounted behind the chopper so that the ir radia- tion is alternately reflected from the sample and from the mirror. The converging radiation was incident at an angle of 6.5�� to the normal in a cone of half angle equal to approximately 6.5��. A correction was ap- plied to the observed positions of the zero crossings of the channeled spectra, as described in Ref. 1, to compensate for both nonnormal incidence and the beam convergence. As all the plastic samples showed some degree of optical anisotropy, all the measurements were carried out in polarized radia- tion. A pile-of-polyethylene-sheets polarizer with an observed polarizance exceeding 96% over the entire spectral region was inserted into the beam to reject the radiation polarized perpendicular to the plane of incidence on the beam splitter. This arrangement utilizes the stronger polarization in the interferome- ter,4 as the beam splitter introduces polarization into the light beam. The 6-,um thick Mylar beam splitter permits continuous coverage of the 50-350-cm-1 spectral region, which is the region for which optical constants are reported in this paper. B. Thickness Measurement The thickness of the sample enters directly into the determination of the refractive index when the channeled spectrum method is used, and therefore the need for precise thickness measurements is ap- parent. Special problems were encountered in mak- ing accurate measurements of these materials be- cause of their softness. It was found that the probe of the electronic comparator used would indent slightly the surface of the sample being measured. The procedure adopted was to place each sample be- June 1975 / Vol. 14, No. 6 / APPLIED OPTICS 1335

tween a pair of parallel-faced sapphire disks, measure the thickness of the sandwich, then remove the sam- ple and measure the sapphire only, and subtract the thickness of the sapphire. The measurements, which were all referred to a set of precision gauge blocks, .yielded thickness values that are considered accurate to about 1 im. All the samples were somewhat wedged, with thickness variations across the surface amounting in some cases to several micrometers, but the average value is a sufficiently good representation of the sample thickness for the precision of the values reported herein. C. Optical Properties of Polymer Films Plastic films have complex structures that can be treated as a mixture of crystalline and amorphous phases. The picture generally accepted for the structure of crystalline long-chain polymers is that portions of many molecules are packed side by side in a precise crystalline fashion, each molecule passing through several crystalline regions, the molecules being much longer than the crystalline regions. In polyethylene, for example, the molecules are typical- ly a few thousand angstroms in length, whereas the crystalline regions are an order of magnitude smaller. The amorphous regions consist of the portions of the chain molecules that tie one crystalline region to the next. Commercial polymer films are oriented as a means of improving their strength and durability. In gener- al, commercial film is produced by quenching extrud- ed film to the amorphous state and then orienting the film by reheating and stretching the sheet approxi- mately threefold in each direction. In order to stabi- lize and reduce the tendency to shrink on heating, the film is then usually annealed under restraint. These operations, in addition to producing orienta- tion of the molecules, also increase the crystallinity of the film by producing small crystallites. Very little work has been published on the optical properties of polymers, probably because these prop- erties are dependent upon many manufacturing pa- rameters that are difficult to measure and are not usually under the control of the investigator. Pa- rameters such as the stretching temperature, stretch- ing rate, amount of stretch, geometry, prior and post- thermal treatments, polymer purity, and molecular weight all contribute to the over-all orientation and crystallinity of the finished film. Examination of our samples under a polarizing mi- croscope showed that all the materials except poly- ethylene and TPX were optically biaxial. The two exceptions were much thicker than the other films in- vestigated, however, and thinner films would almost certainly exhibit the birefringence typical of these or- iented materials. In fact birefringences have been observed in thinner films of polyethylene. In view of the paucity of knowledge about the exact structure of the bulk material and its relationship to the optical properties we concluded that it would be most useful to publish only an average value of the optical con- stants, obtained by measuring at 450 to the principal axes as observed in the visible. All samples were measured in polarized light at the 450 position and in a position perpendicular to that position. No signifi- cant difference in the optical constants could be ob- served for the samples between these two positions, and the results were therefore averaged. This fact seems strong evidence that the principal axes remain unchanged in the far ir and the visible region. II. Results A. High Density Polyethylene By far the commonest plastic material being used for far ir optical components is polyethylene. It has a nearly constant refractive index over a large wave- length range and a reasonably low absorption coeffi- cient. It is inexpensive, readily available in film, sheet, and powder or pellet form, and is easy to work into lenses or other shapes. Its principal drawback is the fact that it is not transparent in the visible, which makes it difficult to align optical systems incorporat- ing thick polyethylene transmitting components. This opacity is a consequence of the high degree of crystallinity of the material. The sample used for the measurements reported here was approximately 1 mm thick and is believed to be relatively unoriented because no birefringence was observed under the polarizing microscope. The optical constants are graphed in Fig. 1 and tabulated in Table I. The absorption coefficient values are somewhat more uncertain in the case of polyethylene than in the other plastics reported herein, owing to the fact that we had only reflection data available 1.53 r w z POLYETHYLENE 1.52 1.51 10 Le 0 0 0 In 5 I I I 100 200 WAVENUMBER 300 (cm1 ) Fig. 1. Optical constants of polyethylene. 1336 APPLIED OPTICS / Vol. 14, No. 6 / June 1975