Elsevier

Optical Materials

Volume 29, Issue 11, July 2007, Pages 1481-1490
Optical Materials

Analysis of the dispersion of optical plastic materials

https://doi.org/10.1016/j.optmat.2006.07.010Get rights and content

Introduction

At present plastic materials find wide application in consumer and high quality optics. Optical plastics (OPs) are used mainly in the visible (VIS) and near-infrared (NIR) spectral regions from 400 nm to 1100 nm. Success in application of OPs depends on knowledge of their optical refraction, transmission, birefringence, haze and homogeneity [1]. The optical properties of polymers are in details considered in [2]. Chromatic dispersion is an important characteristic in the design of optical systems and devices. However, measurements of refractive indices are usually realized at several selected wavelengths. Determination of more extensive refractometric data is possible using dispersion formulae [3], [4].

The measuring methods for determination of OPs’ refractometric characteristic are quite different. The refractive indices of transparent polymers can be obtained using the Federal Test Method Standard [5] in which the Abbe refractometer is applied. It operates with a white light source and Amici prisms as colour compensators. The refractive index value for the sodium D-line can be read directly from the instrument. However, the measuring accuracy is not acceptable for modern optical design projects. Furthermore, determination of refractive indices values can not be realized at different wavelengths. Utilization of Zeiss Pulfrich refractometer (PR2) is possible too [6], [7]. We have measured the refractive indices in the VIS light using the PR2 instrument with its V-type prism [8] and additional goniometric set-up was applied for the entire VIS and NIR regions. The obtained refractive values were compared with the data from Glass catalogues of OSLO [9], ZEMAX [10] and Code V [11]. Laser refractometric measurements of a number of OP specimens have been also accomplished using a He–Ne laser source with 632.8 nm emission wavelength [12].

Most widely used OPs are thermoplastics as polymethyl methacrylate (PMMA), polystyrene (PS), polycarbonate (PC), methyl methacrylate styrene copolymer (NAS), styrene acrylonitrile (SAN) and methylpentene (TPX) [1], [13]. The only thermosetting plastic for optical applications is allyl diglycol carbonate (CR-39) [1]. The available catalogue data for refractive indices and dispersion characteristics of OPs is yet scanty. Useful data of commonly used transparent polymers is presented in Refs. [1], [13]. The refractive indices of the principal OPs are included in many patents [14], [15], [16], [17] and available online web-pages [18], [19]. Companies producing trade-marks of optical polymers provide information on their refractometric and dispersive characteristics [20]. In comparison with glass, OPs have a restricted range of refractive indices and dispersion. The magnitude of the refractive index nD at the sodium D-line (589.3 nm) usually varies from 1.47 to 1.59 [1], [21]. The Abbe number of OPs is in the range from about 100 to a little less than 20 [1], [21]. However, there are some differences in the reported refractometric data. For example, the refractive indices values of such a popular material as PMMA are different in references [15], [16].

Recently it has been noticed considerable interest in the development of OP materials. Plastics replace glasses in products as objectives, lens arrays, aspheric and ophthalmic lenses, displays and lighting fixtures, windows, internally illuminated outdoor signs and skylights [21], [22]. The improvement of manufacturing processes makes possible the utilization of OPs in medicine, military optics, sensors and communications [21], [22], [23], [24]. Chemical companies produce various trade-marks of OPs as NAS-21 Novacor®, CTE-Richardson®, Zeonex®, Optorez®, Bayer®, etc. but some of them have close dispersion data. Our previous refractometric measurements [7] show for example that Optorez 1330® and S – low Styrene® have similar dispersive properties, Zeonex E48R® and COC® are equal, and Bayer® is a PC-type plastic material. Using some new optical materials the designers can improve the performance and balance the production expenses [13]. It seems that OPs can be both a low-cost alternative to glass and an option that provides more degrees of freedom for product and optical design.

In this paper we consider the refractometric and dispersion properties of OP materials in the region of normal dispersion. We have examined various types of OPs including the principal, selected trade marks and some control samples of polymers.

Section snippets

Theoretical analysis of dispersion

The interaction between the electromagnetic wave and the medium (the refractive index n, respectively) depends on its density and individual properties of the molecules on one hand, and the radiation wavelength on the other hand. A characteristic of the materials is the ratio, named a specific refraction r = f(n)/ρ, where ρ is the density of the substance and f(n) is a function of the refractive index. The product of the specific refraction and molar mass is the molar refraction R often used in

Measurement of the indices of refraction

In this study we apply the measuring method described in details in our paper [7]. The OPs’ indices of refraction were measured with the aid of the Carl Zeiss Jena Pulfrich-Refractometer PR2 [8] in the visible spectral region at six standard spectral lines: green e-line 546.07 nm, blue g-line 435.83 nm, yellow d-line 587.56 nm, red r-line 706.52 nm, blue F-line 486.13 nm and red C-line 656.27 nm. We have chosen the V-type SF3 glass prism (VoF3 prism) which usually used for measuring liquids since the

Computer modelling of dispersion

In spectral regions where materials are transparent and normal dispersion occurs (λ  λ0i) Eq. (2.4) is reduced to Cauchy’s equation. In our previous works [7], [12], [25] we have applied a modified Cauchy’s approximation in the form:nλ2=A1+A2λ2+A3λ2+A4λ4+,where A1, A2, A3, A4, … are the calculated dispersion coefficients and λ is the wavelength expressed in microns.

We have studied the precision of this approximation in respect to the number of the involved dispersion coefficients. They were

Summary and discussions

In this paper some new measuring results for refractive and dispersion properties of OPs are presented and discussed. Refractometric measurements of sixteen American, Japanese and German OP materials were accomplished with the aid of the V-type prism on the Zeiss Pulfrich PR2 instrument at standard spectral lines. Additional goniometric set-up with the same V-type SF3 glass prism, white lighting module with interference filters, and a new sensitive photodetector device was also applied in the

Acknowledgement

The authors wish to thank Dr. Wesley R. Hale from Eastman Chemical Company for helpful discussions and providing the OP materials for producing the control samples.

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    This work is supported, in part, by the Burgas University “Assen Zlatarov” under Research Project NIH77.

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