Iterative method for the optical characterization of biaxially oriented poly(ethyleneterephthalate) films Alicia Larena a, * , Felipe Milla ´n b , Gabriel Pinto a a Departamento de Ingenierı ´a Quı ´mica, Industrial y del Medio Ambiente, E.T.S.I. Industriales, Universidad Polite ´cnica de Madrid, Jose ´ Gutie ´rrez Abascal 2, 28006 Madrid, Spain b Departamento de O ´ ptica, Universidad Complutense, 28040 Madrid, Spain Received 10 January 2005; accepted 26 May 2005 Available online 11 July 2005 Abstract We present a method to determine the optical parameters (refractive index, optical thickness, and coherence factor) and the geo- metrical thickness of a weakly absorbing film of poly(ethyleneterephtalate) PET. It is based on the analysis of the interference mod- ulated transmission spectra at normal incidence (in the visible) and of absorbance bands (in the IR). We use successive approximations and interpolations to calculate these properties. To check the results, thickness and refractive index have been deter- mined with an electronic micrometer and an Abbe refractometer, respectively. The method can be applied to other low-roughness films with interference fringes in zones of high transparency. It is non-destruc- tive and allows for a rapid in situ control. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.25.H; 73; 78.66 Keywords: Optical interference; Optical properties; Poly(ethyleneterephthalate) 1. Introduction The investigation described in this paper is part of an ongoing program of research on optical properties of polymer films [1,2]. Previous publications have reported the use of interference fringes in both infrared and visi- ble regions for measuring properties of industrial poly- mer films [3–5]. The Lambert–Beer law have also been employed to evaluate the thickness of photoconductor coatings [6]. Poly(ethyleneterephthalate), PET, is an important material used as substrate for multilayer structures, with a lot of applications [7] in numerous areas, such as pho- tography, radiography, holography, reprography (mate- rials for printers, photocopiers, and so on), optical coatings (coloured, anti-reflecting, electro-optic poly- mers, photoanodes, and others), or protective coatings (adherent, corrosion resistant, and so on). The main goal of this work is to present a non- destructive method that allows for an accurate and rapid characterisation of transparent PET films. We stress that an adequate characterisation of the substrate is usu- ally the previous step in any quality control of multi- layer films. Interference effects give rise to a spectrum, due to the constructive and destructive effects of internally reflected waves inside the sample [3]. In this paper we shall show that far from being a nuisance, as in other spectroscopic experiments, these fringes can be used to calculate the optical constants of the film, as well as the geometrical thickness. Only the spectral interference fringes produced in the optical region are considered in the discussion. All 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.05.010 * Corresponding author. E-mail address: alicia.larena@upm.es (A. Larena). www.elsevier.com/locate/optmat Optical Materials 28 (2006) 1013–1019