Abstract— This paper shows that the four optical parameters [n(metal), k(metal), n(film), and thickness of film] of any metal, semiconductor, or dielectric material covered by a transparent thin film can be determined from ellipsometric data taken at two or more angles of incidence. The method is convenient, non-destructive, and accurate. As an example, an aluminum mirror with a silicon oxide protective coating was characterized. Results are analyzed and probable accuracy determined. The overall range and accuracy to be expected of the method are presented and discussed. Index Terms— Ellipsometry , Spectroscopy , Palomar Mirror I. INTRODUCTION HERE is often a need to determine the optical constants (or dielectric function) of conductor and semiconductor surfaces. It is difficult, however, to prepare and maintain such a surface free of tickle contaminants. On the other hand, it is relatively easy to maintain such a surface in a constant clean condition if it is covered by a hard durable transparent film, such as a tough oxide. The coating may itself be easily cleaned and maintained. Therefore, there is a need to nondestructively characterize the conductor surface in the presence of its protective cover. There is also a need to determine thickness and refractive index of the covering layer, which is generally unknown and which may be of great interest in its own right. Such a case is represented by a dielectric coated mirror. This paper describes in detail the method of determining all four optical parameters simultaneously, i.e., a method for determining refractive index and thickness of the layer, and the complex index of refraction of the underlying material. II. TYPICAL EXPERIMENTAL PROCEDURE AND RESULT In general, upon reflection at any surface, the state of polarization of the incident light changes. Ellipsometry derives its name from the measurement of elliptically polarized light that results from optical reflection. Ellipsometry is a very sensitive method for analysis of specularly reflecting surfaces or of surfaces plus films deposited on them [1]. The change in the state of polarization due to reflection is measured and interpreted in terms of the optical properties of the reflecting surface. In other words, the technique enables one to characterize a surface or thin film in terms of complex refractive index and Amir Bazdar is with the Department of Electronic ,I.A.U Urmia Branch (phone: 0098-441-3440680; fax: 0098-441-3440680; e-mail: a.bazdar@ iaurmia.ac.ir). Saeid Rafizadeh is with the Department of Electronic, I.A.U Urmia Branch (e-mail: saeidrafizadeh@gmail.com). thickness of the film [2]. A single set of ellipsometric measurements Δ and ψ [3] is sufficient to determine the complex index of refraction of a film whose thickness is known, assuming isotropy and homogeneity. However, in the case of the more general problem involving a thin transparent film and an absorbing substrate, at least four data are needed. A solution to the problem is to increase the number of independent experimental measurements by a method such as varying the angle of incidence [4] or changing the refractive index of the first phase. A computer problem based on the least square method was developed to invert a system of nonlinear complex equations such as the following for a three-phase system [5,6] to determine n 2 and h 2 /λ (thickness/wavelength is dimensionless and mathematically convenient to use) of the transparent film, n 3 and k 3 of the highly absorbing and infinitely thick substance from ellipsometric measurements.   i i e r r e r r r 2 23 || 12 || 2 23 || 12 || || 1    (1.1)   i i e r r e r r r 2 23 12 2 23 12 1         (1.2) 2 | |    r R , 2 || || | | r R  (2) 2 / 1 2 / 1 || ) /( ) ( tan   R R  ,       ||     r r arg , arg || ||   (3) and: j j j j j n h       cos , / ) / ( 2    where: j j j ik n n    As an example of a sample, a silicon oxide coated aluminum mirror was used, commercially known as a palomar mirror. It consists of an aluminum film deposited on a glass substrate with the exposed side of the aluminum covered by a thin silicon monoxide film (see Figure 1). The aluminum is highly absorbent and thick such that we can assume it to be homogeneous and infinitely thick. The entire sample will work as a three phase system: The first phase is Optical Properties of Four Phase Systems Amir Bazdar , Saeid Rafizadeh T Proceedings of the World Congress on Engineering 2011 Vol II WCE 2011, July 6 - 8, 2011, London, U.K. ISBN: 978-988-19251-4-5 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) WCE 2011