Nonlinear phase contrast using a bacteriorhodopsin film M. D. Iturbe Castillo, D. Sánchez-de-la-Llave, R. Ramos García, E. Tepichín, and L. I. Olivos-Pérez Instituto Nacional de Astrofísica, Óptica, y Electrónica Apdo. Postal 51, 72000 Puebla, Puebla, México ABSTRACT In this paper we demonstrate a novel phase contrast system that employs a BR film. Since the filter is optically induced by the Fourier transform of the phase object, no alignment is necessary at the filter plane making it extremely robust. Due to the optical properties of BR films the phase filter can be induced with low light intensity levels. The material response allows operation at video frame rates, processing of high spatial resolution objects, and the use of relatively inexpensive laser sources. Such characteristics and the fact that BR films can be produced at a low cost makes the system simple to implement, relatively inexpensive and extremely robust. The effects of varying the illuminating area beyond the phase object area and filter saturation are also analyzed. Keywords: phase contrast, bacteriorhodopsin, spatial filters 1. INTRODUCTION The phase contrast technique, introduced by Zernike in 1934, 1 is based on spatial filtering principles. Basically, a filter modifies only the zero-frequency spectral component of a phase image by a phase shift. He demonstrated that for small phase variations there exists a linear mapping between the contrasted intensity distribution and the phase distribution that originated it. This important property has been exploited in several applications where precise knowledge of the phase object distribution is required. However, phase contrast is not only limited to these type of applications. As has been shown in the past, 2,3 the phase contrast technique can be considered as a common path interferometer where a synthetic reference wave interferes with the input object phase distribution to produce an output intensity distribution. Therefore, in general, the relationship between the output intensity distribution and the input object phase distribution is a nonlinear one. In order to implement the phase contrast technique a disk of few micrometers in diameter was initially used. For the implementation of microscopic systems a ring usually substitutes the disk and an annular light source is employed. For either of these two versions the fabrication and alignment of the filter is not an easy task. In this work we propose the implementation of a Zernike-type filter that does not present fabrication and alignment difficulties 4 . This filter is obtained when a thin nonlinear material, a bacteriorhodopsin film, is illuminated by the intensity distribution of the phase object Fourier transform. Although a similar technique has been reported in the past 5-10 , our proposal presents several novel advantages due to the optical properties of the material employed. In a recent paper 11 we presented a general analysis on the image fill factor role on contrasted images obtained using the Zernike phase contrast technique. The fill factor was defined as the ratio between the phase object area and the total area of illumination. The results obtained shown that the image fill factor plays an important role and in some cases can drastically change the contrast of the output image. Here we analyze experimentally and numerically the fill factor role when a nonlinear filter, BR, is employed. Algorithms and Systems for Optical Information Processing VI, Bahram Javidi, Demetri Psaltis, Editors, Proceedings of SPIE Vol. 4789 (2002) © 2002 SPIE · 0277-786X/02/$15.00 237 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/11/2013 Terms of Use: http://spiedl.org/terms