Digital recording and analysis of optical-polarization images of a domain structure

Abstract

A description is given of a differential two-channel system for digitally recording and analyzing images of the domain structure of ordered media -ferroelectrics, ferroelastics, ferromagnets, and ferrimagnets. The algorithms used tbr the analysis invoh'e segmentation of the images based on an index of the state of polarization t~" light transmitted or reflected by the specimen.Polarized light is widely used as a research tool in materials science. In a number of cases, it offers unique possibilities for observing the domain structure of ordered media (ferroelectrics, ferroelastics, ferromagnets, and ferrimagnets) and structural features of single crystals and polycrystals with natural and induced optical anisotropy [1, 2]. However, the optical-polarization contrast of the structural elements being studied often proves inadequate for direct visual observations, which is impeding the use of the method. Several techniques have been described in the literature that make it possible to increase the contrast of images in observations made in a polarization microscope. In particular, a differential photographic method of analysis was proposed in [3] to detemline the domain structure of ferromagnetic materials by using the Kerr effect. In this method, the image of the noise background (the picture of a one-domain specimen in the state of saturation) is subtracted from the image of the same section of the surface of the specimen in the many-domain state. Thus, the authors of [3] were able to detect a very low-contrast (contrast less than 0.01--0.02) magnetic domain structure that could not be directly observed visually. The study being discussed here furthers develops the differential method in [3] and expands its capabilities by using modem computer-based techniques of recording and analyzing images.Experimental Method. To conduct the study, we created the two-channel optical-polarization unit depicted in Fig. 1. Standard video cameras based on CCD-matrices (CCD -charge-coupled devices) with 768 • 576 elements were connected to a microcomputer via a video card. The optical part of the unit was built on the basis of horizontal metallographic (reflecting) microscope MIM-8, with an attachment for observations of transparent specimens in transmitted light.The method is based on the use of optical effects that change the state of polarization of reflected or transmitted light during its interaction with the specimen (birefringence, the Faraday and Kerr effects, photoelasticity, etc.) [1, 2]. The transmission circuits in each channel differ in their transfer functions, which were chosen so that the difference output signal Ibrmed an image that was segmented based on a certain criterion. The transfer function of each channel is controlled by analyzers 1 and 2 (see Fig. 1).Results and Discussion. 1. We will examine the possibilities of using the phenomenon of birefringence in the system that was developed to record and analyze images. It is known [1, 2] that the intensity of light I i that passes through a "polarizer -birefring...