Novel imaging technique for birefringent materials

Many transparent materials including crystals, polymers, biological tissues and textile fibres are birefringent or optically anisotropic, i.e. the refractive index varies with the direction of vibration of light. Birefringent measurements are important as they provide information about the underlying structure of a material. In general, the most sensitive techniques for measuring birefringence are those that modulate the polarization state in some form, for example, the 'rotating analyzer' method. Most of these 'modulation methods' give a single value for the birefringence integrated over an area of the sample. However, many materials are not uniform in structure and so spatial measurements of birefringence have typically been made with a polarizing microscope. However, the polarizing microscope alone is relatively poor at detecting weak birefringence due to poor intensity contrast at low retardations. Furthermore, such images are complicated by the fact that the contrast within the image is a function of both the magnitude and orientation of any optical anisotropy. A new imaging instrument, Deltascan, has been designed and constructed that makes use of the high sensitivity of the rotating analyzer method and allows spatial measurements of the magnitude of any anisotropy (vertical bar sinδ vertical bar), its orientation, (φ) and transmission (Io) of the material. The instrument separates out and plots as digitized coded colour images these three properties in less than 40 seconds. Retardation and orientation changes of less than 1nm and 1 deg, respectively, can be resolved with a spatial resolution close to that of a conventional polarizing microscope image. A wide variety of optically anisotropic materials have been examined to demonstrate both the quantitative and qualitative nature of this new sensitive polarization microscopy technique. Preliminary measurements have shown that when the system is extended to use two or more wavelengths it is also capable of directly extracting information about the order of the phase difference. (author)