New advances in polarized light microscopy were used to image Congo red-stained cerebral amyloidosis in sharp relief. The rotating-polarizer method was used to separate the optical effects of transmission, linear birefringence, extinction, linear dichroism, and orientation of the electric dipole transition moments and to display them as false-color maps. These effects are typically convolved in an ordinary polarized light microscope. In this way, we show that the amyloid deposits in Alzheimer's disease plaques contain structurally disordered centers, providing clues to mechanisms of crystallization of amyloid in vivo. Comparisons are made with plaques from tissues of subjects having Down's syndrome and a prion disease. In plaques characteristic of each disease, the Congo red molecules are oriented radially. The optical orientation in amyloid deposited in blood vessels from subjects having cerebral amyloid angiopathy was 90°out of phase from that in the plaques, suggesting that the fibrils run tangentially with respect to the circumference of the blood vessels. Our result supports an early model in which Congo red molecules are aligned along the long fiber axis and is in contrast to the most recent binding models that are based on computation. This investigation illustrates that the latest methods for the optical analysis of heterogeneous substances are useful for in situ study of amyloid.T he abnormal transformation of proteins to amyloid fibrils is closely related to the so-called conformational diseases that include the common neurodegenerative disorders such as Alzheimer's disease (AD) and prion diseases. The kinetic and structural bases of fibrillogenesis in these diseases are as yet undetermined. Nevertheless, the presence of amyloid in diseased tissues has been used for the purpose of pathological diagnosis and construction of theories of pathogenesis. A structural characterization and categorization of various forms of amyloid aid accurate diagnosis of amyloid disorders and further our mechanistic understanding of an increasing list of conformational diseases.The principal diagnostic criterion of amyloidosis, established by Divry and Florkin (1), is the detection with a polarizing optical microscope of so-called apple-green birefringence (2-6) from Congo red (CR)-stained tissue sections (7). Despite the durability of this assay, the optical characterization of amyloid has not progressed and is ambiguous (8, 9). The birefringence is rarely quantified, a problem further confounded by the fact that CR does not stain amyloid consistently, and diagnosis by staining depends on the skill of the investigator. Clearly, new opticalcontrast mechanisms are required for simple, reliable amyloid diagnosis (10, 11).Here, we show that recent advances in polarized light microscopy can be used to quickly quantify and refine our description of CR-stained amyloid. In particular, we applied a newly developed imaging system to separate the optical transmission, refractive index anisotropy [linear birefringence (LB)], and optic...

Modulation techniques for measuring changes in optical birefringence, such as the rotating‐polariser method (Wood & Glazer, 1980, J. Appl. Crystallogr. 13, 217), allow one to determine |sin δ|, δ = 2πLΔn/λ, Δn= double refraction, L = light path and λ = wavelength. However, they generally suffer from not providing absolute values of the optical retardance or are limited to relatively low retardance values. In addition, knowledge of the absolute phase is required when establishing the correct values of optical orientation information. In this paper, it is shown how the phase δ, and thus optical retardance, can be extracted from combining measurements of |sin δ| at different wavelengths. The new approach works on each single point of a 2‐D picture without the need to correlate with neighbouring points. There is virtually no limit to the retardance, and the computational efforts are small compared with other methods (e.g. Ajovalasit et al. 1998, J. Strain Analysis 33, 75). When used with imaging techniques, such as the rotating polariser method of Glazer, Lewis & Kaminsky 1996 (Proc. R. Soc. London SeriesA452, 2751) this process has the potential to identify automatically optically anisotropic substances under the microscope. The algorithm derived in this paper is valid not only for birefringence studies, but can be applied to all studies of interfering light waves.

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A birefringence-imaging study of a crystal of SrTiO 3 shows a purely secondorder phase transition close to 105 K with no measurable indications of critical fluctuations. The near perfectly linear development of the birefringence near the m 3m F4/mmm phase transition is consistent with normal Landau mean-field behaviour with an order-parameter critical exponent close to 0.5.

The design and characterisation of a reconfigurable multi-level spiral phase plate is shown. The device is based on a pie-shape liquid-crystal structure with 24 slices driven by custom electronics that allow independent excitation control of each electrode. The electrooptical cell was manufactured using maskless laser ablation lithography and has shown an unprecedented high fill factor. The topological charge can be dynamically changed between 1, 2, 3, 4, 6, 8 and 12. The device has been calibrated and characterised at 632.8 nm but can be employed at any wavelength in the visible and near infrared spectrum, just modifying the driving parameters of the electrodes. The experimental results have been compared to predictions derived from simulations. An excellent correspondence between theoretical and experimental result has been found in all cases.