Optical contrast, defects

Speckle shearing interferometry, or shearography, is a full field optical inspection teclmique that may be used for the nondestructive detection of surface and, sometimes, subsurface defects. Whilst being more sensitive in the detection of surface defects, it may also be considered for pipe inspection and the monitoring of internal conoslon. In contrast, laser ultrasound and other forms of ultrasound, are point by point measurement techniques, so that scanning facilities and significant data processing is required before information on local defects is extracted from any examination of extensive areas [1 - 3]. 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

Acquired immunodeficiency renders a host much more susceptible to secondary infections, including cytomegalovirus, syphilis, herpes zoster, fungi, hepatitis B, tuberculosis, and toxoplasmosis. HIV invades the tissues of the optic nerve and initiates an immune complex-mediated response that results in an optic neittopathy. The primary HIV infection may be responsible for color vision defects, loss of contrast sensitivity, and visual field defects. HIV infection itself may also cause direct degeneration of retinal ganglion cell axons in the optic nerve without a secondary opportunistic infection. 

Discontinue drug at any sign of decreased acuity, CV or VF defect. Consider contrast sensitivity testing. Optic atrophy not usually noted for months (2-5) after onset. 

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

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