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3375 or n?=?1.332 instead of n?=?1.376 to approximate the relative contribution of the posterior cornea). Figure?3 illustrates some examples of patterns of anterior and posterior corneal astigmatism. The toricity of the anterior and posterior crystalline lens surfaces have been measured through the analysis of Purkinje images in both adult[67, 68] and pediatric[69] populations. These studies suggest that the toricity of the posterior lens surface is greater than the anterior lens surface, and that the anterior lens contributes WTR astigmatism, and the posterior lens ATR. Optical modelling on data from young adult subjects, assuming a uniform lens refractive http://www.selleckchem.com/products/DAPT-GSI-IX.html index, has estimated an average contribution of approximately 0.50?D ATR astigmatism from the total crystalline lens.[68] It should be noted that these estimates based upon Purkinje imaging typically involve determination of curvature along a small number of meridians (e.g. four), can be prone to accumulated experimental errors (due to the multiple ocular measures required to determine astigmatic power), and http://www.selleckchem.com/products/Adriamycin.html rely upon assumptions regarding the refractive index of the crystalline lens that may not reflect the exact optical properties of the in-vivo lens.[68] More recently, high speed anterior segment OCT imaging has been used to quantify the three-dimensional structure of the crystalline lens including the topographical characteristics of its anterior and posterior surfaces.[70] This technique, which applies sophisticated methods to correct http://www.selleck.cn/products/dabrafenib-gsk2118436.html for image distortions due to both the instrument scanning architecture (so-called ��fan�� distortion) and refraction of the measurement beam by the ocular components, demonstrated accurate quantitative measures of the crystalline lens in a physical model eye (errors in measured lens surface radii of curvature were