Refractive, corneal, and ocular residual astigmatism: distribution in a German population and age dependency—the Gutenberg Health Study
Dear editor, We would like to thank Dr. Alpins and Mr. Stamatelatos [1] for their interest in our study [2]. Indeed, analysis of astigmatism is a challenging approach and these authors highly contributed to pave its path [3, 4]. In addition to the specific characteristics of astigmatism including amount and orientation, there are (unfortunately) different notations: while in most English-spoken countries a positive notation of astigmatism is in use (cylinder in plus diopters), Germany and other European countries commonly use negative measures of astigmatism (cylinder in minus diopters).While the absolute amount of astigmatism is the same, the orientation has a shift of 90° and spherical power is different. This is in contrast to measurement of velocity, an analogy been drawn byDr. Alpins andMr. Stamatelatos [1]: velocity is defined by the ratio of two units of the International System of Units (SI).
New method of targeting vectors to treat astigmatism
Purpose: To describe a method for optimizing the treatment of astigmatism using vector analysis of both refractive and corneal topographic values. Setting: Cheltenham Eye Centre, Melbourne, Australia. Methods: This study evaluated a method of vector analysis for planning surgery that uses both preoperative topographic and refractive values and determined how to select the relative treatment emphasis to be given to each. In addition, the significance of the phenomenon of ocular residual astigmatism (ORA) was explored. Its presence provides an inherent limitation on eliminating astigmatism from the eye’s optical system. Results: Various comparisons of preoperative and ORA values are plotted in a series of 100 excimer laser photoastigmatic refractive keratectomy patients. These ORA values are equivalent to the expected corneal astigmatism resulting from surgery where treatment is performed by refractive astigmatism values alone. A theoretical example is given in which the corneal astigmatism remaining from surgery is reduced by giving less emphasis to completely eliminating refractive astigmatism and consequently greater emphasis to completely eliminating topographic astigmatism. Conclusion: Using vectors in astigmatism surgery enables the incorporation of topography and refractive values into the surgical plan. This would achieve a greater reduction in corneal astigmatism and potentially a better visual outcome than using refractive astigmatism values alone.
Time behavior of gas pressure and PD activity in insulation cavities under AC voltage
In the framework of developing defect-based life models (i.e. the life model of partial discharge-induced damage growth from a micro-void in polymer matrix), ageing tests were performed on PE layered specimens with artificial cavities under constant gas pressure and constant gas volume conditions, respectively. Time behavior of the discharge repetition rate and amplitude were recorded and compared under these two conditions. Results show that pressure variation has little influence on PD activities at constant gas volume. Under constant gas pressure, the evolution of gas volume has similar characteristic to the product of discharge amplitude and repetition rate.