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).
Magnitude of astigmatism - A comparison between eyes.
PURPOSE
Astigmatism is a highly prevalent refractive error and while studies typically focus to describe the axis symmetry between eyes, little is known about the refractive symmetry. Therefore, this study determined the astigmatic power symmetry between eyes in a large clinic population.


METHODS
A clinical chart review was conducted at three optometric practices in the United States, the United Kingdom and Canada and subjective refraction data from 88,891 patients 14-70 years of age who presented with at least -0.25DC refractive astigmatism in at least one eye were included in the analysis. Data were obtained at these practices between January 2014 and March 2017. The overall distribution (%) and magnitude (DC) of astigmatism was determined and refractive differences between eyes were identified.


RESULTS
The mean age of the patients was 42.1 ± 15.9 years and included 51,685 (58%) female and 37,206 (42%) male patients. In this data pool of 177,782 eyes, 10.9% required zero astigmatic correction, while 56.2% had astigmatism of -0.25 to -0.75DC. In total 23.9% of patients presented with astigmatism of at least -0.75DC in only one eye, while the other eye had 0 to -0.50DC. Overall, the difference in astigmatism between eyes was less than -0.75DC for 82.1% of astigmatic patients. For patients who presented with astigmatism of -1.00DC in the right eye, 80.8% of them had an astigmatic prescription of -1.00 ± 0.50DC in the left eye. For an astigmatic prescription of -4.00DC in the right eye, only 40.6% of patients exhibited astigmatism of -4.00DC ± 0.50DC in the left eye.


CONCLUSIONS
The majority of patients exhibited a difference in astigmatism between eyes of less than -0.75DC, however the refractive cylinder power symmetry was significantly lower in patients with higher refractive astigmatism.
DESIGN CONSIDERATION TO PPM KLYSTRONS FOR INDUSTRIAL LINAC
Toshiba has been developing the C-band and X-band Periodic Permanent Magnet (PPM) focused klystron in collaboration with KEK and manufactured prototypes for the JLC project. [1] The PPM klystron has an advantage to enhance the total efficiency as a result of eliminating the electric power for the focusing magnet. From experience on the development the PPM focused klystrons for the JLC project, we plan to apply the PPM focused klystron to the medical and industrial linac. The periodic magnet focusing scheme has some technical difficulties such as stop band voltage. To design the PPM klystron for the medical and industrial linac, it is necessary to give heed to electron beam instability; so that the tube parameters of the PPM focused klystrons have some constraints. The size of the accelerator system is substantially reduced if a PPM focused klystron is used. The majority of medical and industrial linacs use frequencies of S-band range. We discuss the availability of the S-band PPM focused klystron in this paper.