Validation of an Affordable Handheld Wavefront Autorefractor :

The description of sphero-cylinder lenses is approached from the viewpoint of Fourier analysis of the power profile. It is shown that the familiar sine-squared law leads naturally to a Fourier series representation with exactly three Fourier coefficients, representing the natural parameters of a thin lens. The constant term corresponds to the mean spherical equivalent (MSE) power, whereas the amplitude and phase of the harmonic correspond to the power and axis of a Jackson cross-cylinder (JCC) lens, respectively. Expressing the Fourier series in rectangular form leads to the representation of an arbitrary sphero-cylinder lens as the sum of a spherical lens and two cross-cylinders, one at axis 0 degree and the other at axis 45 degrees. The power of these three component lenses may be interpreted as (x,y,z) coordinates of a vector representation of the power profile. Advantages of this power vector representation of a sphero-cylinder lens for numerical and graphical analysis of optometric data are described for problems involving lens combinations, comparison of different lenses, and the statistical distribution of refractive errors.

To assess the current number of ophthalmologists practicing worldwide in 2010 and to create a system for maintaining, collecting and improving the accuracy of data on ophthalmologists per population, ophthalmologists performing surgery, growth rate of the profession, and the number of residents in training. Between March 2010 and April 2010, the International Council of Ophthalmology emailed a standardised survey of 12 questions to 213 global ophthalmic societies. Missing data and additional information were gathered from direct correspondences with ophthalmologist contacts. The total number of ophthalmologists reported was 204,909. Data are presented for 193 countries. Information was obtained from 67 countries on the number of ophthalmologists doing surgery, entering practice, leaving practice, rate of growth and resident training. The survey results show that despite over 200,000 ophthalmologists worldwide, there is currently a significant shortfall of ophthalmologists in developing countries. Furthermore, although the number of practitioners is increasing in developed countries, the population aged 60+ is growing at twice the rate of the profession. To meet this widening gap between need and supply, it is necessary to aggressively train eye care teams now to alleviate the current and anticipated deficit of ophthalmologists worldwide.

To determine the sensitivity to change and specificity achieved when published test-retest variability (TRV) data are used to determine whether measured changes in ETDRS logarithm of the minimum angle of resolution (logMAR) visual acuity reflect true clinical change or are attributable to measurement error alone. Various degrees of change in visual acuity were simulated in a group of normal subjects by adjusting test difficulty through manipulation of viewing distance. Sensitivity to simulated change and specificity were determined with change criteria derived from published Bland-Altman 95% ranges for TRV. The relationship between viewing distance and measured acuity was as predicted theoretically. Simulated acuity change of 0.2 logMAR (two lines of letters) or greater can be reliably distinguished from no change (both sensitivity and specificity >95%) with the ETDRS chart, but a change of 0.1 logMAR or less cannot. The use of 95% ranges for TRV to establish the smallest measured visual acuity change that can be reliably detected ensures a high specificity but does not take account of sensitivity. Use of change criteria derived from published 95% ranges results in a sensitivity of approximately 50% (assuming identical levels of TRV). Sensitivity may be improved by using a change criterion that is smaller than the minimum change sought, providing the change criterion is still at least as large as the 95% range for TRV, so that specificity is maintained. Reducing TRV allows smaller changes in acuity to be reliably detected.