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Jim Schwiegerling, Carl Chancy, Ramon Carsola Dimalanta; Depth of Focus Measurement of an Ophthalmic Surgical Microscope. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1925.
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© ARVO (1962-2015); The Authors (2016-present)
The purpose of this study is to develop an objective and reliable means for measuring the perceived depth of focus for surgical microscopes.
The depth of focus for an ophthalmic surgical microscope (LuxOR by Alcon Surgical) was assessed. For the depth of field testing, the target, which has a calibrated spatial frequency bar pattern on its surface, is angled at 45° to the surgical microscope objective. The microscope is set to focus at the midpoint of the target. Images of the target are captured through the microscope’s ocular and further processed to extract depth of field information. A profile through the bar pattern in captured images was extracted and digitally analyzed to quantify the depth of field. To quantify the depth of field, the local contrast of the profile is calculated, with contrast calculated by (Imax -Imin)/(Imax + Imin), where Imax is the local maximum pixel value and Imin is the local minimum pixel value. A threshold of 20% contrast was chosen to define the boundaries of the depth of field. The range of image pixels exceeding this threshold was converted to a physical distance using the known pixel scale of the images. The measured depth of field is compared to an empirical perceived depth of field defined by Berek.
Based on this technique, the depth of field of the microscope was measured to be 15.90 mm. This value exceeds the depth of field expected based on the Berek formula.
We have developed a standardized method of capturing images from a depth of field target through a microscope with automatic image processing. The resultant calculations provided an objective measure of the depth of field of a surgical microscope. Our measures exceeded the depth of field empirical equation originally developed by Berek in 1927. Improvements in modern lens design may account for these differences.
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