Abstract
Purpose :
The purpose of this study is to develop an objective and reliable means for measuring the perceived depth of focus for surgical microscopes.
Methods :
The depth of focus (DOF) for a series of ophthalmic surgical microscopes (Alcon LuxOR with 175 mm and 200 mm working distances (WDs); Zeiss 700 and OPMI with and without DOF enhancement; Leica M820 with and without DOF enhancement) was assessed. A target with a calibrated ruling pattern on its surface, is angled at 45° to the surgical microscope objective. Images of the target are captured through the microscope’s objective lens and ocular and processed to extract DOF information. A profile through the tick marks on the ruling in captured images was digitally analyzed to quantify DOF. The local sharpness is calculated as the Full Width Half Maximum (FWHM) dimension of each tick. A threshold width of 20 pixels was chosen to define the boundaries of the DOF. The range of sharp tick marks below this threshold was converted to a physical distance using the known image magnification. The measured DOF is compared across various microscope platforms and accounts for differences in lens design and illumination.
Results :
The LuxOR with a 200 mm WD had DOFs of 6.58 and 3.96 mm, for the 7X and 10X magnifications, respectively. Reducing the WD to 175 mm changed the LuxOR DOFs to 5.87 and 3.73 mm. The Leica 820 with depth enhancement on had DOFs of 5.98 and 2.46 mm. Switching off the depth enhancement modified the DOFs to 4.00 and 2.33 mm. The Zeiss OPMI with depth enhancement on had DOFs of 5.98 and 2.46 mm. Switching off the depth enhancement modified the DOFs to 3.81 and 2.40 mm. Finally, the Zeiss 700 had DOFs of 4.78 and 3.22 mm.
Conclusions :
The DOF of surgical microscopes varies across design. Theoretical descriptions of DOF rely solely on the numerical aperture and magnification of the microscope. These measures do not account for differences in lens design, aberrations and illumination. We have developed an objective means of measuring DOF that captures these additional effects. Based on our results, DOF is enhanced by using collimated illumination in front of the objective lens. DOF enhancement in some microscopes is also achieved through reducing the pupil size which dims the overall image. It was determined that the DOF was highest for the LuxOR microscope using its 200mm objective at both 7X and 10X magnification amongst all systems tested.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.