The operating microscope works on principles similar to those of the slit-lamp biomicroscope. The illumination source of the operating microscope, unlike that of the slit-lamp biomicroscope, is not slit-shaped, and the working distance for the operating microscope is longer to accommodate the specific requirements of ocular surgery. The illumination is said to be “coaxial.” However, it is technically paraxial rather than coaxial (Fig 8-13), as the viewing apertures and illumination path are separated in modern operating microscopes.
Figure 8-12 The split prism in the applanation head creates 2 offset images. A, When the area of applanation is smaller than 3.06 mm, the arms of the inner semicircles remain some distance apart. B, When the area of applanation is greater than 3.06 mm, the arms of the inner semicircles overlap. C, When the area of applanation is exactly 3.06 mm, the arms of the inner semicircles just touch each other. This is the endpoint for measuring intraocular pressure. The value of 3.06 mm was chosen to approximately balance tear-film surface tension and corneal rigidity.
(Courtesy of Neal H. Atebara, MD. Redrawn by C. H. Wooley.)
Figure 8-13 Illumination and viewing paths of the surgical microscope as seen from below the collimating lens, demonstrating paraxial rather than coaxial illumination.
(Reproduced from Guyton DL, et al. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1999. Illustration modified by Kristina Irsch, PhD.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.