2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 8: Optical Instruments
8.1. b. With an ordinary slit lamp, the outlines of individual endothelial cells are best seen by viewing the specular reflection of a narrow slit beam at high magnification. A wider field can be viewed using a specular microscope, with a contact optical system to decrease surface reflections.
8.2. c. Keratometers approximate the refractive power of the cornea by measuring the radius of curvature of the central cornea and assuming the cornea to be a convex mirror. The formula D=(n–1)/r is then used to convert this radius of curvature into an approximate refractive power, where r is the radius of curvature of the reflective cornea, and n is the keratomeric refractive index at 1.3375.
8.3. c. The 1.3375 index of refraction is calculated to compensate for the minus-powered posterior corneal surface that cannot be measured with the keratometer. Myopic keratorefractive surgery primarily flattens the anterior surface, and therefore the assumed index of refraction is no longer correct. Another problem following myopic keratorefractive surgery is that the extreme center of the cornea is usually flattened substantially more than the annulus measured by the manual keratometer (approximately 3 mm). Both errors will lead to overestimating the power of the cornea and a hyperopic postoperative result, if not corrected for in the calculation.
8.4. The condensing lens images the observer’s entrance pupils as 2 very small discs that fall within the patient’s entrance pupil (along with the image of the ophthalmoscope bulb’s filament, which must not coincide with the images of the observer’s pupils, to avoid obscuring the fundus with reflections from the patient’s cornea). Of all the light the ophthalmoscope shines into the patient’s eye, only the emergent light that passes through these pupillary images enters the eyes of the observer and is available for viewing the fundus. As these image discs occupy far less than 1% of the area of the patient’s entrance pupil, and only about 0.1% of the light is reflected from the fundus, the ophthalmoscope “wastes” over 99% of the light that enters the patient’s eye in forming the image visible to the observer, requiring a very bright light source.
8.5. b. With direct ophthalmoscopy, the examiner uses the optics of the patient’s eye as a simple magnifier to look at the retina. The optics of the normal eye are approximately +60 D, so using the formula for a simple magnifier, the magnification is 60/4, or 15×. If the patient’s eye is myopic, a minus lens is dialed in, to overcome the extra plus power “error lens” inside the patient’s eye. Those 2 lenses create a Galilean telescope effect, increasing magnification and decreasing the field of view. Similarly, the retina of a hyperopic eye will be magnified less than 15× because of the reverse Galilean telescope effect created by the minus power error lens inside the patient’s eye and the plus lens of the direct ophthalmoscope. Thus, the size of the optic disc will appear larger (and not smaller) in a myopic eye than in a hyperopic or a normal eye.
8.6. e. When looking through a small pupil, the observer can improve visualization by narrowing his or her effective interpupillary distance. This can be accomplished by several means. Moving the ophthalmoscope’s mirrors or prisms closer to the observer (the “small-pupil feature” available on some ophthalmoscopes) decreases the distance between the light paths to the observer’s left and right eyes, effectively narrowing the observer’s interpupillary distance. Moving the ophthalmoscope’s eyepieces farther apart also decreases the distance between the light paths to the observer’s eyes, similarly narrowing the observer’s effective interpupillary distance. Increasing the distance between the observer and the patient decreases the angle formed by the observer’s eyes and the patient’s eye, thereby allowing the light paths from the observer’s eyes to “squeeze through” a smaller pupil.
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.