until the doubled images touch. At this point, the images are a known distance from each other, and the object size can be measured to calculate the corneal radius of curvature. On most Javal-Schiøtz keratometers, the scale that measures the size of the object has already been converted to its corresponding estimates in diopters of corneal refractive power using the lensmaker’s equation (Chapter 1): P = (n’ − n)/r, where P is the refractive power of the cornea, r is the radius of the corneal curvature (in meters), 1.3375 is the “averaged” corneal refractive index, and n = 1.000 is the refractive index of air.
(Developed b y Neal H. Ateb ara, MD. Redrawn b y C. H. Wooley.)
Topography
Computerized computations of measurements of the reflected image of a Placido disk of concentric circles painted inside a concavity enable corneal topography instruments to produce a detailed map of the shape of the entire outer corneal surface (Figs 7-28, 7-29).
Figure 7-28 A Placido disk. (Courtesy of Neal H. Ateb ara, MD. Redrawn b y C. H. Wooley.)
Figure 7-29 Photograph of a computerized corneal topography system. (Courtesy of Neal H. Ateb ara, MD.)
Ultrasonography of the Eye and Orbit
In ultrasonagraphy, 8–15 million vibrations per second of a piezoelectric crystal are transmitted mechanically through the eye. The time of return of reflected sound waves is measured to create an A- scan graph. Using the presumed speeds of sound in the various parts of the eye, we can measure the location of the sound-reflective interfaces encountered. Their characteristic reflectance (the strength of the echoes) helps us identify the structures. Oscillations of the probe generate a B-scan 2- dimensional section. Scanning the eye while it is immersed in water gives better imaging of its anterior structures.
Macular Function Tests
When contemplating cataract surgery for a patient, we may want to know whether the macula is capable of better vision—so we wish we could let the patient see around the cataract to confirm this. The potential acuity meter does just that; it projects a vision chart into the eye, focusing the beam to be narrow as it passes through one of the clearer parts of the lens and spreading it out again to fall on the retina, where its pencils of light are in focus.
Alternatively, a laser interferometer can be used. It splits a laser beam into 2 parts, which meet and create interference fringes on the retina of variable spacing; the patient is then asked to identify the fringe patterns.
Scanning Laser Ophthalmoscopes
In a scanning laser ophthalmoscope, a rapidly scanning laser illuminates a small spot of retina, while a luminance detector measures the light that is reflected. An image is assembled from the data. The following sections describe a few types of scanning laser ophthalmoscopes.
Confocal scanning laser ophthalmoscopes and microscopes
Optical imaging devices may include a confocal aperture, a small opening through which pencils of light must pass in order to contribute to the device’s image. Pencils of light that converge nearly to a point in the plane of the aperture are able to pass through the opening, but other pencils are mostly blocked (Fig 7-30). In this way, a particular tissue plane of a translucent organ can be imaged, while light reflected by other tissue planes as well as extraneous scattered and reflected light can be excluded.
Figure 7-30 Principle of confocal microscopy. The pencil of light from point A passes through the aperture. Most of the pencil of light emanating from point B is blocked by the aperture. (Illustration developed b y Leon Strauss, MD, PhD.)
Scanning laser polarimeter
The nerve fiber layer is birefringent, which means that polarized light travels through it at different speeds depending on whether the polarization is along or across the fibers. The GDx (Carl Zeiss AG, Oberkochen, Germany), a scanning laser ophthalmoscope, calculates the thickness of the nerve fiber layer based on the phase shift between the slower-traveling light of one direction of polarization and the faster-traveling light polarized perpendicular to the first. The thicker the layer of nerve fibers is, the more the wave peaks of the 2 polarizations of light will be out of phase. Radial symmetry of the orientation of nerve fibers surrounding the fovea is used to correct calculations for the birefringence of the cornea, through which the light must pass twice.
Wide-field scanning laser ophthalmoscope
Mirrors arranged on a rotating polygon enable rapid wide-angle scanning of the retina with red and green lasers, the red being reflected more by the deeper layers and the green by the more superficial layers of the retina. Scanning of the retina far into its peripheral regions is achieved through the use of an ellipsoidal mirror (Fig 7-31).