Figure 7-19 High-power plus lenses for slit-lamp indirect ophthalmoscopy. A, 60 D and 90 D fundus lenses. B, The lenses produce real, inverted images of the retina within the focal range of a slit-lamp biomicroscope. I = image; O =
object. (Both parts courtesy of Neal H. Ateb ara, MD. Part B redrawn b y C. H. Wooley.)
Figure 7-20 A, A panfundoscope lens consists of a corneal contact lens and a high-power, spherical condensing lens. A real, inverted image of the fundus is formed within the spherical glass element, which is within the focal range of a slitlamp biomicroscope. I = image; O = object. B, Photograph of the panfundoscope lens. (Both parts courtesy of Neal H. Ateb ara,
MD. Part A redrawn b y C. H. Wooley.)
Gonioscopy
Unless a gonioscopy lens is placed on the eye, the anterior chamber angle is hidden from view by internal reflection. This problem is solved by the use of a contact lens with a mirror or a contact lens that allows direct viewing at an angle less than the critical angle (Fig 7-21).
Figure 7-21 Gonioscopy. A, At the anterior corneal surface, rays from the anterior chamber angle are incident at greater than the critical angle of cornea in air. Therefore, these rays are totally internally reflected. B, The contact lens curvature approximately matches the cornea, the space between the lens and the cornea being filled with an aqueous solution of methylcellulose or with tears, which have a refractive index close to that of the cornea. Light can then traverse the interface with little lost to reflection, as the critical angle at the interface is now nearly 90°. On the left is a contact lens with an internal mirror, reflecting the image toward the observer. On the right is the Koeppe lens, which gives a direct rather than reflected view of the anterior chamber. (Illustration b y C. H. Wooley.)
Surgical Microscope
The viewing optics of an operating microscope are similar to those of the slit lamp. The illumination is “coaxial,” running near the viewing paths.
Geneva Lens Clock
The Geneva lens clock uses 3 pins to measure the curvature of a spectacle lens. It is calibrated for crown glass but is useful for plastic lenses as well. It is most often used to look for differences in curvatures or to determine whether the cylinder is ground on the front or rear (Fig 7-22).
Figure 7-22 Geneva lens clock. Only the middle pin moves, measuring the curvature. (Courtesy of Tommy S. Korn, MD.)
Lensmeter
To measure the power of a lens using a lensmeter, we place the lens on a nose cone at the top of a cylinder. Farther down, a convex lens is fixed in place such that its secondary focal point is just at the rear vertex of the lens being measured. Still farther down the tube is a movable illuminated target; when the dial of the lensmeter reads 0, the target is at the primary focal point of the fixed lens. If the lens being measured has no power, then parallel rays arrive at the eyepiece, which is an astronomical telescope, and the target appears well focused to the emmetropic nonaccommodating observer. If the lens has power, the target is moved until it appears in focus, and the power of the unknown lens is read on the dial.
The fixed lens serves 2 purposes. First, the lensmeter does not have to be several meters long, and second, its scale becomes linear. That is, you turn the wheel the same amount to get from plus or minus 1 D to 2 D as you do to get from 11 D to 12 D. Badal suggested similar use of such a fixed lens in his version of the optometer, an instrument designed to measure the eye’s refractive error (Fig 7- 23). The observer looks into the lensmeter through an astronomical telescope, through which the
image appears in focus only if the light entering it is collimated. This is a clever arrangement, as the measurement tends to be little affected by the observer’s uncorrected refractive error. The spectacles are placed with the rear vertex of the distance lens on the nose cone. Line bifocals are then turned around so that the front of the glasses rests on the nose cone, and the difference in power between the distance and near portions of the lens is measured to determine the reading add power. By the way, the lensmeter is calibrated by the manufacturer to measure the rear vertex power of the distance spectacle lens. To measure the true power of the lens you would have to locate its principal planes, which is not feasible.
Figure 7-23 A, The optometer principle. T is an illuminated target. When the subject’s eye is placed at the focal point (F2)
of the positive lens (L), the vergence at the eye is directly proportional to the distance (d), measured from the first focal point (F1) of lens (L). B, The optometer principle applied to the lensmeter. The test lens (TL) is placed at F2, and the target
is viewed with an afocal telescope (AT) by the observer. The target is moved along the axis until the vergence at the test lens is equal and opposite to the vertex power of the test lens. The light will then emerge from the test lens with zero vergence (ie, collimated), and the target will be seen in sharp focus by the observer. The afocal telescope magnifies the target, increases the precision of the measurement, and reduces the effect of the observer’s accommodation or refractive
error. (Redrawn from Basic and Clinical Science Course Section 2: Optics, Refraction, and Contact Lenses. San Francisco: American Academy of Ophthalmology; 1986–1987:252. Fig 13.)
To summarize, the target is moved until the light that has passed through the fixed lens and spectacle lens is collimated (in the meridian being studied). The viewer then sees a sharp image of the target in the astronomical telescope, regardless of whether she tends to accommodate or has some uncorrected refractive error.
Automated lensmeters measure the deflection of a light path as it passes through various parts of the lens. Computerized compilation of this information then reveals how those regions of the lens bend the light.