Key Features
- •
The ability of a transparent medium to bend a ray of light is the basis for most of the instruments used in ophthalmology today.
- •
Spherical lenses, prisms, mirrors, slit-shaped illumination, astronomical and Galilean telescopes, and a multitude of other optical components—both simple and complex—have been devised and manufactured for more than two centuries in order to study the human eye and its function.
Introduction
In this chapter, the basic principles that underlie some of the more common instruments used in ophthalmology will be reviewed, including:
- •
Direct ophthalmoscope.
- •
Binocular indirect ophthalmoscope.
- •
Fundus camera.
- •
Optical coherence tomograph.
- •
Slit-lamp biomicroscope.
- •
Slit-lamp fundus lenses.
- •
Goldmann applanation tonometer.
- •
Specular microscope.
- •
Operating microscope.
- •
Keratometer and corneal topographer.
- •
Lensmeter (also known as lensometer).
- •
Automatic refractor.
- •
Magnifying devices.
Direct Ophthalmoscope
The entire retina, if spread out and flattened, is about the size of a large postage stamp. The important structures themselves are rather small. For example, the optic nerve is 1.5 mm in diameter, and the major blood vessels are only 0.1–0.2 mm in diameter. Significant papilledema, with an elevation of the nerve head of 3.00 D, is equivalent to only a 1 mm change in elevation. Most of the important red and yellow details, including blood vessels, hemorrhages, and exudates, are seen against the light red background of the blood-filled choroid. Subtle changes in the pinkish white backscattered light of the optic disc announce major glaucomatous or neuro-ophthalmic alterations. The presence of the corneal reflection and the usual backscattered light of the healthy cornea and lens make the evaluation of fundus changes even more difficult.
In the face of these obstacles, it seems almost miraculous that the examiner is able to make a significant number of diagnoses using the direct ophthalmoscope. Fig. 2.5.1 illustrates how the ophthalmoscope directs the light rays of illumination and observation coaxially, while the observation system is essentially a peephole. The lens and cornea of the patient’s eye actually create the retinal image. Thus the observer does not really see the retina of the patient but an optical image of the retina.
To bring the red fundus reflex into sharp focus for the viewer, the modern ophthalmoscope has a disc of lenses. Because the compensating lens neutralizes the refractive error of both the physician and the patient and the accommodation of each, its total power provides only a rough estimate of the patient’s refractive state. The magnitude of a large amount of astigmatism may actually be estimated if the lens is focused on a blood vessel that travels parallel to the foveal reflex and then refocused on a vessel that travels perpendicular to the first vessel.
Probably the most important advance in direct ophthalmoscopy was the use of the halogen tungsten bulb, which has a number of advantages over the older tungsten bulb. A quartz jacket can withstand higher temperatures than can the glass jacket, thus the filament temperature may be raised higher than that in the conventional tungsten bulb to produce an increased lumen output.
The field of view of the modern direct ophthalmoscope averages about 10° and is limited by the most oblique pencil of rays that can pass from the outer edge of the observer’s pupil to the opposite outer edge of the patient’s pupil. To enlarge the field of view of the direct ophthalmoscope, the investigator’s eye must be brought closer to the patient’s eye with the patient’s pupils dilated.
Because the enlargement capacity of any magnification lens usually is defined as one-fourth of the lens power, the retinal image in the typical emmetropic eye of 60 D may be considered to be magnified by 60/4, or ×15. In aphakic eyes, from which a 20 D natural lens has been removed, the magnification for the observer is reduced to about 40/4 or ×10.
Binocular Indirect Ophthalmoscope
Compared with the direct ophthalmoscope, the binocular indirect ophthalmoscope gives a wide field of view, a stereoscopic impression, and an image of high contrast. Of course, a small price must be paid for these advantages. The patient’s pupil must be dilated, the instrumentation is larger, heavier, and more expensive, and the illumination is almost painfully bright for the patient.
Illumination System
In order to avoid corneal and lens reflection and scatter, the observation beam and the illumination beam is separated at the corneal and lens plane, requiring a dilated pupil ( Fig. 2.5.2 ). The filament of the bulb is actually brought to a focus in a portion of the patient’s pupil. To minimize the loss of light, the condenser lens brings the observer’s pupil to a focus in the patient’s pupil. With patient and observer pupils conjugate, loss of light is minimized, and field of view is maximized.
Observation System
Contrast
Because the observation beam path is different from the illumination beam path, glare degradation from reflection and backscatter is minimized and subtle details are seen more easily. The observer must learn to tilt the handheld lens strategically to avoid reflection from the surface of the lens itself. This reflection is minimized (from about 4% of incident light to 1%) by a lens that has an antireflection coating.
Inverted Image
The handheld condenser lens creates a real, inverted aerial image of the illuminated patient’s fundus, as expected from a positive lens. Thus the examiner must learn to reorient details from where they appear to be to where they actually are located.
Field of View
Fig. 2.5.3 illustrates how the handheld lens produces the aerial image of the fundus. Rays that pass through the nodal point of the patient’s eye and the edge of the handheld lens determine the size of the field of view. The distance of the handheld lens from the patient’s eye also determines the angular subtense of the patient’s fundus caught by the lens. This distance is optimal if it equals the focal length of the lens. Thus the field of view is determined by the expression d / F, where d is the diameter and F the focal length of the handheld lens.
For example, given equal diameters, stronger lenses (e.g., F = 30 D, f = 3.3 cm) provide larger fields of view. However, a weaker lens may be made with a larger diameter because it is less vulnerable to spherical aberration. Thus a 20 D lens of 3 cm diameter yields almost the same field of view as a 30 D lens of 2 cm diameter.
Magnification
Fig. 2.5.3 shows the chief ray that passes from the edge of the fundus view through the nodal point of the eye to the aerial image. The ratio of the fundal object to the aerial image is proportional to the ratio of the focal length of the patient’s eye to the focal length of the condenser lens, or inversely proportional to the power ( F ) of the eye (60 D) and the handheld lens. Thus for an emmetropic eye and a 20 D lens, the magnification = 60 D/20 D = ×3; for a 30 D lens, the magnification = 60 D/30 D = ×2.
Ultimately, the distance of this mildly magnified aerial image from the observer determines the total magnification. If the observer has a large amplitude of accommodation, the aerial image is brought closer and its overall magnification increased.
Stereopsis
The light beam that emerges from the patient’s dilated pupils is directed through the handheld lens and into the two eyepieces (separation usually 15 mm) of the binocular indirect ophthalmoscope. Prisms then redirect the two beams into the examiner’s eyes. A smaller distance between the two eyepieces than the interpupillary distance reduces the stereopsis appreciated by an observer (interpupillary distance of 60 mm) by about one-fourth. However, axial magnification (which equals one-fourth of the square of lateral magnification) augments the stereoscopic appearance. If the lateral magnification of a 20 D lens is ×3, the axial magnification equals 9/4 or ×2.25. Thus the ophthalmoscopic view through the handheld lens amplifies small changes in retinal topography. Using a lower power hand lens further increases this effect: for example, a 15 D lens results in transverse magnification of 60/15 = 4×, but axial magnification of 16/4 = 4-fold.
Variations of indirect ophthalmoscopy include the scanning laser ophthalmoscope and various analyzers of the optic nerve head.
Illumination System
In order to avoid corneal and lens reflection and scatter, the observation beam and the illumination beam is separated at the corneal and lens plane, requiring a dilated pupil ( Fig. 2.5.2 ). The filament of the bulb is actually brought to a focus in a portion of the patient’s pupil. To minimize the loss of light, the condenser lens brings the observer’s pupil to a focus in the patient’s pupil. With patient and observer pupils conjugate, loss of light is minimized, and field of view is maximized.
Observation System
Contrast
Because the observation beam path is different from the illumination beam path, glare degradation from reflection and backscatter is minimized and subtle details are seen more easily. The observer must learn to tilt the handheld lens strategically to avoid reflection from the surface of the lens itself. This reflection is minimized (from about 4% of incident light to 1%) by a lens that has an antireflection coating.
Inverted Image
The handheld condenser lens creates a real, inverted aerial image of the illuminated patient’s fundus, as expected from a positive lens. Thus the examiner must learn to reorient details from where they appear to be to where they actually are located.
Field of View
Fig. 2.5.3 illustrates how the handheld lens produces the aerial image of the fundus. Rays that pass through the nodal point of the patient’s eye and the edge of the handheld lens determine the size of the field of view. The distance of the handheld lens from the patient’s eye also determines the angular subtense of the patient’s fundus caught by the lens. This distance is optimal if it equals the focal length of the lens. Thus the field of view is determined by the expression d / F, where d is the diameter and F the focal length of the handheld lens.
For example, given equal diameters, stronger lenses (e.g., F = 30 D, f = 3.3 cm) provide larger fields of view. However, a weaker lens may be made with a larger diameter because it is less vulnerable to spherical aberration. Thus a 20 D lens of 3 cm diameter yields almost the same field of view as a 30 D lens of 2 cm diameter.
Magnification
Fig. 2.5.3 shows the chief ray that passes from the edge of the fundus view through the nodal point of the eye to the aerial image. The ratio of the fundal object to the aerial image is proportional to the ratio of the focal length of the patient’s eye to the focal length of the condenser lens, or inversely proportional to the power ( F ) of the eye (60 D) and the handheld lens. Thus for an emmetropic eye and a 20 D lens, the magnification = 60 D/20 D = ×3; for a 30 D lens, the magnification = 60 D/30 D = ×2.
Ultimately, the distance of this mildly magnified aerial image from the observer determines the total magnification. If the observer has a large amplitude of accommodation, the aerial image is brought closer and its overall magnification increased.
Stereopsis
The light beam that emerges from the patient’s dilated pupils is directed through the handheld lens and into the two eyepieces (separation usually 15 mm) of the binocular indirect ophthalmoscope. Prisms then redirect the two beams into the examiner’s eyes. A smaller distance between the two eyepieces than the interpupillary distance reduces the stereopsis appreciated by an observer (interpupillary distance of 60 mm) by about one-fourth. However, axial magnification (which equals one-fourth of the square of lateral magnification) augments the stereoscopic appearance. If the lateral magnification of a 20 D lens is ×3, the axial magnification equals 9/4 or ×2.25. Thus the ophthalmoscopic view through the handheld lens amplifies small changes in retinal topography. Using a lower power hand lens further increases this effect: for example, a 15 D lens results in transverse magnification of 60/15 = 4×, but axial magnification of 16/4 = 4-fold.
Variations of indirect ophthalmoscopy include the scanning laser ophthalmoscope and various analyzers of the optic nerve head.
Contrast
Because the observation beam path is different from the illumination beam path, glare degradation from reflection and backscatter is minimized and subtle details are seen more easily. The observer must learn to tilt the handheld lens strategically to avoid reflection from the surface of the lens itself. This reflection is minimized (from about 4% of incident light to 1%) by a lens that has an antireflection coating.
Field of View
Fig. 2.5.3 illustrates how the handheld lens produces the aerial image of the fundus. Rays that pass through the nodal point of the patient’s eye and the edge of the handheld lens determine the size of the field of view. The distance of the handheld lens from the patient’s eye also determines the angular subtense of the patient’s fundus caught by the lens. This distance is optimal if it equals the focal length of the lens. Thus the field of view is determined by the expression d / F, where d is the diameter and F the focal length of the handheld lens.
For example, given equal diameters, stronger lenses (e.g., F = 30 D, f = 3.3 cm) provide larger fields of view. However, a weaker lens may be made with a larger diameter because it is less vulnerable to spherical aberration. Thus a 20 D lens of 3 cm diameter yields almost the same field of view as a 30 D lens of 2 cm diameter.
Magnification
Fig. 2.5.3 shows the chief ray that passes from the edge of the fundus view through the nodal point of the eye to the aerial image. The ratio of the fundal object to the aerial image is proportional to the ratio of the focal length of the patient’s eye to the focal length of the condenser lens, or inversely proportional to the power ( F ) of the eye (60 D) and the handheld lens. Thus for an emmetropic eye and a 20 D lens, the magnification = 60 D/20 D = ×3; for a 30 D lens, the magnification = 60 D/30 D = ×2.
Ultimately, the distance of this mildly magnified aerial image from the observer determines the total magnification. If the observer has a large amplitude of accommodation, the aerial image is brought closer and its overall magnification increased.
Stereopsis
The light beam that emerges from the patient’s dilated pupils is directed through the handheld lens and into the two eyepieces (separation usually 15 mm) of the binocular indirect ophthalmoscope. Prisms then redirect the two beams into the examiner’s eyes. A smaller distance between the two eyepieces than the interpupillary distance reduces the stereopsis appreciated by an observer (interpupillary distance of 60 mm) by about one-fourth. However, axial magnification (which equals one-fourth of the square of lateral magnification) augments the stereoscopic appearance. If the lateral magnification of a 20 D lens is ×3, the axial magnification equals 9/4 or ×2.25. Thus the ophthalmoscopic view through the handheld lens amplifies small changes in retinal topography. Using a lower power hand lens further increases this effect: for example, a 15 D lens results in transverse magnification of 60/15 = 4×, but axial magnification of 16/4 = 4-fold.
Variations of indirect ophthalmoscopy include the scanning laser ophthalmoscope and various analyzers of the optic nerve head.
Fundus Camera
Lighting
The present illumination system of the Zeiss fundus camera is shown in Fig. 2.5.4 . Light from the incandescent lamp (for observation) and the flash lamp (for photography) is superimposed by means of the beam splitter, so that the light from the flash travels along the same path as that of the observation light. The lamp filaments are imaged in the vicinity of the holed mirror, which deflects the light toward the eye. The holed mirror is similar to the mirrors used in many retinoscopes, with a central hole to allow observation. In contrast to the retinoscope arrangement, the holed mirror in the fundus camera is imaged onto the plane of the patient’s pupil by the objective lens to ensure the necessary separation of illumination and observation pathways at the pupil. The objective lens corresponds to the condenser lens in the indirect ophthalmoscope; both lenses are designed with aspherical surfaces to provide the best possible image quality over a wide field of view.
Photographing through an undiluted pupil was achieved by observation of the fundus using infrared light, which does not stimulate the retina. In such a system, the infrared light penetrates the 4–5 mm diameter pupil in a dark setting, is reflected off the retina, and is displayed on a monitor. Once the retina has been focused and framed, an electronic flash illuminates the retina before the pupil constricts.
Reducing Reflections From the Cornea and Instrument
Anyone who has tried to evaluate subtle macular detail using a direct ophthalmoscope is familiar with the annoyance of corneal reflection. Gullstrand’s table-mounted ophthalmoscope embodied his principle that the illumination system should not intersect the cornea in the same area as the rays that come from the observation system. In the Zeiss system, this principle is satisfied by a mirror that has a central hole. The mirror reflects a circle of light through the pupillary periphery while the fundus is viewed through the central hole of the mirror. Although such a system is not able to completely eliminate ocular and camera lens reflections, it successfully reduces them to a significant degree.
The Observation System
Fig. 2.5.4 illustrates all the elements of the fundus camera. As noted before, light from the incandescent lamp and that from the flash lamp are folded into a common path, which ultimately strikes the holed mirror and is reflected into the patient’s eye. This illumination system and the observation system are very similar to those of the indirect ophthalmoscope. In the case of the fundus camera, light reflected from the patient’s retina passes through the hole in the mirror and focuses a real image in the film plane of the camera. A beam splitter diverts a portion of the light directed to the camera and sends it to the eyepiece. In essence, the eyepiece is like a simple microscope. It receives the real image of the fundus and processes it such that parallel light exits for the observer.
Field of View
In theory, a 180° aerial image of the retina can be captured, even through a small pupil. In practice, however, because the rays from the equator exit the pupil at a very sharp angle, the collection of these rays can be accomplished only by a lens held very close to the pupil or a lens of very wide diameter. Of course, although wide-diameter lenses produce a greater field of view, they also introduce significant spherical aberration. For fields of view greater than 100°, therefore, the only sensible way to collect the sharply bent rays that come from the retinal periphery is to move the front lens close to the pupil. Thus in the equator-plus camera, the front lens of the system is a contact lens. Because the aerial image of such a large expanse of retina follows the globe’s curvature, special lenses must be introduced into this system to flatten the image.
To photograph different fields of view, three different focal length lenses are used, much as the ophthalmologist might switch between +14 D, +20 D, and +28 D handheld lenses. The lens system that has the longer focal length (less dioptric power) produces a more magnified image. Thus the amount of field captured in a frame is smaller than that produced by the more powerful, shorter focal length lens. Theoretically, both the larger field and higher magnification could be obtained if the size of the film could be doubled.
Portions of the retina can be photographed beyond the traditional central 30° if the camera is directed to the peripheral area of interest. However, a camera aimed off axis by 30° or more with the 60 D ocular optical system induces 10–15 D of oblique astigmatism and results in fuzzy pictures. Fortunately, well-designed fundus cameras anticipate off-axis photography and include a large range of cylindrical corrections with which to sharpen the peripheral views.
Lighting
The present illumination system of the Zeiss fundus camera is shown in Fig. 2.5.4 . Light from the incandescent lamp (for observation) and the flash lamp (for photography) is superimposed by means of the beam splitter, so that the light from the flash travels along the same path as that of the observation light. The lamp filaments are imaged in the vicinity of the holed mirror, which deflects the light toward the eye. The holed mirror is similar to the mirrors used in many retinoscopes, with a central hole to allow observation. In contrast to the retinoscope arrangement, the holed mirror in the fundus camera is imaged onto the plane of the patient’s pupil by the objective lens to ensure the necessary separation of illumination and observation pathways at the pupil. The objective lens corresponds to the condenser lens in the indirect ophthalmoscope; both lenses are designed with aspherical surfaces to provide the best possible image quality over a wide field of view.
Photographing through an undiluted pupil was achieved by observation of the fundus using infrared light, which does not stimulate the retina. In such a system, the infrared light penetrates the 4–5 mm diameter pupil in a dark setting, is reflected off the retina, and is displayed on a monitor. Once the retina has been focused and framed, an electronic flash illuminates the retina before the pupil constricts.
Reducing Reflections From the Cornea and Instrument
Anyone who has tried to evaluate subtle macular detail using a direct ophthalmoscope is familiar with the annoyance of corneal reflection. Gullstrand’s table-mounted ophthalmoscope embodied his principle that the illumination system should not intersect the cornea in the same area as the rays that come from the observation system. In the Zeiss system, this principle is satisfied by a mirror that has a central hole. The mirror reflects a circle of light through the pupillary periphery while the fundus is viewed through the central hole of the mirror. Although such a system is not able to completely eliminate ocular and camera lens reflections, it successfully reduces them to a significant degree.
The Observation System
Fig. 2.5.4 illustrates all the elements of the fundus camera. As noted before, light from the incandescent lamp and that from the flash lamp are folded into a common path, which ultimately strikes the holed mirror and is reflected into the patient’s eye. This illumination system and the observation system are very similar to those of the indirect ophthalmoscope. In the case of the fundus camera, light reflected from the patient’s retina passes through the hole in the mirror and focuses a real image in the film plane of the camera. A beam splitter diverts a portion of the light directed to the camera and sends it to the eyepiece. In essence, the eyepiece is like a simple microscope. It receives the real image of the fundus and processes it such that parallel light exits for the observer.
Field of View
In theory, a 180° aerial image of the retina can be captured, even through a small pupil. In practice, however, because the rays from the equator exit the pupil at a very sharp angle, the collection of these rays can be accomplished only by a lens held very close to the pupil or a lens of very wide diameter. Of course, although wide-diameter lenses produce a greater field of view, they also introduce significant spherical aberration. For fields of view greater than 100°, therefore, the only sensible way to collect the sharply bent rays that come from the retinal periphery is to move the front lens close to the pupil. Thus in the equator-plus camera, the front lens of the system is a contact lens. Because the aerial image of such a large expanse of retina follows the globe’s curvature, special lenses must be introduced into this system to flatten the image.
To photograph different fields of view, three different focal length lenses are used, much as the ophthalmologist might switch between +14 D, +20 D, and +28 D handheld lenses. The lens system that has the longer focal length (less dioptric power) produces a more magnified image. Thus the amount of field captured in a frame is smaller than that produced by the more powerful, shorter focal length lens. Theoretically, both the larger field and higher magnification could be obtained if the size of the film could be doubled.
Portions of the retina can be photographed beyond the traditional central 30° if the camera is directed to the peripheral area of interest. However, a camera aimed off axis by 30° or more with the 60 D ocular optical system induces 10–15 D of oblique astigmatism and results in fuzzy pictures. Fortunately, well-designed fundus cameras anticipate off-axis photography and include a large range of cylindrical corrections with which to sharpen the peripheral views.
Optical Coherence Tomography
Optical coherence tomography (OCT) is based on the Michaelson interferometer invented in the late 1800s. Originally the instrument was used to make extremely accurate measurements of length. A single beam of white light is split into two beams moving in perpendicular directions. The beams are reflected back to, and recombine at, the beam splitter. When the beams recombine, interference fringes are observed, provided that the difference in optical path length (OPL) between the two arms of the interferometer is less than the coherence length of the light utilized. Michaelson used white light with a coherence length of 1–2 µm. One arm of the device had a fixed, known length, and the length of the other arm was varied until interference fringes were observed, at which point the difference in OPL between the two arms had to be less than 2 µm. Submicron accuracy could be achieved by counting fringes.
If this technique were directly applied to retinal imaging, only a single layer’s thickness could be measured because of the low coherence of white light. However, if a laser is used, the coherence length is too long and the position of a retinal layer could be localized only to within a few centimeters. The OCT utilizes a superluminescent diode that operates in the near-infrared and has a bandwidth of about 50 nm or about six times more coherent than white light but far less than the coherence of a laser.
The original clinical instruments incorporated two moving mirrors. One varied the length of one arm of the interferometer and was used every time the device scanned a narrow section of retina ( Fig. 2.5.5 ). After a single scan was completed, the other mirror shifted the beam to the neighboring section of the retina and the next scan was repeated. An important limitation was the amount of time required to complete a scan. The time required to scan a single line of retina a few millimeters in length far exceeded even the best patient’s fixation. Moreover, the eye itself is not dimensionally stable due to choroidal blood flow that varies with the cardiac cycle. Sophisticated software can overcome some fixation requirements, but limitations exist to this technique.
In spectral domain instruments one of the mirrors (which scans the reference arm) is replaced by a spectrometer that measures the reflection from each wavelength simultaneously, producing a much faster scan that not only improves accuracy but also enables a larger region of retina to be interrogated. It is important to realize that OCT measures optical path length not physical thickness. The presence of edema or other pathologies that differ in refractive index from the retina can distort the apparent thickness of the pathology, leading to imaging artifacts that can complicate interpretation of OCT images.
Slit-Lamp Biomicroscope
The slit lamp is the piece of equipment most frequently used by the ophthalmologist. With the addition of auxiliary lenses, it can give unique, magnified views of every part of the eye. In conjunction with auxiliary devices it can be used to take photographs and to make quantitative measurements, including intraocular pressure, endothelial cell counts, pupil size, corneal thickness, anterior chamber depth, and others.
Illumination
The modern slit lamp produces an intensity of about 200 mW/cm 2 . When operated at the rated voltage, halogen lamps have a higher luminance and color temperature than do conventional incandescent lamps. For slit-lamp work, a high color temperature (e.g., a greater amount of blue light) is useful. Because many of the ocular structures are seen via light scatter, and because the shorter wavelengths are scattered most, a light with a high blue component illuminates the structures best. The light is first brought to a focus at the slit aperture ( Fig. 2.5.6 ), and the light within the slit is focused by the condensing lens onto the patient’s eye.
Improving Tissue Contrast
One of the great strengths of the modern slit lamp is the way in which contrast can be improved by various maneuvers:
- •
Optical sectioning: As the beam is narrowed, the scattered light of adjacent tissue is removed and greater detail of the optical section is seen.
- •
Tangential illumination: When the light is brought in from the side, highlights and shadows become stronger, and the texture (i.e., elevations and depressions) is seen better.
- •
Pinpoint illumination: The cells and flare in the anterior chamber in a patient who has iritis are best seen using a narrow beam focused into the aqueous, so that the black pupil becomes the background. The combination of the narrow beam and the dark pupillary background eliminates any extraneous light that would reduce contrast. The same principle holds when the examiner pushes the lower lid up to examine the tear meniscus. For example, the stagnant cell pattern of an obstructed tear duct is best seen using a narrow beam with the dark iris in the background.
- •
Specular reflection: In this technique the angle of observation is set to equal the angle of illumination. In this way, the structure of the front surfaces of the cornea (i.e., ulcers, dry areas) and the rear surfaces (endothelial pattern) may be assessed.
- •
Proximal indirect illumination: In this technique a moderately wide beam is directed to the areas adjacent to the area of interest. Against a dark background, the backscattered light from the lesion yields a higher contrast, which often allows the observer to see the borders of the lesion more precisely. For example, when this technique is used, subtle corneal edema—with its minute pools of fluid—stands out more distinctly against a dark pupil.
- •
Sclerotic scatter: With the slit illuminator offset from its isocentric position, light is directed to the limbus. The light then follows the cornea as if it were a fiberoptic element and reaches the other side of the limbus. However, if a lesion or particles within the cornea exist, the backscattered light from the lesion or particles is seen clearly against the dark pupillary background.
- •
Retroillumination from the fundus: Light sent through the pupil to the fundus is reflected and yields an orange background. Holes in the iris or subtle wrinkles in the cornea become silhouetted and much easier to see.
Observation System
The observation system of the slit-lamp biomicroscope has a long working distance of about 3.9 inches (10 cm), which is about 100 times longer than that of a laboratory microscope. Prisms take the divergent rays from the patient’s eye and force them to emerge as parallel pencils from each eyepiece. Thus a stereoscopic appreciation of the patient’s eye is achieved without convergence of the observer’s visual axis. Most slit-lamp microscopes offer magnifications between ×5 and ×50, with ×10, ×16, and ×25 being the most popular. Image resolution is ultimately limited by diffraction.
Illumination
The modern slit lamp produces an intensity of about 200 mW/cm 2 . When operated at the rated voltage, halogen lamps have a higher luminance and color temperature than do conventional incandescent lamps. For slit-lamp work, a high color temperature (e.g., a greater amount of blue light) is useful. Because many of the ocular structures are seen via light scatter, and because the shorter wavelengths are scattered most, a light with a high blue component illuminates the structures best. The light is first brought to a focus at the slit aperture ( Fig. 2.5.6 ), and the light within the slit is focused by the condensing lens onto the patient’s eye.
Improving Tissue Contrast
One of the great strengths of the modern slit lamp is the way in which contrast can be improved by various maneuvers:
- •
Optical sectioning: As the beam is narrowed, the scattered light of adjacent tissue is removed and greater detail of the optical section is seen.
- •
Tangential illumination: When the light is brought in from the side, highlights and shadows become stronger, and the texture (i.e., elevations and depressions) is seen better.
- •
Pinpoint illumination: The cells and flare in the anterior chamber in a patient who has iritis are best seen using a narrow beam focused into the aqueous, so that the black pupil becomes the background. The combination of the narrow beam and the dark pupillary background eliminates any extraneous light that would reduce contrast. The same principle holds when the examiner pushes the lower lid up to examine the tear meniscus. For example, the stagnant cell pattern of an obstructed tear duct is best seen using a narrow beam with the dark iris in the background.
- •
Specular reflection: In this technique the angle of observation is set to equal the angle of illumination. In this way, the structure of the front surfaces of the cornea (i.e., ulcers, dry areas) and the rear surfaces (endothelial pattern) may be assessed.
- •
Proximal indirect illumination: In this technique a moderately wide beam is directed to the areas adjacent to the area of interest. Against a dark background, the backscattered light from the lesion yields a higher contrast, which often allows the observer to see the borders of the lesion more precisely. For example, when this technique is used, subtle corneal edema—with its minute pools of fluid—stands out more distinctly against a dark pupil.
- •
Sclerotic scatter: With the slit illuminator offset from its isocentric position, light is directed to the limbus. The light then follows the cornea as if it were a fiberoptic element and reaches the other side of the limbus. However, if a lesion or particles within the cornea exist, the backscattered light from the lesion or particles is seen clearly against the dark pupillary background.
- •
Retroillumination from the fundus: Light sent through the pupil to the fundus is reflected and yields an orange background. Holes in the iris or subtle wrinkles in the cornea become silhouetted and much easier to see.
Observation System
The observation system of the slit-lamp biomicroscope has a long working distance of about 3.9 inches (10 cm), which is about 100 times longer than that of a laboratory microscope. Prisms take the divergent rays from the patient’s eye and force them to emerge as parallel pencils from each eyepiece. Thus a stereoscopic appreciation of the patient’s eye is achieved without convergence of the observer’s visual axis. Most slit-lamp microscopes offer magnifications between ×5 and ×50, with ×10, ×16, and ×25 being the most popular. Image resolution is ultimately limited by diffraction.
Slit-Lamp Fundus Lenses
Because the cornea has such a high refractive power, the slit-lamp microscope can view only the first one-third of the eye. Special lenses, in conjunction with the slit-lamp microscope, can be used to view the posterior vitreous and the posterior pole retina. The two ways to overcome the high corneal refractive power are (1) nullifying the corneal power, or (2) utilizing the power of the cornea as a component of an astronomical telescope in a manner similar to that exploited by the indirect ophthalmoscope.
The Goldmann contact lens ( Fig. 2.5.7 ) and other similar lenses work in conjunction with the slit-lamp microscope to nullify the dioptric power produced by the corneal curvature and to bring the retina into the focal range of the slit-lamp microscope. These plano-concave contact lenses are placed on the cornea, forming virtual, erect, and diminished images of the illuminated retina near the pupillary plane within the focal range of the slit-lamp microscope.
The Hruby lens is a powerful plano-concave lens, −58.6 D in power. It is held immediately in front of the cornea, forming a virtual, erect, and diminished image of the illuminated retina near the pupillary plane, bringing it within focal range ( Fig. 2.5.8 ).
The 60 D, 78 D, and 90 D funduscopic lenses ( Fig. 2.5.9 ) use a different approach to view the posterior vitreous and posterior pole retina. These lenses act as high-powered, biconvex, condensing lenses, projecting an inverted, real image in front of the lens within focal range. This is the same optical principle used by the indirect ophthalmoscope: the higher the power of the lens, the lower the magnification of the image.
The Goldmann three-mirror contact lens ( Fig. 2.5.10 ), as its name implies, incorporates three internal mirrors. The contact lens nullifies the refractive power of the patient’s cornea, and the three mirrors then reflect light from the patient’s midperipheral retina, peripheral retina, and the iridocorneal angle, respectively. The posterior pole of the fundus can be visualized, also, in a manner similar to that of the Goldmann posterior pole contact lens.
The Quadraspheric, SuperQuad, and similar lenses are corneal contact lenses ( Fig. 2.5.11 ).
A real, inverted aerial image of the fundus is formed a few millimeters outside the large aspherical condensing lens, which is within the focal range of the slit-lamp microscope. Because the condensing lens is so close to the eye and has such a high power, the field of view is very wide, making these lenses specially suited for a wide-angle view of the posterior pole and midperipheral fundus.
Goldmann Applanation Tonometer
The applanation tonometer ( Fig. 2.5.12 ) is used to measure intraocular pressure. It relies on an interesting physical principle. For an ideal, dry, thin-walled sphere, the pressure inside a sphere is proportional to the force applied to its surface. Unlike an ideal sphere, however, the human eye is not thin-walled and it is not dry, producing two confounding forces: (1) a force produced by the eye’s scleral rigidity (because the eye is not thin-walled), directed away from the globe; and (2) a force produced by the surface tension of the tear film (because the eye is not dry), directed toward the globe ( Fig. 2.5.13 ). Goldmann determined that when a flat surface is applied to the cornea with enough force to produce a circular area of flattening 3.06 mm in diameter, then the force caused by scleral rigidity exactly cancels out the force caused by surface tension. Therefore it is a very useful fact that the applanating force required to flatten a circular area of cornea exactly 3.06 mm in diameter is directly proportional to the intraocular pressure. Specifically, the force (measured in dynes) multiplied by 10 is equal to the intraocular pressure (measured in millimeters of mercury).
How does the observer know when the area of applanation is exactly 3.06 mm in diameter so that the intraocular pressure can be measured? The applanation tonometer is mounted on a biomicroscope to produce a magnified image. When the cornea is applanated, the tear film, which rims the circular area of applanated cornea, appears as a circle to the observer. The tear film often is stained with fluorescein dye and viewed under a cobalt-blue light to enhance the visibility of the tear film ring. Higher pressure from the tonometer head causes the circle to have a wider diameter because a larger area of cornea becomes applanated ( Fig. 2.5.14 ). Split prisms, each mounted with their bases in opposite directions, are mounted in the applanation head, creating two images offset by exactly 3.06 mm. The clinician looks through the applanation head and adjusts the pressure until the half circles just overlap one another ( Fig. 2.5.15 ). At this point, the circle is exactly 3.06 mm in diameter, and the reading on the tonometer (multiplied by a factor of 10) represents the intraocular pressure in millimeters of mercury ( Fig. 2.5.16 ).
Specular Microscope
A number of significant obstacles stand in the way of easy microscopical observation of the living corneal endothelium. First, the reflection from the front corneal surface interferes with a sharp view of the endothelium. Second, the intervening stromal layers backscatter light, which decreases the contrast of the endothelial details. In addition, when the stroma becomes thick and edematous, the views of the endothelium become hazy. Finally, because of the small difference in index of refraction between the cornea (1.376) and the aqueous (1.336), only 0.02% of the incident light (for most angles of incidence) is reflected from the interface between corneal endothelium and aqueous.
To eliminate the bothersome reflection from the front corneal surface, two approaches are used. An increase in the angle of incidence moves the anterior reflection to the side so it covers less of the specular reflection from the endothelium. This approach alone is used in the noncontact technique. If the cornea could be thickened artificially (without an increase in light scatter), this would move the surface reflection further to the side. With use of a contact lens that has a coupling fluid of index of refraction similar to that of the cornea, the surface reflection is eliminated and the corneal thickness may be assumed to include the contact lens thickness also. The reflection from the surface of the contact lens replaces that of the corneal surface. However, because of the thickness of the contact lens, the surface reflection is moved well over to the side ( Fig. 2.5.17 ).
