Kevin Greuloch, MD
Introduction
The field of ophthalmology relies heavily on the development and use of instruments to obtain measurements that allow a clinician to diagnose and treat (and, hopefully, improve the lives of) patients. Some instruments are highly specialized and seldom used. However, other instruments are indispensable and would cause a clinic to grind to a halt if removed. Herein will be discussed four of the latter: the lensometer, the slit-lamp biomicroscope, the keratometer, and the indirect ophthalmoscope.
The Lensometer
Almost every examination of a new patient includes an evaluation of existing spectacle corrections. The lensometer is the instrument clinicians use to determine the spherical and cylindrical power and the cylindrical axis of an unknown lens (Slide 1). The goal of the lensometer is to determine the vertex power, or effective power. The reciprocal of vertex power is the back focal length, which is defined as the distance from the back surface of the lens to its secondary focal point.
Slide 2 is a schematic of a highly simplified lensometer. The illuminated target to the left of the diagram is in a grid pattern, and the distance is varied from the lens of unknown power. The target's position is controlled by a dial, which is rotated by the clinician. The light from the target is refracted by the unknown lens and is viewed through the telescopic eyepiece. By definition, when the target is moved to the secondary focal point of the unknown lens, the light reaching the eyepiece will be parallel. Because the telescope is focused for optical infinity, only parallel rays of light will be clear to the observer looking through the eyepiece, and the target will be seen in sharp focus when it is positioned at the secondary focal point of the unknown lens. This distance (U) is measured and converted to diopters on the scale on the instrument's dial. However, challenges exist with this type of arrangement. For a lens of 0.5 D, one would need an instrument that is 2 m long, which is an impractical size for an office.
A clinical lensometer includes an additional lens (the field or optometer lens) to eliminate this issue. The lens is placed so that its focal point falls on the posterior vertex of the unknown lens. Compare Slide 3 to the simple lensometer. The illuminated grid target is moved back and forth until the target appears in sharp focus to the observer. At this point, several things occur. The target grid is imaged by the fixed field lens. The target is between the field lens and the field lens' focal point; thus, a virtual image is created (to the leftmost portion of Slide 3). This distance is exactly the focal point of the unknown lens, because that is the only place the image could be and still produce parallel light rays exiting the system and entering the telescopic eyepiece. Overall, the field lens places a virtual image at the distance required, thus eliminating the need to physically place it there and, therefore, allowing a much more compact and practical instrument. The other advantage of this setup is that because the fixed field lens has a focal point placed at the vertex of the unknown lens, the distance from the fixed field lens and the target varies linearly with the dioptric power of the unknown lens, meaning that the lensometer scale is linear, which makes it straightforward to use. Badal's principle is the term for this alignment and is described with examples in Rubin's text.1
To determine the actual power of a lens that has a cylindrical component, first the principal meridian is aligned and focused. Most lensometers use a trilinear target to facilitate obtaining the most accurate axis of the cylinder. The second target, which is set 90° from the first target, is then refocused. These two powers, along with the cylindrical axis, make up the final prescription of the unknown lens.
Many offices now use an automatic lensometer. An automatic lensometer measures all of the meridians' powers, and the meridians with the greatest power difference are used to calculate the prescription of the lens without a need for human input. This system is quite sensitive to any tilting of the lens. For a detailed explanation of the inner workings of the automatic lensometer, see Miller's text.2
The following is a clinical point regarding the determination of bifocal add power. As opposed to the distance prescription, the front focal length is a better estimate of the lens' effective power. To assess this power, one must turn the glasses around, so that the temples are facing the observer. If this is not done, then the error is most pronounced on high dioptric correction glasses, such as aphakic spectacles. In this case, the indicated add power would be higher than the actual effectiveness of the bifocal segment of the lens.
The Slit Lamp Biomicroscope
History
The slit lamp biomicroscope (Slide 4) has evolved into its present form over many centuries. Optics and lens technology advances led to the binocular loupe system, which was used for many years. Purkinje attempted to develop a two-handed system in 1823, with a hand-held magnifying lens and an additional hand-held lens to focus the illumination source. Gullstrand developed the slit-lamp illuminator in 1911, whereas Nernst had developed a bright light source. Czapski's binocular corneal microscope was combined with Henker's illuminator in 1916, and the slit-lamp biomicroscope was born. Comberg developed the co-pivotal and isocentric relationship between the slit illuminator and the biomicroscope in 1926, allowing for the various methods of illuminating the eye. Goldmann added the multi-functional "stick" control in 1938. The slit-lamp biomicroscope used today is quite similar to Goldmann's models from almost 70 years ago.2
Construction
All slit lamps have two basic components: a light source and a viewer. The light source is the slit-lamp illuminator and allows for the flexibility of beam height, width, angle, and intensity. The intensity is about 200 mW/cm2, which allows for visualization of the ocular structural changes that may be too subtle to see with a non-varying direct light source.
As can be seen from Slide 5, a condensing lens is used to focus the light source onto the slit aperture. The light then passes through the filter tray (and, thus, whichever filter is being used), and then through another condensing lens that focuses the slit beam onto the reflecting mirror and then to the patient's eye. These lens surfaces have anti-reflective coatings that contribute to a 20% brighter light on the patient's eye.
Additional features of the slit lamp include filters, gauges, and light beam aiming adjustments. The filter options usually include a red free, a blue (approximately 490 nm for exciting fluorescein), and a neutral density for decreasing the light intensity. Gauges are calibrated to indicate the exact height of the beam to make ocular size measurements (of a corneal ulcer, for example). The slit lamp usually contains an adjustment knob that allows alteration of the isocentric relationship, thus allowing for indirect forms of illumination of the structures being examined. Additionally, the beam can be rotated from its usual vertical orientation.
The viewer is the biomicroscope, which is a binocular instrument mounted to a movable arm that enables magnified and three-dimensional viewing. The magnification range is typically from 6X to 25X. Magnifications >25X tend to increase image blur because of fine eye movements and do not augment resolution. The interpupillary distance can be tailored to the observer, as well as the dioptric correction to allow for non-emmetropes to use the instrument without their refractive correction.
The two separate components can only synergistically aid the observer if they provide illumination in the field of the observer's view regardless of the position of the individual components. The slit lamp and the biomicrosope must be aimed at the same point in space no matter what angle exists between the two components, which is accomplished by their copivotal mounting that allows them to be centered on the same point, thus ensuring an isocentric relationship. Additionally, these two components must be positioned such that the slit beam of light falls into sharp focus at the plane of focus of the biomicroscope. This is called the parfocal relationship of the slit beam and the biomicroscope.3
Accessories can be added to the slit-lamp instrument including an applanation tonometer, a pachymeter, a potential acuity meter, and a beam splitter for an observer sidepiece or a camera.
The Keratometer
The curvature of the cornea is important to determine clinically, both to assess disease states such as keratoconus as well as for refractive purposes, as in contact lens fittings and IOL calculations. The anterior surface of the cornea acts as a convex mirror (a strong one, with a power of approximately -250 D), reflecting light projected onto it. This reflection can be seen with any hand-held light, and can be used to estimate high degrees of astigmatism or irregularity. The design of the keratometer provides a measurement of the size of the reflected image, and leads to the determination of the radius of curvature of the anterior corneal surface (Slide 6).
On a historic note, in 1619, Christopher Scheiner used the above concept to determine the radius of curvature of an unknown by comparing the reflection of a window frame on a patient's cornea to the size of the same window frame on a series of metal balls.2 By matching the size of the reflection, and knowing the diameter of that specific shiny metal ball, he could determine the radius of curvature of the patient's cornea. This is essentially all that a keratometer does, although several tricks are used to put all of the "metal balls" on a dial in numerical format to be easily read by the clinician.
The radius of curvature of any mirror determines the size of the images it produces from a given object. Thus, knowing the object's size and distance from the cornea, and measuring the image's size, leads to the solution of the unknown: the radius of curvature, which is best illustrated with Slide 7.
The object "x" in Slide 7, to the left in the slide, represents the lighted mires inside the keratometer. The object distance (U in Slide 7, 75 mm in a keratometer) and the reflected, virtual image (x prime) distance (F, or r/2) are fixed, which is accomplished by the use of a small, fixed-focus, high-powered eyepiece telescope that only allows clear focus of the corneal reflection at a precise and fixed distance from the corneal surface. By measuring the size of the image, one can use these relations to solve for the image distance and, thus, the radius of curvature:
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Solving this equation for r
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Because the distance "2U" and the size of the object mires "O" are fixed and known, then the radius of curvature "r" is directly proportional to the size of the corneal image "I." Therefore, by finding the size of the image "I," one can easily calculate "r" and, thus, know the curvature of the anterior surface of the cornea. The next few paragraphs explain how image size is determined.
As can be seen from Slide 7, the image size (I) is small relative to the object (the mires in a keratometer are only 64 mm to start with). But, the real challenge is that the patient's eye is moving to a small degree no matter how hard he or she tries to hold still. The concept of doubling was used to counter this difficulty and make the keratometer possible.
Doubling uses two prisms that are mounted with their bases attached to each other. This biprism doubles the view of whatever source enters the setup. Objects will be doubled and by varying the distance from the biprism to the object, the two images can be made to touch. This point allows one to determine the exact size of the image. When the two images are in contact with each other, the biprism splits the single image into two images separated by exactly one-half the length of the object's size (thus allowing the two images to touch). Therefore, because the image-object distance, object size, and prism power are known, making the images touch will allow one to solve for the image size. The reason this makes keratometry possible is that any fine movement of the cornea affects the two split images equally. Therefore, adjusting the knobs and lining up the targets (usually a – and a ’’’) is not reliant on perfect stillness of the cornea because the separation between these doubled images does not change with small oscillations.
The keratometer takes this concept one step further and has two biprism-like setups that are 90° apart. One target is split along the radius of greatest curvature, the other along the radius of least curvature, allowing for cylindrical measurement.
The image size obtained from the doubling process is used to calculate the radius of curvature. Taking the refractive index of the cornea into account, the radius of curvature is then converted to the dioptric power of the cornea, which is the number indicated on the side of the dial. Simply lining up the mires by rotating the dials allows the clinician to quickly and accurately determine the corneal curvature, as long as it is between 36.0 D and 52.0 D, the range of the keratometer. Auxiliary lenses can be added to measure outlying curvatures, such as in keratoconus and cornea plana.
The Indirect Ophthalmoscope
The indirect ophthalmoscope (Slide 8) is used to illuminate and view the retina in a magnified and stereoscopic fashion, and provides the advantages of a wide field of view with a high contrast. The indirect ophthalmoscope includes a light source and two prismatic convex viewing lenses that are worn on the observer's head and a condensing lens. The reflected light is projected out of a patient's eye from its retinal source. This light can then be condensed into a viewable image for the observer. This image is inverted because it is located at the secondary focal plane of the condensing lens (eg, the +20 D or +14 D lenses). The image has depth due to the axial magnification, which varies as a square of the lateral magnification of the lens. Therefore, condensing lenses with longer focal lengths (lower "numbers") will have more lateral magnification and axial magnification and, therefore, will produce a thicker image, giving an exaggerated stereoscopic view.
To reduce glare and backscatter, the light rays that form the observation beam path must be separated from the light rays that are illuminating a subject's retina (Slide 9). Thus, the illuminating light rays exit the indirect headpiece above the viewer apertures. A slight tilting of the lens also assists with this goal, as do antireflective coatings.
Slide 10 illustrates the different components of the indirect ophthalmoscope and the tracings of the light rays as they reflect from a patient's retina and through the condensing lens (note that the depth of view increases as a square function of the lateral magnification).
The inverted, floating image at the secondary focal plane of the condensing lens is not of any value until it is directed onto the observer's retina. Slide 11 shows that there exists only a small number of actual light rays from the image that enter the observer's pupils.
The small area in Slide 11 corresponds to a cone of light rays that emanated from the patient's pupil and, as a result, the closer the observer's eye is to the patient, the larger the pupils must be. As a corollary, if a patient has a relatively small pupil size, then the observer can back away and still successfully examine the patient's retina. The disadvantage to this relationship is that the image is farther from the observer's retina and, therefore, subtends a smaller space on the observer's retina (the image looks smaller).
The stereoscopic nature of the image is provided by the viewing headpiece, which creates an image for each eye of the observer. Because the observer's interpupillary distance (P.D.) is approximately 60-70 mm and the pupil is only 10 mm wide with maximal dilation, then optically reducing the interpupillary distance is required. This reduction is accomplished by totally internal-reflecting prisms that decrease the effective interpupillary distance to about 15 mm (Slide 12).
These prisms can be adjusted to any P.D. to fit the observer. The 15 mm optically effective P.D. remains unchanged, as can be seen from Slide 12. Also built into these viewing pieces is a plus lens of approximately 2 D of power. This is to lower the observer's accommodative requirement while viewing the aerial image (thus lowering eye strain as well as reducing the accommodation-convergence of the observer, which is unnecessary with this configuration). One can imagine, and observe, that depth perception is reduced because the effective P.D. is only 15 mm.
The diameter of the field of view obtained with indirect ophthalmoscopy is tripled compared with direct ophthalmoscopy (25° versus 8°). This means that the total area viewable at one time is increased almost ten-fold. For a given condensing lens, the field of view is directly proportional to the diameter of that lens. So, although a 28-D lens will provide a wider field of view than a 20-D lens, a wider 20-D lens can compete while giving a more magnified view.
Conclusion
The challenge of glare and backscatter was mentioned briefly above. More specifically, three major reflections can interfere with a clear image. Two are from the anterior and posterior surface of the condensing lens and the other is from the patient's cornea. The images of the light source reflected by the lens surface can be moved by tilting the lens, but more tilting is required for higher power lenses. The corneal reflection (of the light source) is also dealt with by slight lateral movements of the lens. These techniques do not need to be consciously planned for as the observer will experiment and almost automatically find the lens position that gives the best image quality.
Slides
Slide 1. A lensometer being used in a clinical setting.
Slide 2. A schematic drawing of a simplified lensometer.
Slide 3. A schematic drawing of a lensometer.
Slide 4. A modern slit lamp biomicrosope being used to examine a patient.
Slide 5. A schematic drawing of a slit lamp biomicroscope.
Slide 6. A patient's corneal power being measured with a keratometer.
Slide 7. Light ray tracings and schema of a keratometer.
Slide 8. A patient's retina being examined with the indirect ophthalmoscope.
Slide 9. The separation of the illuminating light path and the viewing path within the patient's pupil.
Slide 10. Light ray tracings and image representations using the indirect ophthalmoscope.
Slide 11. Path of light rays from the patient's retina to the observer's retina.
Slide 12. Reduction of inter pupillary distance by the prismatic viewing system of the indirect ophthalmoscope.
Slides were produced specifically for this publication, though modified from other texts.
References
- Rubin ML. Optics for Clinicians. 2nd ed. Gainesville, Fl: Triad Scientific Publications; 1974:291-323.
- Miller D. Optics and Refraction, A User Friendly Guide. Vol 1. New York, NY: Gower Medical Publishing; 1991:14.1-14.36.
- Martonyi C, Bahn CF, Meyer RF. Clinical Slit Lamp Biomicroscopy and Photo Slit Lamp Biomicrography. Ann Arbor, Mi: Time One Ink, Ltd; 1985:4-8.