Preliminary examination saves the doctor time in assessment of the patient. A preliminary examination of the eyes trains and alerts the ophthalmic assistant to the numerous variations and abnormalities that occur around the eye and the eyelid. It provides a fascinating change from routine duties and challenges the assistant to sharpen diagnostic acumen and develop an interest in the many major and minor diseases and disorders of the eye.
Vision should be assessed both with and without glasses on a standardized chart, and each eye should be tested independently. It has been found that the normal eye can easily distinguish two points separated by an angle of 1 minute to the eye. By convention, most visual acuity charts are constructed so that the sections of a letter subtend 1 minute of arc. Each letter is printed on squares made up of five parts in each direction so that the whole letter to be identified subtends a 5-minute angle to the eye ( Fig. 8.1 ).
Visual acuity (VA) is determined by the smallest object that can be clearly seen and distinguished at a distance. The commonly used Snellen charts consist of letters carefully designed to subtend a 5-minute angle to the eye at certain specified distances ( Fig. 8.2 ). Generally speaking, 20 feet (6 m) has been considered a practical distance for assessing vision for distance, and the charts have been calibrated with this in mind ( Fig. 8.3 ). At 20 feet, the distant rays of light from an object are practically parallel and very little effort of accommodation is required. In rooms that are shorter than 20 feet, mirrors may be used to achieve the required distance. Also charts may be proportionately reduced in size to compensate for a room with a shorter working distance.
The results of vision testing are expressed as a fraction. The numerator denotes the distance the patient is from the chart letters and the denominator denotes the distance from the chart at which a normal person can see the chart letters. For example, if a person reads the 20/20 line at 20 feet, visual acuity is 20/20 (VA = 20/20). If the person reads the 20/60 line at 20 feet, visual acuity is 20/60 (VA = 20/60). This actually means that the person can see at 20 feet a letter that a normal person can see at 60 feet (18 m).
In general, in the western hemisphere, visual acuity charts are designated in feet, whereas in Europe the metric system is used ( Table 8.1 ).
A quiet area should be selected for testing visual acuity. The chart should be fastened at eye level on a light, uncluttered wall that has no windows nearby to avoid glare. The recommended illumination on the wall chart is 10 to 30 footcandles, but many offices use projected types of vision charts or retroilluminated charts (see Fig. 8.3 ). The general illumination in the room should not be less than one-fifth the amount of illumination on the chart.
In assessing vision, the examiner places an occluder over one of the patient’s eyes without exerting any pressure on the eye. The patient is then asked to read the chart. The smallest line of letters identified is noted. Adjacent to the line is a notation, such as 20/20 or 20/40. The line read clearly is recorded as 20/20 or whatever the case may be. If one or two letters are missed in the line, this may be recorded. For example, if the patient sees the 20/20 line but misses one letter, visual acuity should be recorded as 20/20 − 1.
The patient who is unable to read the largest letter is asked to walk toward the chart; the distance at which he or she begins to read the large letter is recorded as the numerator. For example, 4/200 indicates that the patient was 4 feet from the 20/200 letter. If it is impossible for the patient to distinguish the large letter, the examiner holds his or her fingers before the patient’s eye in good light and the vision is recorded as the farthest distance at which the fingers can be counted. For example, if the patient can accurately count the number of fingers the examiner is holding up 3 feet (1 m) away, this is recorded as counting fingers at 3 feet. If the patient cannot distinguish fingers, the examiner should wave a hand in front of the eye. If the patient perceives hand movements, the vision is recorded as HM, or hand movements. If the patient cannot even detect hand movements, the room is darkened, a test light is shone into the eye from the four quadrants, and the patient is asked to point in the direction of the light. If the patient can accurately point to light, vision is recorded as light projection. If the patient cannot distinguish the position but is able to just detect the light, the visual acuity is recorded as light perception. If the patient is unable to detect light at all, the vision is recorded as absent light perception.
Illiterate patients and preschool children (ages 2–5 years) may be tested by charts made up of numbers, pictures ( Figs. 8.4 and 8.5 ), tumbling E’s ( Fig. 8.6 ), or Landolt’s broken rings. HOTV ( Fig. 8.7 ) and LEA symbols (see Fig. 8.4 ) are reliable tests for preschool children. The child can either match the projected optotype to a handheld chart or name the letter or shape, depending on their developmental level. In the tumbling E test, the child points in the direction of the E either with a finger ( Fig. 8.8 ) or with a handheld cutout E. With Landolt’s broken-ring test, the child merely identifies where the break in the ring occurs ( Fig. 8.9 ). Other optotypes, such as Allen figures, do not conform to accepted parameters of optotype design and are less used nowadays.
For near vision, the examiner holds the near vision card at normal reading distance, and the patient indicates on the key card the letters that he or she can identify.
Preverbal children are tested by preferential looking tests, such as Teller Acuity Cards II (Stereo Optical Co, Chicago, IL) ( Fig. 8.10 ) and Cardiff Acuity Cards, if available. The basis of these tests is that a child presented with two different patterns will fixate on the pattern or picture rather than on a plain stimulus. Each eye is tested at a time and the child’s preferential looking at the stimulus is observed by the experienced examiner and recorded. If these tests are not available, more crude methods of visual assessment can be used, such as monocular fixation, following of a moving object and objection to monocular occlusion. Simply flash a light into each eye consecutively. If the child is able to fixate on the light centrally and steadily, vision may be assumed to be grossly normal. If the child’s fixation is eccentric but steady, the child’s vision is probably below normal. If the fixation pattern is unsteady and eccentric, vision is probably extremely poor and the eye defective. An infant should be able to follow a light by the age of 3 months and reach for toys by the age of 4 to 6 months.
When visual acuity is tested, the following points should be noted:
For the preliterate child, most pediatric ophthalmologists consider Snellen or Sloan optotypes to be the most accurate, followed in decreasing order of accuracy by HOTV and LEA symbols, tumbling E, Allen figures, and fixation behavior. The examiner should use the most sophisticated test a child can perform.
A false idea of visual acuity will be obtained if an isolated letter is presented to the patient rather than a line of letters. This is particularly true in persons with amblyopia, who may have 20/40 or 20/50 vision when tested with isolated letters and only 20/200 vision when asked to identify letters in a series. This is known as the crowding phenomenon . Single optotypes can be surrounded by crowding bars to overcome this problem.
There can be differences in recognition of letters in the same line. The letter L is considered the easiest letter in the alphabet to identify and B the most difficult. The letters T, C, F, and E are progressively more difficult.
Vision should always be tested with and without the patient’s glasses so that a comparison between the two can be made.
In children, visual acuity testing should not be prolonged and fatiguing. Children are easily distracted and may fail to respond to conventional visual acuity tests because of loss of interest or short attention span.
In all visual acuity measurements, the assistant should note any consistent pattern in the letters missed by the patient. For example, failure to see the nasal or temporal half of the chart may indicate a serious field defect, with loss of vision of half the visual field of each eye.
If both eyes are tested together, it is usually found that each eye reinforces the other, so that binocular vision tends to be slightly better than the vision of each eye tested separately.
A false visual acuity will be obtained if the patient partially closes an eye or squints. This causes a decreased pupillary aperture and thus allows only central rays to enter the eye, giving much better vision than the patient would normally have. It is important for the patient to keep the eyes wide open.
The patient should be observed during testing to prevent peeking around the occluder. Illiterate patients often say they cannot see rather than admit ignorance. It is important to obtain their confidence and coax them to read a number or illiterate E chart.
Early treatment diabetic retinopathy study chart
The early treatment diabetic retinopathy study (ETDRS) chart was developed in 1982 and then revised in 2000 by the National Eye Institute for use in the Early Treatment Diabetic Retinopathy Study. The chart ( Fig. 8.11 ) comprises a set of letters originally created by Louise Sloan, using the design of the LogMAR visual acuity chart that consists of letters (optotypes) arranged in standardized typeface, spacing, and size. It differs from the Snellen chart in that there is an equal number of letters per row, the rows and letters are equally spaced on a log scale, and the individual rows are balanced for letter recognition difficulty. Experts believe that the chart provides a more accurate and reproducible measurement of visual acuity than Snellen chart testing. The ETDRS chart has the same number of letters on each line (five), but the size of the letters on a line decreases based on a geometric progression. The ETDRS chart has become the preferred or required chart used in most clinical trials. The standard test is performed at a distance of 4 meters.
Use of pinhole
The pinhole disc, if placed before the eye, eliminates peripheral rays of light, improves contrast, and generally improves vision to almost within normal limits if the patient has a refractive error. The pinhole disc thus serves to differentiate visual loss caused by refractive errors from poor vision resulting from disease of the eye. In the latter condition, vision is not improved when a pinhole disc is placed before the eye ( Fig. 8.12 ).
Dynamic visual acuity
Visual acuity measured in an office setting is artificial. The eyes are steady, the body is still, and the target is immobile. In real life, as we walk down a street, the eyes are in motion, the body is displaced both forward and vertically, and the object of regard is rarely still. We look at things in action. This type of acuity is sometimes called kinetic vision or dynamic visual acuity.
Kinetic vision, or moving vision, cannot be measured but it is known that acceleration reduces acuity. The faster one travels, the worse one’s vision becomes. Body displacement spoils good vision. Try reading on a truck with poor shock absorbers. Fast eye movement is also a detriment to seeing clearly. It is impossible to follow a tennis serve traveling at more than 100 miles (160 km) per hour.
Another aspect of vision that has proven of interest is contrast gradient visual acuity. This is a measure of the acuity when hampered by poor contrast. A person can have 20/20 Snellen acuity and complain of poor vision. Snellen acuity measures only an individual’s ability to see small, high-contrast images. The visual contrast test can assess the entire spectrum of images and contrast. An individual with cataracts or night blindness may see well in daytime but see poorly at night or on cloudy days when there is little contrast. Vision in the real world can be evaluated more realistically.
Contrast sensitivity testing measures vision that resembles real-life situations more closely than the Snellen chart. Contrast sensitivity could probably be detected with photographs of real-life situations with different variations in their contrast; however, this is not practical and reproducible. Consequently, contrast sensitivity charts or machines are used. The contrast sensitivity chart presents a pattern of stripes of varying contrast, size, and orientation. The patient is asked to describe the orientation of the stripes. If the patient answers correctly several times, it is assumed that he or she is able to see these objects at that particular size and contrast. Many sizes and contrasts are presented to determine whether the patient has normal or decreased contrast sensitivity.
Contrast sensitivity tests may be presented as a wall chart with grids of varying size and contrast and a recording pad and instruction for analyzing the results. A smaller chart for near vision testing is also available. The Pelli-Robson chart ( Fig. 8.13 ) determines the contrast required to read large letters of a fixed size. In this chart, the contrast varies but the letter size remains the same. In the Regan contrast sensitivity chart, low-contrast letters of different sizes are shown to the patient. In the Ginsburg Functional Acuity Contrast Test chart, sine-wave gratings tests special frequency (sizes), and levels of contrast are used to plot a contrast sensitivity curve.
In some practices, patients are evaluated for contrast sensitivity before and after fitting contact lenses. If the fit is incorrect or the lens is not properly designed, the contrast sensitivity may decrease. This may be even truer with bifocal contact lenses. In addition, lenses often spoil with protein accumulation. Protein deposition reduces their contrast sensitivity while providing good Snellen acuity. In addition, contrast sensitivity testing can help to determine improve-ment in macular degeneration after use of nutritional supplements and to evaluate treatment response in patients with glaucoma.
The use of contrast sensitivity tests has become important in refractive surgery. The use of lasers to alter the shape of a normal eye so that there is a reduction in myopia must be accompanied by an evaluation. Contrast sensitivity is an important hallmark of the final visual acuity in a person and is much more reliable than Snellen acuity.
Visual acuity may degrade considerably in the presence of bright light. This is particularly true if there are opacities in the media, such as a posterior polar cataract. A number of glare test devices are available on the market (true visual acuity [TVA], brightness acuity tester [BAT], Eye Con) that create a dazzle effect and identify the person whose vision is reduced by glare. The BAT ( Fig. 8.14 ), developed by Jack Holladay, delivers three controlled degrees of light when the eye is viewing a Snellen target. Vision with opacities in the ocular media, cornea, lens, posterior capsule, and vitreous, when under the effect of bright light, degrades considerably and provides a true visual acuity in ambient lighting.
In glare testing, the patient looks into the machine or at some Snellen letters arranged on a wall chart. The examiner then turns on lights that shine directly into the patient’s eyes. The lights have been calibrated to imitate the brightness of headlights coming toward the patient at night, both high and low beams. With the lights on, the patient is instructed to read the letters on the chart. The acuity is measured after glare testing is recorded. With high-beam light, this usually falls off considerably if lens opacities are present.
The Miller-Nadler glare tester is commonly used to test for visual discrimination during bright daylight conditions. It consists of a tabletop viewing screen and a slide projector with 17 slides of varying sizes of land, dot, sea, and rings. The slides are projected onto the viewing screen. With each successive slide, the background is made progressively darker, thus decreasing contrast. The projector screen acts as a glare around the edge of the slide and shines into the patient’s eyes. The ability or inability of a patient to detect breaks in rings of smaller size and lower contrast correlates with loss of functional vision outdoors in bright sunlight.
Macular photostress test
This is a sensitive test for detecting macular dysfunction, such as cystoid macular edema, central serous retinopathy, and senile macular degeneration. Under conditions of bright light, such as produced by the BAT (see Fig. 8.14 ), these disorders are slow to recover vision. Normal recovery to bright light is 0 to 30 seconds but it becomes prolonged to more than 1 minute in patients with maculopathies.
Potential acuity meter
It is often difficult to see behind a dense cataract, or even an early cataract, to give a good estimate of the potential visual acuity of any particular eye. The cataract often partially obscures the fundus so that evidence of optic atrophy, retinal detachment, and macular disease cannot be determined. Although B-scans can sometimes determine retinal detachments, the subtle retinal defects, such as macular edema, macular degeneration, and other vitreoretinal defects are often difficult to determine.
The Guyton-Minkowski potential acuity meter (PAM) is a small apparatus that attaches to a slit lamp. The patient looks into a small aperture in the machine and sees the Snellen acuity chart. The examiner can control the position of the acuity meter, shine it through the pupil, and direct it through particular sections of the patient’s crystalline lens. Even patients with mild to moderate cataracts are not totally dense to this light and there are small breaks between opacities. The examiner shines the light through one of these small breaks; the patient can see it unobstructed by the cataract and can then read down the Snellen chart. This has important prognostic significance for determining what the acuity will be following cataract surgery. It lets the physician know that the retina and media are intact and gives an estimate of potential visual acuity.
The interferometer is an apparatus similar to the PAM. Instead of a Snellen chart that is imaged on the retina through breaks in the cataract, the interferometer shines red laser light or white achromatic light directly through the opaque portion of the cataract. The light is not blocked by lens opacity and passes through unchanged. Laser light in a pattern of stripes, either red or white depending on the type of machine, is separated by black stripes of equal size. The width of the stripes can be changed. The patient is asked to name the orientation of each grid as the width is changed, to estimate acuity. As the stripes become smaller, it becomes more difficult to detect which way they are pointing. If the patient can name the orientation of several grids with very thin stripes, it is assumed that the retina can resolve images at that visual level and should approximate good postoperative acuity.
The Heine (Heine Optotechnik GmbH & Co., Germany) Lambda 100 Retinometer (interferometer) operates on the principle of the Maxwellian view: a microaperture is illuminated by a halogen bulb through a red filter and imaged by an optical system into the patient’s pupil. The optical system consists of two lenses between which optical grids with variable spacing can be positioned in the parallel beam that passes through them. The resulting diffraction forms a circular test pattern with equally spaced red and black lines on the retina. The distance between the lines corresponds to that of the Snellen E (Visus 1 = 33 lines/degree of visual angle). The orientation of the lines can be selected by means of a prism in 45-degree steps. Because the beam in the pupillary plane is very narrow (a few tenths of a millimeter), a tiny “window” in the opacity of the lens is enough to allow the light to pass through for a successful examination.
The PAM, the interferometer, and the retinometer give only an estimate of the potential acuity. A patient’s acuity may be much better or worse than what was expected.
Near vision testing
Near vision charts are designed to be read at 14 to 16 inches. In patients with accommodative loss, as in patients with early presbyopia, a corrective lens is required to record the near vision. The near vision is recorded as the smallest type that can be comfortably read at the distance at which the card is held. Test cards are available in a wide variety of forms, such as printed paragraphs, printed words, music, numbers, pictures, and E’s ( Fig. 8.15 ).
Near vision for normal individuals may be recorded as 14/14, J2, or N5. The term 14/14 has the same meaning as the Snellen fraction in that the patient is able to read, at 14 inches (35 cm), small print that is easily seen by a normal individual. The term J2 refers to the Jaeger system. In the latter part of the 19th century, Jaeger designed a system of readable print and arbitrarily assigned numbers, beginning J1, to the various sizes. The term N5 refers to the printers’ point system; the print ranges in size from N5 to N48.
Near vision in children does not always correspond to the vision taken in the distance. Children can usually read despite significant refractive errors because they can hold reading material close to their eyes and thereby obtain magnification by the powerful range of accommodation.
Measurement of glasses
Before the refractive status of a patient is evaluated, it is important to know the previous prescription. The ophthalmic assistant should be very familiar with the technique of neutralizing lenses and arrive at the prescription of the glasses the patient is wearing.
The lensmeter is an instrument designed to measure the prescription of an optical lens ( Figs. 8.16 and 8.17 ). (Lensometer is the trade name of the American Optical Company. All other manufacturers refer to lens-measuring equipment as lensmeter, Vertometer, Vortexometer, or Focimeter.) Lenses are made up of either spheres or cylinders or a combination of both. By using a target area on the lensmeter, one can determine the exact prescription of any lens. All targets have some means of identifying two meridians that are at right angles to each other.
Many types of lensmeters are available. Each manufacturer publishes a manual showing how their instrument is used because the instruments vary in approach. Some work in minus cylinders with plus spheres. Some manual instructions are in plus cylinders for all readings; others are in minus. Therefore the user may be confused when confronted with an unfamiliar instrument.
Although the instruments vary, the eyepiece on all lensmeters (except the projection type) must be adjusted to compensate for the user’s refractive error, if any exists, or all readings will be inaccurate. The examiner should perform the following procedure:
Turn the power-focusing wheel until the target is not visible.
Turn the eyepiece fully counterclockwise.
Look through the eyepiece and turn it slowly clockwise until the grid or reticule just comes into focus. The correct position is the place where it first comes into focus. If in doubt, repeat.
Bring the target into sharp focus. The reading should be zero. If it is not, repeat. If zero cannot be obtained, set the power wheel to zero, turn the eyepiece counterclockwise to blur the reticule, then clockwise until the target and reticule just come into clear focus. Note the number on the scale around the eyepiece for quick, future adjustment.
If the examiner wears distance glasses while making this adjustment of the eyepiece, they should be worn every time the lensmeter is used. If the examiner prefers to use the instrument when not wearing glasses, then the eyepiece adjustment should be made without them. It is important to be consistent.
Lensmeters fall into two categories: those using the American crossed-line-type target ( Fig. 8.18 ) and those using the European dot-type target ( Fig. 8.19 ). Both types are accurate, providing the correct technique is used. The target type therefore is a matter of individual choice and of the operator’s familiarity with a specific type.
The American crossed-line-type target consists of solid straight lines at right angles to one another. A single line runs in one direction and three parallel lines run in the opposite direction ( Fig. 8.20 ). The whole target can be rotated 360 degrees for determining cylinder axis. At the beginning, it is easiest to measure a lens with this type of lensmeter by following five simple rules:
Place the spectacles on the base so that both left and right lenses are resting on the holder. This prevents rotation of the lens and inaccurate axis reading.
Focus the single line. Rotate the lines by using the axis wheel so that the single line gives readings closest to zero for both the plus and the minus spheres. This is then marked down as the sphere component.
Focus the triple line and record the difference from the single line to the triple line. This is the cylinder portion. For example, if the single line is at +1.00 and the triple line is at +3.00, the prescription will be + 1.00 + 2.00.
Rotate the axis wheel so the target is on axis when the triple lines are continual. Mark down this axis; this is the axis of the cylinder (e.g., +1.00 +2.00, axis 90). If the single line and the triple line are in focus at the same time, the lens is a sphere.
In determining the reading addition in a bifocal lens, move the lens up to the reading segment and then focus again on the triple lines. The difference from the recording of the last triple line to the new triple line in focus is the reading addition. This is always recorded as plus.
If the examiner initially focuses the single line so that it is closest to zero, both the cylinder and the sphere will have the same sign (plus or minus).
In some cases, the examiner may wish to record all prescriptions in terms of plus cylinders. In doing so, it is necessary to first bring the single and then the triple lines in focus. By rotating the axis wheel, the examiner can arrange that the triple lines come into focus when more plus is introduced, thereby ending up with cylinders recorded as plus.
The European dot-type target consists of a circle of dots (or variation thereof) that does not rotate. Instead, a protractor grid rotates in the field of view to determine the cylinder axis. The power of a spherical lens is determined by bringing the dots into sharp focus and then reading the power (see Fig. 8.19 ).
The power of a toric or cylindric lens is determined as follows:
If a lens contains a cylinder, the target will appear as a system of focal lines. These lines focus in two positions, one perpendicular to the other. First, focus the target so that one of these lines is in focus. The reading closest to zero is marked down as the spherical component (e.g., +1.00).
Focus the target so that the second set of lines is now perpendicular to the first reading and record the difference in dioptric powers between the first and second readings. This is the cylinder portion. For example, if the first focal point is at +1.00 and the second focal point is at +3.00, then the prescription will be +1.00 +2.00.
Adjust the cross line so that it is parallel to the focal lines on this second reading. This is the axis of the cylinder (e.g., +1.00 +2.00, axis 90).
Note: It is impossible with a very weak cylinder to identify a cylindric lens and its axis when the dots are in focus. (When the user is unfamiliar with this instrument, the most common error is that the axis is off by 90 degrees.) There will be no difficulty with stronger cylinders because the dots will smear into distinct lines and aligning the axis grid offers no problems.
The technique for neutralizing a lens with a weak cylinder is as follows. First, rock the power-focusing wheel on either side of the target’s focus point and note how the individual dots “bloom” or go out of focus. If the blooming is spherical, you have a spherical lens. If the blooming is oval, you have a cylindric lens. While rocking the focusing wheel, rotate the target protractor or reticule to line it up with the direction (axis) of the oval blooming or smearing of the circular dots. After establishing the cylindric axis, determine the spherical and cylindric power.
In determining the additions in a reading bifocal lens, move the lens up to the reading segment and then focus again on the lines at the second focal point. The difference between the value of the second focal point in the distance prescription and that found in the reading segment is the reading addition. This is always recorded as plus.
With all lensmeters it is important: (1) to center the lens well before reading the prescription and (2) to measure the prism if present. The lens has a prism if the center of the lens does not coincide with the center of the target. There are circles surrounding the central target of the lensmeter to measure the amount of prism. The distance between each circle represents 1 prism diopter. It is easy to see at a glance how far the optical center of the lens is displaced in prism diopters from the center of the target. Fig. 8.20 illustrates a prism in a lens with the American type of target.
Universal method of using any lensmeter
Many projection instruments make reading of the lens prescription easier. The following method works with any lensmeter, standard or projection type, using any type of target:
Place the lens to be measured (in frame or otherwise) in the lensmeter on the table, convex side toward you, with the lens surface firmly against the instrument and with no tilt. Tilting a lens will introduce an error that may result in an inaccurate axis and cylinder power. This is a common error made by the inexperienced user in measuring the bifocal segment. Finding the addition of a fused-glass bifocal requires a special technique, which is covered in detail later.
Center the target, then set the power to zero. Move the power wheel from zero to a point well beyond the focus of the target and then back toward zero to the point where the first target meridian or dots come into focus. Take the reading. This is the spherical power.
Continue rotating the power wheel in the same direction (do not reverse direction) to bring the second meridian into focus. The algebraic difference is the cylindric power. The sign of the cylinder is opposite that of the sphere. Note the axis of the second meridian. This may be correct or exactly 90 degrees off.
Make this check. If the target line or smeared dots are nearly horizontal, the axis is going to be near zero or 180 degrees. If the target line or smeared dots are nearly vertical, the true reading will be near 90 degrees.
This procedure is the same with all makes of lensmeters, and the user actually has a check on a possible cylinder axis error. The user may be in doubt, however, when the axis is near 45 or 135 degrees, in which case another pair of glasses should be tried (or a cylinder from a trial case) to identify which line on the target, or the grid, gives the true axis.
This universal method gives plus cylinder results with minus spheres and minus cylinder results with plus spheres.
Once the user has mastered the instrument, he or she will have no difficulty in modifying the method to work only in plus or only in minus cylinders if this is wished, rather than transposing mathematically from one to the other.
Prism, with the base in any direction, is measured by concentric circles and the displacement of the target. The circles may be in 0.50 or 1.00 prism diopter steps. In addition, some lensmeters have a Risley rotary-type prism as an integral part of the instrument; its secondary use is to center the target for accurate axis reading in a prismatic lens.
To find the addition of a bifocal lens less than 3.00 diopters, the difference in power between the distance and the reading portions must be found. First, the distance power should be found in the conventional manner, holding the lens being tested in the instrument with the rear surface (usually concave) away from the eye. Second, the reading power should be found, holding the lens in the same position, rear surface away from the eye. If the bifocal is less than 3.00 diopters, the previously mentioned method will produce a correct reading to within 0.06 of a diopter, which is an insignificant error.
For all bifocals greater than 3.00 diopters, the following is the only procedure that gives accurate results:
Check the distance portion in the manner indicated previously. This step is identical in checking all lenses and gives an accurate result as to the power of the distance portion.
The bifocal must now be reversed in the instrument. Hold its front (usually convex) surface away from the eye and check the distance portion at a spot about the same distance above the center as the point at which you will check the reading power is below the center. In the case of aspheric surfaces, measure the distance through its optical center.
Note the finding made through the distance portion. Now check the power through the reading, again with the convex surface away from the eye. Subtract the power of the distance portion found from the reading power to determine the addition.
When making all readings, the lens must be firmly against the lensmeter stop, its surface at right angles to the axis of the lensmeter. In some lensmeters, it is advisable for the operator to hold the lens in position with his or her fingers and not rely on the lens holder.
Space-age technology has once again simplified the operator portion of arriving at spectacle measurements. Although costly, this technology will save time and improve accuracy in a busy office. A lens analyzer measures in a single operation the sphere, cylinder, axis, and prism of a lens ( Fig. 8.21 ). The values are digitally displayed and can be recorded, if desired, on a paper tape by the built-in printer. This instrument eliminates the focusing and target alignment tasks, which require a fair amount of skill with a conventional lensmeter. It also does the mathematic tasks of computing cylinder power and of computing the add value of the bifocal segment. The lens analyzer has the unique ability to position spectacle lenses at a given interpupillary distance and then to use prism information collected to compute the net prismatic effect of the spectacle pair. These measurements are made very rapidly. The instrument uses a white light source and a ray trace-type system to make its measurements. It can also measure the amount of ultraviolet lens transmission to let one know exactly how much protection the wearer has.