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CONTINUING EDUCATION PROGRAM
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The Marchon Training Center is sponsored by Marchon's Department of Education and Research as a service to the ophthalmic community. It is intended as a correspondence course for practitioners seeking continuing education credit (CEC) from the American Board of Opticianry (ABO) and from various state board agencies. In addition to the Training Center's Home Study program Marchon sponsors a number of other educational publications, among them 101 Dispensing Tips and Procedures® - a quick-and-easy reference guide packed with information to help dispensers do their job more easily and effectively, and The Eye-Glassery¨ Ñ a review book for ophthalmic professionals and a working manual for new-to-the-field aspiring dispensers interested in improving their optical skills. For information about our educational materials contact your Marchon sales representative or write: Marchon Eyewear, Inc., 35 Hub Drive, Melville, NY 11747-3500.
Marchon is a registered trademark of Marchon Eyewear, Inc. ©1997 by Marchon Eyewear, 35 Hub Drive, Melville, NY 11747-3500. |
Dr. Weber has published more than 60 papers in national and international ophthalmic journals and is the author of several books and manuals on optometry and opticianry. |
Page1
INTRODUCTION
| You could call this course a "hands-on
lesson in a textbook" since it offers a workshop method for a better
understanding of ophthalmic lenses. As with all hands-on procedures, you
will need some materials to work with. In our case, you'll require a few
experimental lenses that can be borrowed from your stock of uncut lenses.
We start the workshop by gathering our working tools. Obtain the following lenses for your experiments:
|
You will also need a felt-tipped pen.
Although each lens is packaged individually with its power indicated on its envelope, there are, of course, no markings on the lenses themselves. Therefore, should you inadvertently mix the lenses as you work with them, it will be much easier to re-identify them if you mark every lens with India ink or paste small labels on them as you remove each one from its individual envelope. Before we begin our lenses-in-hand procedures, we will review some pertinent definitions. You're probably familiar with the following terms, so you may wish to skip over them. Be aware, though, there may be some useful descriptions explained below, so we suggest you skim through them on your way to the text's main sections. |
| PURPOSE: This section presents a refresher
course of commonly used words, terms, and procedures pertinent to the
study of ophthalmic lenses. We begin with a review of the action of light
rays through ophthalmic lenses, as follows:
A. Refraction. The term refraction has a double meaning: The definition we are concerned with in this handbook has to do with the passage of light rays through eyeglass lenses (the other meaning has to do with eye examinations). Simply stated, refraction of light means the bending and deviation of light rays as they pass from one medium into another medium of a different density. How and how much these rays are deviated and refracted (bent) depends upon (1) the angle between the two surfaces of the lens, and, (2) the relative density of the two media through which the rays travel (called the index of refraction). |
(1) The angle between the two surfaces of
the lens. A ray of light striking the surface of a lens perpendicularly
(at 90 degrees) will pass through the lens without deviation, provided
the two sides of the lens are parallel to one another. Although the ray
slows down while it's in the lens, it's not bent from its original direction
of transmission (Fig. 1).
(2) However, if the ray strikes that same lens at some angle other than 90 degrees, the ray is refracted (bent) from its original path while passing through the lens. When it emerges from the lens's second surface into air it's refracted again and, even though it travels in the same direction as its original path of transmission, note that the ray has been displaced in the same direction as the original ray of propagation (Fig. 2). |
Page 2
| On the other hand, if a ray of light strikes
a lens whose sides are not parallel, the ray will be refracted and will
emerge in a different direction than its original path of propagation.
This is always true whether the light ray approaches the glass perpendicularly
or at some other angle to the surface (Fig. 3). Moreover, the greater
the angle of entry, the greater the amount of refraction.
You can prove the above by trying this experiment: a. Place a pencil in a half-glass of water so that it is perpendicular to the water's surface (Fig. 4,a). b. Ignore the apparent magnification (that's another subject), and slowly shift the pencil so that its top rests against the top rim of the glass (Fig. 4,b) while, at the same time and at eye level, observing the action that's taking place. You'll see that the more the pencil is tilted, the more refraction (bending) takes place, thus proving the rule that the greater the angle of entry, the greater the amount of refraction. B. The index of refraction. The importance of density in relation to refraction of light can be proven by noting the speed of light as it passes through various media. When light passes from air to, say, water, glass, or some other denser but still transparent medium, the rays slow down (for example, like moving an object through water as compared with moving the object through molasses). How slowly they pass through various transparent media is called the index of refraction. The breakthrough in recognizing there is such a phenomenon as an index of refraction occurred when Dr. Wildebrod Snell, investigating the speed of light in 1621, |
noted that light rays are bent in a direct
ratio to the density of the substance they are passing through.
In other words, rays are bent more and more sharply as they pass through
denser and denser media. Thus, the higher the index of refraction, the
greater the "bending" or "focusing" power which,
in effect, means that the higher the index of refraction, the thinner
a prescription lens can be ground.
Dr. Snell reasoned that, because the index of refraction is actually the ratio of the speed of light in air as compared with the speed of light in any other transparent medium, the relationship could be calculated mathematically, as follows: The index of refraction of the medium equals the speed of light in air divided by the speed of light in the medium. Let's see how this works out with an ophthalmic lens; for example, a lens made of crown glass: We know the speed of light in air is 186,000 miles per second. It has also been determined that the speed of light in crown glass is 122,000 miles per second. Therefore: The index of refraction for crown glass equals 186,000 miles per second divided by 122,000 miles per second, which equals 1.524. Using the same formula, the refractive index for CR-39 plastic is 1.498; for high-index plastic, 1.595; for polycarbonate, 1.586; and for titanium glass, 1.701. C. The dioptric system. The diopter represents a unit of measure for lenses. It defines a one-diopter (1.00 D.) lens as a lens whose focal length is one meter (40 inches), with its power being the reciprocal of its focal length. In other words, a one-diopter lens focuses parallel rays of light at a distance of one meter; a two-diopter lens focuses parallel rays of light at a distance of one-half meter; a one-half diopter lens focuses rays of light at a distance of two meters; and so on (Fig. 5). D. Optical centering (O.C.) It was pointed out earlier that light rays striking any transparent medium at an oblique angle will be refracted, and those that enter perpendicularly to the medium's surface will not be so affected (refer to Figs. 1 and 2). |
Page 3
| Put another way, light rays passing at an angle
through a refracting medium are bent to help form a focus, but rays that
enter "head-on" pass through without being bent or displaced
(Fig. 6). The tiny area in which no refraction takes place is called the
optical center (see also page 11).
E. Geometrical centering (G.C.) Often also referred to as the "mechanical center," the geometric center is the one point in the middle of a "boxed" lens that is equidistant from all sides of the box (Fig. 7). F. Orientation of cylindrical axes. Dispensers sometimes overlook the fact that the axes of cylindrical lenses are opposite the dispenser's side as compared with viewing the axis from the patient's side. |
 Fig. 8,a. Orientation of cylindrical axes on a protractor. The upper register indicates the cylinder axis while the lower register shows the axis from the ocular (patient's) side; Fig. 8,b. Another way to orient cylindrical axes. Recall that convex surfaces of cylindrical axes (facing the patient) shown on the upper scale of an optical protractor go clockwise from 180 degrees to zero. From the opposite (patient's) side, the lower scale of the protractor reverses the axes (Figs. 8,a and 8,b). A good way to recognize cylindrical axes from the dispenser's side is to view the back of your left hand; your thumb will point to zero degrees (Fig. 9). |
| PURPOSE: In this section, we review various
functions of ophthalmic lenses. Following, in order, are analyses of (A)
spherical (convex and concave) lenses, (B) cylindrical (plus, minus, and
compound cylinders), and, (C) prismatic lenses.
(A) Review of convex lenses. Because convex lenses are more sharply curved on their front surfaces than on their back surfaces, they are necessarily thicker in their centers than they are at their edges. They also magnify the size of objects and make things seem to move when the lenses are moved. Note the following hands-on procedure for reviewing convex lenses: 1. Take a +1.00 sphere from your experimental lenses and, holding the lens several inches from your eye, observe a line of print at arm's length. Notice how the lens magnifies the print. |
2. Next, hold the lens in front of your eye
so that a small object is viewed from a few feet away. Notice, now, that
moving the lens upward causes the object to be displaced downward,
and moving the lens downward causes the object to be displaced
upward. Moreover, moving the lens to the right seems to
move the object to the left and vice versa. This phenomenon
of an object being displaced against or opposite the direction
of motion of a convex lens is called against movement.
Convex lenses are prescribed for the correction of hyperopia (farsightedness). In this vision defect, rays of light are not refracted sharply enough by the eyes, so the rays tend to focus behind the retina (Fig. 10,a). Farsighted eyes function more efficiently at far distances than they do up close. |
| The defect is corrected with plus lenses, which
shorten the focal length, thus enabling images to focus on the retina
(Fig. 10,b).
B. Review of concave lenses. Concave lenses have characteristics that are opposite to those of convex lenses. Because concave lenses are more sharply curved on their back surfaces than they are on their front surfaces, the lenses are necessarily thinner in their centers than they are at their edges. Also, objects seen through a concave lens seem to look smaller and things seem to move in relation to any movement of the lens. Try this lens-in-hand procedure for reviewing concave lenses: 1. Take a -1.00 sphere from your lens kit and, holding it a few inches from your eye, observe a line of print two to three feet away. Notice how much smaller the letters appear. 2. Then, move the lens from side to side and up and down. Notice, now, that the object seems to move in the same direction the lens is moved. This is called a with movement. Concave lenses diverge light rays to help correct myopia (nearsightedness). In this vision defect, the eye refracts so sharply in relation to its length that the rays are brought to a focus before they have a chance to reach the retina (Fig. 11,a). Patients with myopia have difficulty seeing things clearly at distances, but they are able to see things clearly up close. Concave lenses correct the defect as shown in Fig. 11,b. |
Fig. 11,a. In nearsightedness, the eye refracts so sharply in relation to its length that light rays are brought to a focus before they have a chance to reach the retina.
C. Review of cylindrical lenses. Cylindrical lenses are often referred to as cylinders or toric lenses because of their out-of-round surfaces (much like the surfaces of a spoon). Cylinders may be ground in simple form (plano cylinders) or in compound form (sphero- or compound-cylinders). A plano cylinder has no refractive power in its axis, but does develop power as it leaves the axis until reaching full strength 90 degrees away. Such a cylinder then reverses itself, gradually losing power until it returns to zero or plano power at its original axis. For easier and better understanding of cylinders, try this experiment: 1. Take the -1.00 plano cylinder from your kit. 2. Holding the lens a few inches from your eye, sight a straight-lined object, such as a window or door frame. 3. Looking through the cylinder lens, slowly rotate it as you would turn the steering wheel in your automobile - first to the right (clockwise), then to the left (counterclockwise). 4. Notice, upon turning the lens, how a section of the door's frame appears slanted (Fig. 12,a). |
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| This is called a "scissors movement"
5. Continue rotating the lens as you simultaneously move it up and down. You soon will find one position on rotation in which the frame will neither look slanted nor will the up-and-down movement displace the target (Fig. 12,b). The meridian without movement is at axis 90 degrees on your plano cylinder while the opposite meridian shows movement (power) 90 degrees away (that is, at 180 degrees). 6. The weaker meridian is called the primary meridian while the stronger meridian (90 degrees away) is called the secondary meridian. In the above illustration, the 90th meridian is the primary meridian and the 180th meridian is the secondary meridian. These two meridians (primary and secondary) are called the principal meridians of a cylindrical lens (Fig. 13). Compound - (sphero) - cylinders are merely plano cylinders to which spheres have been added. |
In astigmatism, because the eye is out-of-round
(shaped like a football rather than, say, a baseball), the eye focuses
objects in two planes, only one of which is able to focus on the retina
(Fig. 14). Cylinders ground with out-of-round curves opposite in design
to those of the astigmatic eye are prescribed to correct the condition.
Plano cylinders are prescribed to correct simple astigmatism while sphero-cylinders
are used to correct more complicated astigmatic cases.
D. Review of prismatic lenses. By definition, an ophthalmic prism is a wedge-shaped lens of glass or plastic that is thicker at one edge (the base) than at its opposite edge (the apex). Prisms usually are prescribed to correct faulty eye-muscle imbalances in which both eyes do not act as a team. In these cases, the refractionist prescribes the proper amount of prism and the position of the base in order to correct the affected muscles. Prisms are sometimes inadvertently introduced when prescription lenses are not centered properly in their eyeglass frames. These are called unwanted prisms (more about these unwanted prisms on page 12). For teaching purposes, an ophthalmic prism is traditionally illustrated in the form of a pyramid or triangle that dramatizes the difference in thickness between its base and its apex (Fig. 15,a). |
| In practice, however, unless present in strong
amounts, prisms incorporated in ophthalmic lenses are cosmetically inconspicuous.
In our discussion, we will illustrate prisms as shown in Fig. 15,b (base
down) and Fig. 15,c (base up).
Light rays entering and leaving a prism are bent toward the base of
the prism and away from the apex. This causes objects to become displaced
away from the base of the prism and toward its apex. Thus, if an object
is viewed through a base-down prism, it will seem to become displaced
upward. Similarly, objects appear to shift downward when viewed through
a base-up prism, to the right or left when seen through a base-out or
base-in prism, and so forth. Let's examine this concept by means of
our workshop method, as follows: 1. Take your +1.00 D. sph. and -1.00 D. sph. from your lens kit and cover the bottom half of each lens with paper, leaving a "half-moon" design on the lenses (Fig. 16,a). This is known as veiling. 2. With the bottom halves of the lenses obscured, the convex lens now reveals a base-down prism, since its "half-moon" look is thicker in its diameter than at its edge (Fig. 16,b). The concave lens shows a base-up prism (Fig. 16,c) since its edge is thicker than its diameter. |
3. Next, with your naked eye, view a small,
nearby object and slide the lens abruptly into your line of sight. Since
the convex lens's prism bends the object's image toward the base but
projects it toward the apex, the object appears to "jump"
upward; that is, away from the base and toward the apex (Fig. 17,a).
4. Repeat the above procedure with your -1.00 D. sph. Because the "half-moon" concave sphere contains a base-up prism, the object's image now appears to "jump" downward; that is, toward the apex (Fig. 17,b). |
| PURPOSE: In the discussion that follows,
we will be repeating some of the definitions and explanations offered
earlier in this manual. While this may seem redundant, it will save you
from flipping back through the pages should you reach a word or phrase
whose definition you may not readily recall.
One of the best methods for fully understanding ophthalmic lenses is an old and almost-forgotten procedure called hand neutralization - a method for checking or determining the power of a lens if it's unknown. Before we discuss this technique, recall (from your school days) that figures can be added algebraically; that is, you can add pluses and minuses together, as in these examples: +2 +2 = +4; -2 +4 = +2; -4 + 2 = -2; -2 + 2 = 0, and so forth. Recall also that objects do not appear to move when viewed through a moving plano lens. However, when the process is repeated with a convex sphere, the object appears to move in a |
direction opposite to the movement of
the lens. Conversely, when the process is again repeated with a concave
sphere, the object appears to travel in the same direction as
the lens movement.
From the above, it follows that if you take a convex lens and place it in contact with a concave lens of the same power, the movement will be arrested; that is, the object will not appear to move and the lens under examination will be considered neutralized. The above concept is the principle behind hand neutralization. Compare the procedure with the act of weighing an object on a scale: To obtain the weight of an unknown substance, one platform is balanced or neutralized by the known values on the other platform. Today, neutralization is quickly and easily performed on manual and high-tech lensometers. All one has to do is place a lens on the instrument's stage, turn a couple of wheels, or push a button. |
| The lens's power will instantly reveal itself.
But this method is strictly mechanical - it doesn't teach much about the
nature of ophthalmic lenses. What will help you understand the concepts
behind ophthalmic lenses is our up-close and personal approach to the
subject of hand neutralization, as follows:
A. The method for hand neutralization: The technique begins when a lens of known power (called the neutralizing lens) is placed against a lens of unknown power (called the Rx lens) in such a way as to reveal the nature and power of the unknown lens. To perform the procedure, you will need a crossed-line chart. Here's how to construct the chart: On a piece of 8 1/2" x 11" plain, white paper or cardboard, draw two crossed lines bisecting each other, thus leaving four arms extending horizontally and vertically as shown in Fig. 18. This is your target. The procedure starts with checking whether the lens to be neutralized is a sphere, plano cylinder, or sphero-cylinder by holding it about one foot away from your eye and viewing a vertically lined object such as the vertical post on a door frame. If rotating the lens causes the doorpost to look slanted, it is a cylinder; if no distortion or break in the line is observed, the lens is a sphere. Let us assume the lens is spherical - the lens we'll discuss first - and then get into cylinders. 1. Procedure for neutralizing spheres: a. Hold the lens to be neutralized at arm's length between thumb and forefinger, with the concave surface facing you and the convex surface facing the target (Fig. 19,a). b. Mount the target on a wall or similar vertical surface, on a line with your eye, and placed about four feet from the lens to be neutralized (Fig. 19,b). c. Take a +1.00 D. spherical lens from your experimental lenses and consider this to be your unknown Rx lens. Then, move the lens up and down and side to side. Notice how the target moves opposite to the direction in which you move the Rx lens. |
d. Next select the -1.00 D. spherical lens and
regard this as your neutralizing lens. Observe the target through
this lens and, as you did with the unknown lens, move the neutralizing
lens up and down and side to side. Notice that the target appears to move
in the same direction as the movement of the -1.00 D. lens.
e. Finally, place the front surface of the neutralizing lens (-1.00 D. sph.) in contact with the back surface of the unknown lens (+1.00 D. sph.), and move the combination up and down and side to side. As you now expect, there will be no movement, indicating that the unknown lens has been neutralized and its power revealed. You can record this result as follows: Unknown (Rx) lens = +1.00 D. sph. 2. Method for neutralizing cylinders: Hand neutralization of cylinders depends upon the same principles as spheres do, but cylinder neutralization requires more steps. For the purpose of simplicity, we will discuss plano and sphero-cylinders separately as follows: Neutralizing simple plano cylinders. As you well know, a plano cylinder has power in all of its meridians except one, and that meridian is called the axis. Let's examine this definition through our workshop method: a. Select your -1.00 D. cylinder, and consider this to be the unknown or Rx lens. b. Hold the lens at arm's length from your eye. While looking through it, sight the crossed lines on your target which you have placed about four feet from your eye (see Fig. 18). |
Fig. 20,a. As the lens is rotated, the crossed lines become distorted and break in a "scissors-like" motion. Fig. 20,b. Upon continuing lens rotation, the lines become unbroken. c. Rotate the lens in a wheel-like movement. Notice how the lines distort and "break" in "scissors-like" motion upon rotation (Fig. 20). d. Keep turning the lens until you locate the meridian in which the lines are unbroken (as shown in Fig. 19,a). Then, keeping the lines unbroken, move the Rx lens up and down. If the target does not move in reaction to the movement of the lens, it means there is no power in the vertical meridian and, therefore, the lens is positioned at axis 90 degrees. On the other hand, if there is movement, the axis is located at 180 degrees (90 degrees away) and, therefore, must be rotated a quarter-turn to position it at 90 degrees. e. With the lens positioned at axis 90 degrees, use your felt-tipped marker to place three dots on the lens's 90-degree axis (Fig. 21), and allow the dots to dry so that you will not lose this axis position when performing the remainder of the procedure. (Incidentally, the center dot, located on the intersection of the vertical and crossed lines of the tar get, represents the lens's optical center.) f. You now have the -1.00 plano cylinder positioned so that there is no power (0.00 D.) in the 90th meridian but full power (-1.00 D.) in the 180th meridian (Fig. 22). g. Neutralize the movement caused by the resulting -1.00 D. power in axis 180 with the +1.00 D. sph. from your experimental lenses. |
Fig. 22. The lens is positioned so there is no power (0.00 D.) in the 90th meridian but full power (-1.00 D.) in the 180th meridian. h. Since you know there is no power in the 90th meridian of your Rx lens, you can express this as 0.00 D. at axis 90. And, since you've proved there is -1.00 D. power 90 degrees away from the 90th meridian (axis 180), the lens that's been neutralized can be written as: -1.00 D. cyl. axis 90 degrees. The technique for hand neutralizing compound-cylinders is as follows: a. Select the -1.00 D. sph. c/w -1.00 D. cyl., and consider this to be your unknown Rx lens. b. Observe the crossed-line target, and rotate the lens as you did with the plano cylinder. Notice how the crossed lines not only "break," but they move in a scissors-like direction as the lens is turned clockwise and counterclockwise (see Fig. 20, a). However, unlike the plano cylinder that has no motion in its plano meridian, the sphero-cylinder retains some motion in all its meridians. c. Understand this more clearly by considering the sphero-cylinder as two separate lenses; specifically, a sphere and a cylinder - each to be neutralized separately, as follows: d. Square up the -1.00 D. sph. c/w -1.00 D. cyl. by rotating it until the crossed lines on your target do not show any "scissors" effect, as in Fig. 20, b. Its principal meridians are now at 90 and 180 degrees. e. Neutralize the spherical component of the lens being tested by means of a +1.00 D. sph. This now reduces the lens to the status of a plano -1.00 D. cyl. f. With your second +1.00 sph., follow Steps a through h on pages 10 and 11 (neutralization of plano cylinders) to complete the neutralization process. g. As with your neutralization of plano cylinders, the power of the sphero-cylinder is revealed and you now can write it: -1.00 c/w -1.00 axis 90 |
Page 9
| Recapping: Neutralization by hand employs
a lens of known power (called the neutralizing lens) to stop, or
neutralize, the movement seen through a lens of unknown power (called
the Rx lens). As lenses are moved up and down, side to side, and
rotated, the crossed lines appear to move as follows:
A plano lens causes no movement of the target; a convex sphere produces against movement; a concave sphere triggers with movement; a plano cylinder has no movement in its axis but produces movement away from its axis; and a sphero-cylinder has different powers in all its meridians. Neutralization of a multifocal is first handled in its distance portion; the near-vision |
segment is then treated as a convex sphere and is neutralized with a concave sphere. B. Movement vs. lens power The extent of apparent image movement indicates lens power: Low-powered lenses produce slow-moving image displacement, but objects appear to move more and more rapidly as lens power increases. For example, note the displacement speed with your +1.00 lens; then add the second +1.00. The increase in displacement speed with the (total) +2.00 compared to the +1.00 becomes markedly noticeable. |
| PURPOSE: To recognize and interpret the
special signs and symbols of an eyeglass prescription in order to permit
a clear understanding of what an Rx represents.
As pointed out earlier, lenses for the correction of hyperopia (farsightedness) are indicated by plus signs; those for myopia (nearsightedness) are shown by minus signs; lenses for correction of astigmatism are designated by the abbreviation cyls, cx, or merely x, followed by the axis designation. Prisms are expressed by a raised triangle or pyramid followed by the base notation. As a rule, words, abbreviations, and symbols for diopter (D), sphere (sph), and cylinder (cyl), are incorporated into the prescription. However, the axis degree indicator itself (o) is seldom used because it could be confused with the figure "zero." For example, 5o, if carelessly written, could easily be mistaken for axis 50 degrees. |
Abbreviations for the right, left, or both eyes
are: O.D. (from the Latin words oculus dexter), or plain
R., for the right eye; O.S. (from the Latin oculus sinister),
or plain L. for the left eye; and O.U. (from the Latin oculi
uniter) for both eyes when used together.
Let's say a prescription reads: O.D. -1.00 D. sph. c/w -2.00 cx 90. This Rx describes the right eye of a patient who has 1.00 D. of nearsightedness and 2.00 diopters of myopic astigmatism. You will find that most prescriptions are written in minus cylinders. There are two reasons for this: For one thing, most refractionists (mainly optometrists) use minus cylinders when examining eyes. Secondly, today's uncut lenses are stocked with minus cylinders. On the other hand, some refractionists (mainly ophthalmologists) prefer plus cylinders. However, as you are well aware, plus-cylinder corrections can easily be converted to minus cylinders by a process called transposition and still have the same power. |
| PURPOSE:The relationship between the
optical center of an ophthalmic lens and the geometric center of an ophthalmic
frame cannot be overstated. Incorrect positioning of the optical center
probably causes more grief and expense than any other error in fitting
eyeglasses. In this section, we review proper lens centration.
Look back to Figs. 17,a and 17,b on page 9, and note that light rays passing through a lens at an angle are bent away from their course by prismatic action. But observe that rays entering a lens head-on are not bent since they pass through the lens's optical center. To avoid unwanted prism in prescription lenses, it is essential that the lenses be designed and inserted into ophthalmic frames in such a way that their optical centers come out in the exact centers of the patient's pupils. To do this, the fitter must measure the distance between the centers of the patient's pupils (the patient's P.D.) and match that measurement against the centers of the eyeglass rims (the frame's P.D.). A mismatch between the patient's P.D. and the frame's P.D. introduces unwanted prism which, in turn, gives rise to patient discomfort and his or her inability to wear the glasses. Among |
other symptoms produced by unwanted prism are:
double vision, dizziness, headaches, and blurred vision. Also, because
human eyes are extremely sensitive to prismatic effects, even the positioning
of unwanted prism could lead to bizarre symptoms. Some examples:
Unwanted base-down prism causes a floor or other flat surface to seem concave, and the patient feels like he or she is standing in the bottom of a depression. Unwanted base-up prism makes the floor look convex, so the patient experiences a sensation of walking downhill. And, unwanted base-in or base-out causes horizontal objects, such as a table top, to look too high on one end and too low on the other, with the too-high side toward the base of the prism. A handy device for measuring the patient's P.D. and the frame's P.D. as well as for taking a number of other important optical measurements is the Marchon Versa-Rule - a tool for measuring lenses and frames simply and accurately. If you don't have a supply of these rules in your office, ask your Marchon representative for a complimentary supply. (Incidentally, optical rules are called "rules" and, although they resemble common stationery-store rulers, they are always called "rules.") |
Page 10
1. A ray of light striking the surface
of a plano lens at a 90 degree angle:
2. What is the index of refraction of a lens in which the speed
of light is 122,000 miles per second?
3. Convex lenses:
4. A base-up prism:
5. The weaker meridian of a cylindrical surface is called:
6. A +1.00 D. lens focuses light rays at a distance of:
7. Unwanted base-down prism causes a floor or other flat surface
to appear:
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8. Light rays are not refracted when:
9. Which material has the strongest bending power?
10. The index of refraction equals:
11. The Latin term "oculus dexter" refers to:
12. This drawing represents:
13. Concave lenses:
14. A prescription lens is neutralized with a +2.00 D. sph.
and a +1.00 D. cyl. x 90. What is the power of the Rx lens?
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Page 11
15. True or False? How much light
rays are refracted depends upon the angle between the two surfaces
of a lens and the index of refraction.
16. This illustration proves that the greater the angle
that light rays enter a medium:
17. Light rays are bent more and more sharply as they pass
through:
18. How much, and in what direction, should lenses be decentered
if the customer's P.D. is 65 mm. and the frame's P.D. is 65 mm.?
19. The Latin term "oculus sinister" indicates:
20. A +0.50 D. lens focuses light rays at a distance of:
21. Unwanted base-up prism causes a floor or other flat
surface to seem:
22. A good way to recognize cylindrical axes from the dispenser's
side is to view the back of your left hand. In this test, your thumb
will point to _______ degrees:
23. Cylindrical lenses:
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24. True or False? Light rays entering
and leaving a prism are bent toward the base of the prism and away
from the apex.
25. If an object is viewed through a base-down prism, it
will seem to be displaced:
26. True or False? In hand neutralization, as lenses are
moved up and down, side to side, and rotated, objects viewed through
the lens appear to move as follows:
27. The Latin term "oculus uniter" indicates:
28. A mismatch between the customer's P.D. and the frame's
P.D. could give rise to any of the following symptoms:
29. Unwanted base-in or base-out prism causes a floor or
other flat surface to seem:
30. How much, and in what direction, should lenses be decentered
if the anatomical P.D. is 69 mm. and the geometric center is 64
mm.?
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