Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

1.True or False? Lenses are surfaced in a finishing laboratory.

2.Which of the following lens types has the same power over the entire lens?

a.A single vision lens

b.A segmented multifocal lens

c.A progressive addition lens

3.Which of the following terms is a synonym for a “finished lens”?

a.Single vision lens

b.Semifinished lens

c.Uncut lens

d.Progressive addition lens

e.Multifocal lens

4.True or False? It is possible to individually cast mold both segmented multifocal lenses and progressive addition lenses to the prescribed power without surfacing the lens.

5.True or False? Some edgers operate in both patterned and patternless modes.

6.A “frame tracer” is often used in conjunction with which of the following?

a.Lensmeter

b.Lens blocker

c.Lens edger

7.Of the following steps in lens fabrication, which process occurs last?

a.Blocking

b.Grooving

c.Edging

d.Spotting

8.Arrange the steps in the edging process in their correct order.

1.blocking

2.centration

3.edging

4.finding lens axis and MRP location

a.2, 3, 1, 4

b.2, 4, 1, 3

c.1, 2, 3, 4

d.4, 2, 1, 3

e.4, 3, 2, 1

Selecting the Most Appropriate

Lens Blank

A lens either is “pulled” from available stock or must be obtained from a surfacing laboratory. A stock lens is appropriate if it fits the following criteria:

1.Fulfills all the optical requirements of the written prescription

2.Is big enough to cover the frame’s lens opening

3.Is appropriately thin for the lens material and frame type1 and size selected

SELECTING THE MOST APPROPRIATE LENS

In the decision as to whether a stock lens is the most appropriate lens, the first question concerns availability. Is the lens in stock in the finishing laboratory? If not, is it made as a stock lens? It may be appropriate to order a stock lens instead of having one surfaced. However, even if a stock lens is available in the right material and power, it may or may not be used. Stock lenses may or may not work if any of the following are true:

•The prescription requires prescribed prism.

•The lens blank is too small for the frame.

•A plus lens blank is larger than needed, resulting in an unnecessarily thick center and edge.

Size of the Lens

If a lens is too small and gets edged anyway, a gap will exist between the lens and the edge of the frame, making the lens unsuitable (Figure 2-1). Several ways exist to determine whether the lens blank will be large enough.

A blank size determiner may be used in combination with the frame. Another method is to use the following formula:

MBS = ED + 2(dec.) + 2

where:

MBS = Minimum blank size

ED = Effective diameter of the frame (dec.) = Decentration per lens

1For grooved mountings, the lens edge must not end up too thin for grooving. For more information on this, see Chapter 14 on groovedlens mountings.

FIGURE 2-1 A lens blank is too small if, when properly decentered, it will not cover the lens opening of the frame.

(The exact use of this formula and the concept of minimum blank size is explained in Chapter 4.)

Minus Lens Center Thickness

Lenses come in both plus and minus powers. Minus lenses are thinnest in the center and get progressively thicker toward the edge. Therefore the final size of a minus lens does not affect center thickness of the lens. Figure 2-2 shows that regardless of whether the lens blank chosen was large or small, edged minus lenses have exactly the same center and edge thickness.

Even though center thickness for minus lenses remains the same if the lens gets larger, edge thickness does increase with increasing lens sizes.

Effect of Blank Size in Plus Lenses

With minus lenses, edge thickness increases with lens size. With plus lenses, center thickness increases with lens size. The larger the plus lens, the greater the center thickness will be (Figure 2-3).

Unnecessary use of large plus lens blanks results in thick centers, thick edges, glasses that magnify the wearer’s eyes, and heavier lenses. Using a smaller lens blank when the frame size is small is much better. Sometimes no stock lens is small enough, and the lens should be ordered from the surfacing laboratory. By knowing the size and shape of the frame and the distance between the centers of the wearer’s pupils, the surfacing laboratory personnel are able to grind the lens so that the center and edge thicknesses are minimal.

The worst-looking examples of inappropriately used plus stock lenses are for small children’s frames. Using

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A

C

B

C H A P T E R 2 S P O T T I N G O F L E N S E S

A

B

C

D

FIGURE 2-3 Lens blank A and lens blank C both have the same plus power. When a larger-than-necessary plus lens blank is used (A), the result is a lens that is too thick in both center and edge (B). When the smallest possible blank is chosen (C), a much better functional and cosmetic result is obtained (D). Dotted lines indicate the diameter being cut from the blank to produce the finished lens.

a large stock lens results in extremely thick edges that are entirely unnecessary. The lenses are much thicker than they need to be, and the size of the child’s eyes is magnified. For plus lenses, magnification increases as center thickness increases.

VISUAL INSPECTION OF THE LENS FOR FLAWS

Before edging, the lens should be checked to ensure it is free from flaws. A scratched lens may have the correct power but still be unacceptable. Once a lens has been edged, it is too late to return it to the manufacturer because of a flaw, whether it is a stock or custom surfaced lens. During inspection of an uncut lens for flaws, its front surface quality, back surface quality, and internal lens characteristics are checked.

One method used to inspect surface quality is by use of an unfrosted incandescent bulb. The lens is held as if it were a mirror (Figures 2-4 and 2-5). It is tilted so that all areas of the lens surface are inspected. The image of the light bulb filament must be sharp and clear on all areas of the lens surface. The lens is then turned over and the second surface is inspected in a similar manner.

Internal lens properties may be checked by looking at the lens with a dark background and a light such as a 40-watt, incandescent clear (unfrosted) bulb positioned about 12 inches from the lens, striking it at an angle from behind (Figures 2-6 and 2-7). Any foreign substance in or on the lens scatters the light and causes the area of the foreign substance to be visible.

FIGURE 2-5 To inspect the entire surface rapidly, the inspector looks at the bulb filament and tilts the lens slightly.

Assuming that the lens is free of surface deficiencies or internal foreign matter, it may be checked for irregular power variations at this point. This check involves observation of a straight line, such as the edge of a fluorescent tube through the lens. If the straight edge is vertical, the lens is moved left and right along one of its major meridians (Figure 2-8).2 If the lens has any refractive power, the line observed through the lens

2The major meridians of a lens are along the cylinder axis and 90 degrees from the axis.

will appear to curve as the lens is moved (Figure 2-9). It should curve evenly. Unevenness in the curve indicates a power variation within the lens. Possible causes of unevenness are a wavy surface or nonuniformity of refractive index within the lens material. A wavy surface usually results from a surfacing problem and is less likely to occur in a quality stock lens.

Using the Lensmeter

The power of a lens may be measured using a lensmeter. A lensmeter also may be called a lensometer, focimeter, vertometer, and vertexometer. For clarity, the author of this book uses the word lensmeter when referring to any of these instrument types. Lensmeters may be manual or automated.

When the power of a lens is measured using a lensmeter, many people refer to the process as neutralizing the lens because the instrument is adjusted until the lens system within the lensmeter cancels out, or neutralizes, the power of the lens. For manual lensmeters, this brings the illuminated internal target in focus.

Lenses may be measured for power with the lensmeter before they are edged or after they have been mounted in the frame. The following explanation begins with lenses already edged and mounted in the frame.

FOCUSING THE EYEPIECE

Before attempting to read the power of a lens using a conventional manual lensmeter, the practitioner must first focus the eyepiece. An eyepiece that is not focused for the individual may cause an inaccurate reading.

FIGURE 2-6 A defect within the lens causes a scattering of light. By positioning the light source off to the side and viewing the lens against a matte black background, the main body of a clean, unblemished lens almost will disappear. Any defect becomes easily visible. Inspectors wearing white gloves for factory quality control use this method.

The lensmeter is focused by first turning the eyepiece outward. The practitioner looks into the instrument and rotates the eyepiece slowly inward until the crosshairs and rings within the instrument appear to first focus. The eyepiece location should be noted for future reference because it varies from individual to individual (Figure 2-10). When more than one person uses the same lensmeter, it may be helpful to put a colored mark on the eyepiece. Each person has a different color and will be able to quickly turn the eyepiece back to this colored mark each time.

FIGURE 2-7 A lens is inspected in oblique illumination against a matte black background.

READING A LENS IN MINUS CYLINDER FORM

Lenses with a cylinder component may be written with that cylinder as a plus cylinder or as a minus cylinder. When written with a plus cylinder, the prescription is said to be written in plus cylinder form. When written with a minus cylinder, the prescription is in minus cylinder form.

To read the power of a lens with a lensmeter such that the prescription may be written directly in minus cylinder form, the lens is placed in the instrument and the power wheel turned to high plus (Figure 2-11). Looking through the eyepiece, the practitioner turns the power wheel slowly in the minus direction until the target within the instrument begins to focus.

The target consists of two sets of lines that run at right angles to each other, forming a cross. One set is a narrowly spaced set of lines (Figure 2-12). (Older instruments have a single line.) This set of lines is known as the sphere lines. The set at right angles is a broadly spaced triple set (Figure 2-13). These lines are referred to as the cylinder lines.

As the power wheel is being turned in the minus direction, one of the following two things will happen:

1.Both sets of lines will focus simultaneously, indicating that the lens is spherical (Figure 2-14).

2.One set of lines will begin to focus before the other, meaning that a cylinder component is present.

Spherical Lens

In the event that the prescription is spherical, all lines—sphere line and cylinder line sets—come into focus at once. In this case the refractive power is read directly from the power wheel.

Spherocylinder Lens

For a lens with cylinder power the procedure begins the same. The power wheel is turned into the high plus powers and slowly turned back in a minus direction. However, this time the sphere lines may not come

FIGURE 2-12 The exact configuration of the sphere instrument. (Some instruments do not use lines at all, elongate into a circle of short lines when cylinder power

line(s) varies from instrument to but rather a circle of dots that is present in the lens.)

FIGURE 2-13 Cylinder lines appear at right angles to the sphere lines and are usually visible simultaneously, except in the case of extremely high cylinder values. (Instruments using a circle of dots will show elongation of the dots 90 degrees from the original direction of elongation. The original direction of elongation was seen first when the target was viewed for the correct sphere power.)

immediately into clear focus. Using the axis wheel of the lensmeter is necessary when a cylinder component is present (see Figure 2-11). As one set of lines begins to clear, rotation of the axis wheel probably will be necessary to increase clarity. As the axis wheel approaches one of two major meridians, one set of lines begins to clear. The sphere lines must be brought into focus first. If the cylinder lines clear up first instead of the sphere lines, the axis wheel is rotated 90 degrees, which brings the sphere lines into focus.

Once the sphere lines are clear, the sphere power and cylinder axis of the prescription are correct. They may be recorded directly from the power and axis wheels.

Next the power wheel is turned slowly once more in the minus direction until the cylinder lines clear. (The axis wheel should not be rotated.) The power wheel reading is noted. The cylinder value is the difference between the first reading (sphere lines) and the second reading (cylinder lines). It is recorded as a minus value (Box 2-1).

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4

3

2

1

1

2

3

4

5

FIGURE 2-14 When both sphere and cylinder lines focus at the same time, the lens has a uniform power in all meridians and is spoken of as being spherical. (Instruments using a circle of dots will show no elongation in any direction. The focused target appears to be the same as when no lens is present in the instrument and the power wheel registers zero.) If the sphere and cylinder lines do not intersect at the center of the mires, the lens optical center is not centered in front of the lensmeter aperture and prism is being manifested.

Example 2-1

A lens is placed in the lensmeter and the power wheel rotated to a high plus power—+10.00 or +12.00, for example. (The plus power need only be high enough to be certain that it is more plus than the power of the prescription lens.)

While the power wheel is rotated slowly back in the minus direction, the cylinder lines begin to clear. The

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BOX 2-1

Finding Spherocylinder Lens Power with a Standard, Crossed-Line-Target Lensmeter

1.The eyepiece is focused.

2.The power wheel is turned into the plus until the illuminated target blurs out.

3.The power wheel is turned slowly in the minus direction until the sphere lines clear.

4.The axis wheel is adjusted for optimum sphere line clarity.

5.Sphere power and cylinder axis are recorded.

6.The power wheel is turned farther in the minus direction until the cylinder lines clear.

7.The difference is taken between the two power wheel readings and recorded as a minus cylinder.

axis wheel reads 180. Because the cylinder lines are the wrong lines to start with, the axis wheel is rotated from 180 degrees to 90 degrees. Rotating the axis wheel 90 degrees will blur the cylinder lines and cause the sphere lines to clear. As the power wheel is turned slowly toward minus (away from plus) and the axis wheel slightly adjusted, maximum clarity is obtained. At maximum clarity the power wheel reads +2.50 D and the axis wheel reads 87 degrees. Two parts of the prescription can be recorded as follows:

Next the power wheel is rotated further in the minus direction. Now the cylinder lines come into focus when the power wheel reaches +1.00 D. The cylinder value is the difference between the two major meridians. The difference between +2.50 D and +1.00 D is 1.50 D. This is the correct cylinder value. It is recorded as a minus number. The prescription now reads as follows:

C H A P T E R 2 S P O T T I N G O F L E N S E S

Direct plus cylinder power readings are carried out as follows:

1.The power wheel is turned into the high minus numbers.

2.The power wheel is advanced slowly in the plus direction.

3.The axis wheel is rotated to cause the sphere lines to come into focus first. (Note: The sphere lines must always come into focus first, regardless of whether the lens is being read in plus or minus cylinder form.)

4.When the sphere lines are in focus, the sphere and axis values are recorded.

5.The power wheel is moved a second time in the plus direction, until the cylinder lines come into clear focus.

6.The difference between first and second power readings is the cylinder power. It is recorded as a plus value.

The plus cylinder procedure is identical to the minus cylinder procedure, with the exception of the direction of power wheel movement.

Spotting Lenses without Prism

POWER VERIFICATION AND SPOTTING OF SPHERES

When the power of the lens to be verified is of known power, rather than the entire neutralization process being performed, the power simply is checked as the lensmeter is set for the expected sphere value. If the lens is a sphere, the target should be immediately clear, which indicates a lens of the correct power. If the target is unclear, the lens power is incorrect. The actual power may be found through adjustment of the lensmeter power wheel.

Occasionally the lensmeter target will not appear clean and crisp even after being focused correctly. The best focus nevertheless may indicate a correct power reading. If this is the case, the lens is not well polished and should not be used.

When the lens has been determined to be of acceptable quality, it is centered optically in the lensmeter as the lens is moved until the center of the illuminated target crosses the center of the crosshairs in the lensmeter eyepiece or screen (see Figure 2-14). The marking device is then swung into position and the front surface of the lens spotted (Figure 2-15).

READING A LENS IN PLUS CYLINDER FORM

If a prescription is to be written in plus cylinder form, the lens can be read by the lensmeter in plus cylinder form. This way the power may be written directly from lensmeter values without having to convert or transpose the prescription from minus to plus cylinder form.

POWER VERIFICATION AND SPOTTING OF SPHEROCYLINDERS

When verifying spherocylinder lenses, the practitioner turns the lensmeter power wheel to the expected sphere power. In addition the cylinder axis wheel is turned until the axis indicated for the prescription is correctly