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

A BLEMISHED SPHERE

The most versatile lens type is the spherical lens. Because a sphere is of uniform power in all meridians, it may be rotated around its OC without changing its optical characteristics.5 Therefore if a large chip were to be broken from its edge as shown in Figure 2-29, A, the lens must be turned only so that during the edging process this chipped portion will be edged away (Figure 2-29, B).

A BLEMISHED SPHEROCYLINDER

A spherocylinder lens is less adaptable. It has only two possible orientations. The decision on how best to orient a slightly damaged spherocylinder is made after lens spotting but before it is marked at the top with an L or R.

Example 2-7

A prescription for the right eye has a of power +2.00 -1.00 × 10. The lens is verified and spotted as shown in Figure 2-30, A. In recognition that the lens is scratched, the frame shape is checked against the lens. If the scratch appears within the frame shape, the lens is unacceptable (Figure 2-30, B). Can this lens be used?

Solution

A cylinder axis goes from one side of the lens to the other. Axis 180 is the same as axis zero. Axis 90 is the same as axis 270. For this reason, cylinder axis is specified only up to 180 degrees. Turning a cylinder lens upside down does not affect the optics. In this example, a 10-degree axis is the same as a 190-degree axis. In the example, turning the lens also repositions it

5This does not apply to a polarized lens.

B

FIGURE 2-29 A, The lens will be ruined if edged as marked. By turning this spherical lens, the same optical endpoint is achieved without a sacrifice in quality. B, The lens should be remarked so that in marking and blocking the chipped portion will be positioned as shown.

so that the scratch will be ground completely away. This is shown in Figure 2-30, C. The lens may now be marked with the appropriate R and used.

A BLEMISHED PRISM LENS

When prescribed prism is present, standard lens blanks may be rotated only before the MRP is marked for the correct amount of prism. Once the lens is marked for prism, the lens may no longer be rotated.

Lenses that have been specifically surfaced to obtain a prismatic effect that could not be achieved by decentration of a standard lens blank cannot be rotated at all.

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Scratch

A

Scratch

B

C

FIGURE 2-30 A scratch on a lens may not make the lens unusable. Whether it can be used depends on scratch location, frame size, and—in the case of cylinder lenses—axis orientation. A, The lens is marked for edging (no prism). If used as oriented, the scratch will appear on the edged lens, as shown in B. However, because a spherocylinder without prism can be rotated 180 degrees without any change in optical effect, as C demonstrates, this lens is still useful. (Any rotation of the lens must be done after spotting and before the lens is marked.)

Spotting of Polarizing Lenses

Polarizing lenses block horizontally polarized light that reflects from glare surfaces and allow vertically polarized light to pass through the lens. The lens does this because it has a laminated layer sandwiched within the lens. This laminated layer must be oriented correctly in the frame; otherwise the lens will not work correctly.

To ensure that the lens is oriented correctly, notches are located in the laminated layer on either side of the lens (Figure 2-32). These notches must fall on the 180-degree line.

For sphere lenses, the OC of the lens is located and spotted. Then the outer two lensmeter dots are removed, leaving only the center dot. When the lens is

FIGURE 2-32 Polarizing lenses must be spotted so that the two notches in the laminated layer of the lens are oriented on the 180-degree line. If this is not done, the lens will not cut out reflected glare.

ready to be blocked, the central spot indicating the OC and the two notches are used to position the lens. Because it is likely that a polarizing lens will have been custom surfaced from a semifinished lens, it is probable that a number of these lenses will have their OCs displaced nasally. After the lens is spotted, it is turned so that the displaced OC will indeed be nasal before marking an R or an L on the lens. (This is the same procedure described in the previous section.)

Spherocylinder lenses and most lenses with prescribed prism have to be surfaced so that the cylinder axis is correctly placed in relationship to the lens’ direction of polarization.

Prescription Verification and

Spotting of Multifocal Lenses

Multifocal lenses are checked for surface defects and internal deficiencies in the same manner as are single vision lenses. They also should be spotted. This marks the MRP and the 180-degree line. This 180-degree line indicates the horizontal plane of the lens. As with single vision lenses, the 180-degree line is not the axis of the cylinder but rather the line from which the axis of the cylinder is measured.

METHOD FOR SPOTTING FLAT-TOP–SEGMENT MULTIFOCALS

For multifocals, the bifocal should be placed in the lensmeter as it will be when mounted in the frame. This

means that for flat-top bifocals, the segment top should be horizontal. The sphere power is dialed into the lensmeter. If the lens has a cylinder component, the axis of the cylinder should be dialed in as well.

Next the MRP of the lens is located. When the lens is spherical, the lens may be spotted. With spherical lenses, if the segment was not placed in the lensmeter exactly straight, the lensmeter-spotted 180-degree line will not be horizontal. It may need to be tilted during the next step of centration so that the segment line will be horizontally straight. This is explained in Chapter 6.

For multifocals with spherocylinder powers, the axis of the cylinder has been custom ground for that particular lens. The lensmeter is set for the axis ordered and the lens rotated to the correct axis. With MRP and cylinder axis correct, the lens is spotted, just like a single vision lens. After the lens has been spotted, the three dots on the 180-degree line should be parallel to the upper edge of a flat-top segment (Figure 2-33, A). If they are not parallel to the top of the bifocal segment, the cylinder axis is off and the lens was surfaced improperly. An example of this is shown in Figure 2-33, B. When this happens, mounting the lens in the frame with both its segment top straight and cylinder axis correct will be impossible. (Only in the case of an extremely low-powered cylinder may an error like this fall within acceptable quality standards.)

To precheck the lenses as a pair, the lenses are held front-to-front with the segments overlapping (Figure 2-33, C). If two different MRP heights or two different segment insets do not exist, the center spots of both lenses should be at the same place. If they are not, a problem with unwanted horizontal or vertical prism is likely after the lenses are edged.

Box 2-3 provides a summary of spotting flat-top multifocals.

HOW TO SPOT ROUND-SEGMENT MULTIFOCALS

For multifocals with round segments and a spherically powered distance portion, the MRP and 180-degree line are marked first by rotation of the lens to the estimated segment position. Segments are rotated inward toward where the nose would be. With a right lens in the lensmeter, the lower part of the lens is rotated inward so that the segment is slightly right of center (Figure 2-34, A). The OC is located and the lens spotted (Figure 2-34, B). The left lens segment will be rotated likewise somewhat left of center. Even though these left and right inward lens rotations are not likely to be exact, any inaccuracies can and will be corrected later when the lenses are being prepared for blocking.

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A

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

For spherocylindrical round-segment multifocals the lens is placed in the lensmeter. The correct sphere power and cylinder axis are dialed into the lensmeter. The OC is found, and the lens is rotated to the correct cylinder axis. Now the lens is spotted. Whether the cylinder axis is within tolerance in reference to the near segment position can be judged only during the centration process that follows.

Invisible segment or “blended bifocal” lenses have round-segment areas with borders that have been

smoothed out or “blended” to make them unnoticeable. These blended round-segment lenses are spotted in exactly the same manner as regular round-segment lenses. (Blended bifocals should not be confused with progressive addition lenses. Progressive addition lenses will be considered shortly.) Box 2-4 summarizes the procedure used to spot round-segment lenses and blended bifocals.

To conclude the spotting/verification process, all segments are verified for accuracy of the near addition

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

BOX 2-3

Spotting Flat-Top Multifocals

1.The lens sphere power and lens cylinder axis are dialed into the lensmeter.

2.The lens is placed in the lensmeter.

3.The major reference point is located.

4.If the lens is spherical, the lens is spotted.

5.If the lens has a cylinder, the lens is rotated until the sphere lines are clear.

6.If the lens has prescribed prism, the illuminated target is moved until it is located at the position where the prism equals that called for in the prescription.

7.The lens is spotted.

8.For spherocylinder lenses and lenses with prescribed prism, the practitioner must verify that the segment top and three lensmeter dots are parallel to one another.

9.When both lenses have been spotted, the lenses are lined up front-to-front to check for R-L spotting accuracy. The central spots should overlap.

power after the lens is spotted. The lens is removed from the lensmeter and marked with an L or R.

DISAPPEARANCE OF THE MAJOR REFERENCE POINT INTO THE SEGMENT PORTION

When bifocal and trifocal lenses are surfaced, the distance OC could be ground into the lens so that it will be halfway between the top and bottom of the edged lens shape (Figure 2-35). When the distance OC is placed here, edge thickness will be equal at the top and bottom of the edged lens.

Sometimes bifocal or trifocal lenses are ordered with the bifocal or trifocal segment especially high. In fact, the segment tops occasionally may be higher than the middle of the edged lens. If the distance OC is placed on the 180-degree midline and the segment is higher than the 180-degree midline, the distance OC will be in the segment area (Figure 2-36, A). With use of the lensmeter, the distance OC becomes “lost” in the segment.

If the distance OC falls within the segment of the lens, distance power measurements cannot be made at the OC. They must be made above the OC and above the segment. For measuring power this is not serious. The process of measuring power above the MRP location is standard for progressive addition lenses.

Unfortunately when the distance OC or MRP cannot be located easily, the lens cannot be spotted and

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A

B

FIGURE 2-34 A, To spot a round segment lens for edging, the segment for this right lens is rotated slightly in a direction toward the nose. B, The optical center is located, and the lens is spotted. With spheres, this estimated nasalward segment rotation places the lens more as it should be and makes the centration and blocking process easier.

checked for the wearer’s interpupillary distance (PD) accurately. Prismatic effects from the segment interfere with distance lens optics and cause the location of the distance OC to appear displaced. For all except round segments, though, lens centration can still be carried out normally. When flat-top lenses are laid out for edging, the segment borders are used for reference instead of the distance OC.

Leaving the OC on the horizontal midline of a multifocal lens, even if the segment is at or above the center of the lens, is not standard procedure for all surfacing laboratories. In cases in which equality of upper and lower lens edge thicknesses is not a factor, the surfacing laboratory commonly places the OC slightly higher than the top of the segment line (Figure 2-36, B).

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BOX 2-4

Spotting Round-Segment Lenses and

Blended Bifocals

Spherically Powered Round-Segment Lenses

1.The sphere power is set in the lensmeter.

2.The lens is placed in the lensmeter.

3.The segment is rotated inward so that it is somewhat nasally located.

4.The lens is moved until the major reference point is located.

5.The lens is spotted.

6.Because these lenses are spheres, the two outside lensmeter dots are wiped off to avoid confusion later.

Spherocylindrically Powered Round-Segment

Lenses

1.The sphere power and cylinder axis are set in the lensmeter.

2.The lens is placed in the lensmeter.

3.The lens is moved until the major reference point is located.

4.The lens is rotated until the cylinder axis is correct

5.The lens is spotted.

1/2

FIGURE 2-35 A common location for the distance optical center (OC) of a lens is halfway between the top and bottom of the edged lens.

An Alternative Round-Segment Spotting Method

When the distance OC is within the segment area of a round-segment lens, the following process may be used:6

1.Distance power is verified at a location just above the segment. This distance should be the same for both left and right lenses.

6Several factors can prevent this process from being entirely accurate, such as the horizontal prism that may be induced by the presence of a strong oblique cylinder. Yet because the near add can, with higher powers, affect the apparent cylinder axis as well as induce horizontal and vertical prism of its own, the method remains a viable compromise.

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

Distance optical center

A

Distance optical center

B

FIGURE 2-36 A, A high bifocal segment combined with conventional vertical placement of distance optical center (OC) “loses” the OC in the segment. It cannot be accurately found with a lensmeter. B, Some surfacing laboratories routinely place the distance OC above a highly placed segment top unless it will cause thickness differences between upper and lower lens edges to be cosmetically objectionable.

2.Care should be taken to keep the cylinder axis correct. The lens is moved laterally until no horizontal prism exists. (If prism is prescribed, the lens is adjusted until the correct prism amount appears.)

3.The lens is spotted at this location. As long as the three dots on the lens are kept horizontal, the cylinder axis will be right.

Unless a strong oblique cylinder is present, the OC should be located correctly.

Progressive Addition Lenses

Progressive addition lenses have certain “hidden” markings used in establishment of lens orientation. Lenses coming from the surfacing laboratory also are marked with non–water-soluble ink. If the visible inked marks are applied correctly, the lenses do not need to be spotted. However, they should be verified before edging.

VERIFICATION OF PREMARKED

PROGRESSIVES

To check distance lens power, the lens is positioned in the lensmeter to view through the circled area above the MRP (Figure 2-37). (The MRP usually comes marked with a dot.) The center of this circled area used to locate the point for verifying distance power is called the distance reference point, or DRP (Figure 2-38). Incidentally, some prism will almost always be at the DRP because the DRP of the lens is not the OC of the lens.

To check distance power, the power wheel is set to the sphere power and the cylinder axis wheel to the ordered cylinder axis. The lens is rotated until the target lines are clear and unbroken. The non–water- soluble horizontal reference marks on the lens should be oriented horizontally and not tilted. If they are tilted, the axis of the cylinder is incorrect.7

Near lens power is checked through a point well into the near zone so that no intermediate progressive

7It is possible that the cylinder axis is correct, but the visible lens marking was applied incorrectly at an angle.

power is measured. This point usually is marked with a non–water-soluble inked circle and is called the near reference point, or NRP. The near addition is verified in this specified NRP area.

To check for prism, the lens is centered in the lensmeter at the MRP. A synonym for MRP is prism reference point (PRP). However, the lensmeter target may not be altogether clear at the MRP because the progressive zone of the lens starts here. The lens power in the lower half of the measuring area is increasing and may blur the lower half of the target.

Progressive lenses often come with equal amounts of vertical prism in both right and left lenses. This allows the lenses to be made thinner. Equal amounts of “yoked” vertical prism for “prism thinning” purposes are both allowable and usually expected. For example, both right and left lenses may read 1.5 base down at the PRP. Because the prism thins the lens and the net binocular prismatic effect is zero, the lenses are considered free of unwanted vertical prism.8

8For more information on prism thinning and its workings, consult Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann (Chapter 11).

FIGURE 2-39 When the engraved circles on the lens are located and dotted, the guide marks can be reconstructed for use in layout and power verification.

As stated previously, if the lenses are correct and have non–water-soluble progressive lens markings, the lens does not need to be spotted. The existing markings will be used in the blocking process. If the lenses do not come with markings, or if it appears that the markings were applied inaccurately, then the markings must be reapplied.

PROGRESSIVE LENSES THAT ARE NOT PREMARKED

If a progressive addition lens leaves the surfacing laboratory without visible markings, the finishing laboratory should reconstruct the manufacturer’s recommended system of identifying marks. This is done as follows:

1.The hidden marks are located on the lens surface. This may be done in several ways.

•The lens is held under an incandescent bulb. For maximum visibility, the background should be matte black. Two small, etched marks usually are found at about 17 mm from either side of the lens center.

or

•The practitioner holds the lens up to a light and looks through the lens.

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

or

•A commercially available lens mark finder that illuminates and magnifies is used.

2.With a marking pen, a dot is placed on the centers of the small marks on the front surface of the lens.9

3.The lens is placed on a verification card and turned so that the dots fall at the indicated “engraved circle” points of the card. The lens manufacturer provides verification cards.

4.The appropriate lines are drawn on the lens from the master markings found on the verification card (Figure 2-39). Some lens manufacturers may provide easily removable decals that may be placed on the lens using the hidden circles for reference (Figure 2-40). This saves drawing the marks on the lens.

Chapter 5 explains more on progressive addition

lenses.

9Marking the etchings on the back of the lens when the etchings are actually found on the front can cause a significant amount of error because of parallax.

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FIGURE 2-40 A decal placed on the lens in accordance with previously located hidden circles is quick and neat.

1.True or False? If a stock lens is available, it is normally used instead of a custom surfaced lens.

2.For high plus lens powers, as center thickness increases, which of the following is true?

a.Magnification of the wearer’s eyes increases.

b.Minification of the wearer’s eyes increases.

c.No relationship exists between lens power, center thickness, and either magnification or minification.

3.Given the generalized formula for minimum blank size, which of the following is the minimum blank size required for a frame having an effective diameter of 53 and a decentration of 3 mm per lens when no allowance is made for chipping?

a.54 mm

b.59 mm

c.62 mm

d.68 mm

e.71 mm

4.Apart from price considerations, in which of the following instances is it critical that the smallest possible lens blank be used?

a.When the prescription is minus in power

b.When the prescription is plus in power

c.Apart from economic considerations, neither plus nor minus prescription power is a consideration.

d.Using the smallest possible blank size is critical for both plus and minus prescriptions.

5.Internal lens deficiencies are inspected for in which of the following ways?

a.Looking at the filament of an unfrosted light bulb as it reflects from the surface of the lens

b.Observing a straight line through the lens as the lens is moved back and forth

c.Looking through the lens at a black background under indirect illumination

d.All the above

e.Both b and c

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6.A prescription is –2.00 –1.25 × 30. Which of the following two power wheel readings appear when first sphere, then cylinder lines, are brought into focus?

a.–2.00

b.–0.75

c.–3.25

d.–1.25

e.+0.75

7.True or False? When the lensmeter is used to neutralize a lens of unknown power and obtain results directly in plus cylinder form, the power wheel is turned into the high minus numbers and slowly moved in the plus direction until the cylinder lines first are brought into sharp focus.

8.The lensmeter power wheel is turned into the high plus power. The power wheel is then turned back slowly, reducing plus power until the sphere lines are clear. (The power wheel now reads +2.00 D; the axis wheel reads 12.) The power wheel is then turned further into the minus until the cylinder lines become clear. (This causes the power wheel to reads –1.00.) Which of the following is the prescription?

a.+2.00 –1.00 × 12

b.–1.00 +2.00 × 12

c.+2.00 –1.00 × 102

d.+2.00 –3.00 × 12

e.–1.00 +2.00 × 12

9.The lensmeter power wheel is turned into the high plus power and slowly returned until the cylinder lines are clear. (The power wheel reads +4.00 D; the axis wheel reads 180.) The power wheel is turned further into the minus until the sphere lines become clear. (The power wheel reads +3.00 D.) Although this is not the correct procedure for lensmeter use, which of the following is the prescription?

a.+4.00 –1.00 × 90

b.+4.00 –3.00 × 180

c.+4.00 –1.00 × 180

d.+3.00 –1.00 × 90

e.+3.00 –1.00 × 90

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

10.When a single vision lens is spotted for edging, in reference to edged lens orientation, the lensmeter ink dots will be on which of the following?

a.The sphere meridian

b.The cylinder meridian

c.The 180-degree meridian

d.The cylinder axis

11.True or False? When a lens is spotted by using plus cylinder notation instead of minus cylinder notation, the lens is turned 90 degrees from where it would otherwise be located.

12.By convention, lenses in the finishing laboratory normally are marked for right (R) or left (L) on which of the following?

a.Outside surface, in mirror image, on the upper half

b.Inside surface, on the lower half

c.Inside surface, on the upper half

d.Outside surface, on the lower half

e.Outside surface, on the upper half

13.True or False? Lenses should never be placed convex-side-down in the job tray.

14.True or False? Lenses are placed convex-side-up in the laboratory tray. The wearer’s right lens will be in the lower left half of the tray and the left lens in the lower right half of the tray.

15.For high cylinders in which either the lensmeter sphere or the cylinder lines are visible one group at a time, but not simultaneously, the MRP is found by which of the following?

a.Centering on the sphere line

b.Centering on the center cylinder line

c.Alternately centering on first the sphere line, then on the center cylinder line

d.Using a lens center locator

e.Cannot be found by any of the above methods

16.Which of the following points should always appear either exactly in front of (or somewhat below) the wearer’s pupil?

a.OC

b.DBC

c.Geometric center

d.MRP

e.IOP