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

positioned. The lens is placed in the instrument. The lens holding device is not allowed to touch the lens. The lens is rotated (Figure 2-16) until the sphere lines of the lensmeter target are sharp and unbroken. When these lines are clear, the cylinder axis is correct. (The

lens also may be moved horizontally and vertically in an effort to begin centering the target lines over the central crosshairs of the eyepiece or screen).

With the lens correctly rotated for axis position, the lensmeter power wheel is turned in the appropriate

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FIGURE 2-17 The lens designation (R or L) is always marked on the upper half of the lens so that the lens will not be blocked upside down. Although not as critical for nonprismatic single vision lenses, an inverted prism lens or multifocal would be useless when inverted. (The lens is pictured from the back side.)

direction to check the cylinder power. (When minus cylinder notation is used, this is always in the minus direction.) The power wheel should read them as the sum of the sphere and cylinder powers. For example, if the lens power is +5.00 –1.00 × 180, the power wheel should read as +4.00 because plus 5.00 minus 1.00 equals +4.00.

Next the lens is moved carefully left, right, upward, or downward until the target is accurately centered. (The lens holding device must be pulled away from the lens surface so that the lens will not get scratched.) If the lens has an especially high cylinder power, it may be necessary to rock the power wheel between sphere and cylinder readings to achieve a correct centration. This is because only one set of target lines may be visible at a time. The lens should not rotate during this process. Rotating the lens causes the axis to be off. When the target is accurately centered, the lens may be spotted (Figure 2-17).

The power verification in a spotting procedure for a spherocylinder lenses is summarized in Box 2-2.

Marking the Lens Right or Left

As soon as the lens is spotted, it should be removed from the lensmeter and marked for the right or left eye.

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 2-2

Spotting Single Vision Sphere or Spherocylinder Lenses with a Standard, Crossed-Line-Target Lensmeter

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 prescription prism, the illuminated target is moved until it is located at the position at which the prism equals that called for in the prescription.

7.The lens is spotted.

Lenses are commonly marked on the back surface with a wax pencil. The letter R or L in uppercase letters is written in the upper half of the lens, above the three spots. Figure 2-17 shows that the letters are written normally (not in mirror image as is commonly done in surfacing procedures).

The lens is then returned to its tray with the back (concave) side down. Placing the lens front side down risks scratching the front lens surface as the lens slides in the tray. Traditionally the right lens always is placed in the right side of the tray and the left lens in the left side (Figure 2-18). In the production process, everyone expects the right lens to be on the right side. If it is not placed correctly in the tray, lenses easily can be marked or edged for the wrong eye.

A Lens Prescription that Includes Prism

OPTICAL CENTER OF A LENS

Up to this point the procedure described has been limited to single vision lenses with no prism power indicated in the prescription. The procedure detailed here includes centering of the illuminated lensmeter target in the middle of the crosshairs. By centering the target in the crosshairs, the optical center (OC) can be found. Locating the OC and spotting it ensures that after the lens is edged, the OC will be positioned before the pupil of the eye. No prism exists at the OC of a lens. When no prescribed prism is in the prescription, the needed point of reference is the OC. The OC becomes

FIGURE 2-18 By convention the right lens is placed on the right-hand side of the tray and is always face up to prevent scratches on the front surface. In spite of this convention the lenses should still be checked before each step in the fabrication process to verify that the correct lens is being used.

the reference point. It is of major importance in aligning the lens. Therefore it is known as the major reference point, or MRP. So when no prism is in the prescription, the OC is the MRP.

OPTICAL CENTERS NOT WITHIN THE LINE OF SIGHT

Whenever the eye looks through a lens at a place other than the OC, the object appears to be displaced from its actual location.3 This apparent displacement is caused

3In the case of a plano cylinder, no prismatic effect occurs anywhere along the axis of the cylinder. It might be said that the “optical center” is really an “optical line.”

by the prismatic effect of the lens. When the eye looks through the OC of a lens, no apparent displacement exists. However, when the eye looks through an offcenter point on the lens, the object does appear displaced.

Most spectacle lens prescriptions have no prescribed prismatic effect. This means that the OC of the lens needs to be in front of the eye. Unwanted prism requires one eye to turn away from the normal direction of gaze to keep from seeing double. This can be very uncomfortable.

Sometimes a prescription includes prescribed prism. The lens must be positioned so that the amount of prism called for will be in front of the wearer’s pupil, in the eye’s line of sight. When prism is called for in the prescription, the point on the lens with the correct amount of prism becomes the point of reference; it is not the OC. This prismatic point is important in alignment of the lens, and now it becomes the MRP. So when the prescription contains prescribed prism, the OC and MRP are two separate points.

A synonym for the MRP that is perhaps even more descriptive is prism reference point, or PRP. MRP and PRP are the same.

Prentice’s Rule

A relationship exists between prism power and the distance between the OC and the MRP. For a desired prismatic effect the needed distance in centimeters between the OC and the MRP depends upon the power of the lens. It can be calculated using Prentice’s rule for decentration. Prentice’s Rule states the following

= cF

where:

= Prism diopters at the point of reference c = Distance in centimeters

F = Power of the lens

For spherical lenses the calculation is straightforward.4

Example 2-2

How far from each other will the OC and MRP be for a -3.00 D lens when a 1.5 prismatic effect is desired?

4For decentration of plano cylinder lenses along major meridians, the power used is the power of the cylinder in the meridian of decentration. If a cylinder is oriented at an oblique axis and the direction of decentration is horizontal or vertical, prism will be induced with its base oriented obliquely. (For more information on the optical effects of decentration, the reader is encouraged to see Brooks CW, Borish IM: System for ophthalmic dispensing, Boston, 1996, Butterworth-Heinemann.)

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Solution

In this example,

F = -3.00 D

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

sphere/cylinder target lines must be positioned to correspond to the location of the desired prismatic effect.

Example 2-3

and

= 1.5

Prentice’s Rule states that

= cF

This may also be written as

c =

F

Therefore

c = 1.5 = 0.5 cm 3

So the OC is OC = 5 mm from MRP

Spotting Lenses with Prism

SINGLE VISION LENSES

The procedure of spotting single vision lenses with prism is nearly identical to that of nonprism lenses. The only difference is in how the illuminated target is centered. Instead of placing the center of the illuminated target at the center of the crosshairs, the illuminated

A right lens calls for 2.0 base out prism. How would it be positioned for spotting?

Solution

To correctly position this lens the following steps must be taken:

•The sphere/cylinder target intersection must be on the circular mire marked 2.0.

•Because the prism is horizontal, the illuminated target must be on the 180-degree line.

•Base out for the right eye is to the left. Therefore the center of the illuminated target must be on the 2 prism circle where it crosses the 180-degree line to the left.

(Note: Before reading prism, the practitioner must be sure that the internal horizontal and vertical lines that are a part of the black lensmeter mires are really horizontally and vertically aligned. If no internal or external degree references are available to use, the cylinder axis is set at zero and the mires are lined parallel to the illuminated target in the lensmeter.)

When the lens is correctly positioned the lensmeter target appears as shown in Figure 2-19.

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

X

FIGURE 2-20 The major reference point (MRP) of a lens ultimately will be positioned before the wearer’s pupil center. If prism is indicated in the prescription, the optical center (OC) is displaced purposely. Therefore the point that will be important in centration and that is consequently spotted is the major reference point—not the optical center.

Once this position is achieved and the cylinder axis is correct, the lens may be spotted. Figure 2-20 shows the lens spotted with the three lensmeter dots. The center lensmeter ink spot is no longer at the center of the uncut lens, but the center dot still indicates the location of the MRP.

PRESCRIBED PRISM WITH BOTH HORIZONTAL AND VERTICAL COMPONENTS

In a case in which both horizontal and vertical prisms are called for simultaneously in the same lens, the target must be moved both laterally and vertically until it reaches the desired position. That position is one where the target center is directly above (or below) the required horizontal prism reading. It is also exactly left or right of the required vertical prism reading.

Example 2-4

A right eye requires 4.0 D base out and 2.0 D base up. How would the lens be positioned for spotting?

Solution

To correctly position the lens, the target must be four full prism diopter units to the left of center and two full prism diopter units above center. (The half diopter prism ring should not be counted as a full prism diopter.) This is shown in Figure 2-21.

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PRISM RESTRICTIONS WITH ASPHERIC AND ATORIC LENSES

When aspheric or atoric lenses are used, the lens design changes power in a concentric-ring–like manner from the center to the edges of the lens. The center of the “ring” must be in front of the wearer’s eye. This means that finished single vision aspheric or atoric lenses cannot be decentered to create prescribed prism. When prescribed prism is present in the prescription, it is necessary to begin with a semifinished lens. The prism must be ground onto the lens precisely at the center of the aspheric “ring.” Once a semifinished aspheric or atoric lens has been surfaced for the correct prism amount, it may be spotted in the usual manner as described, with no adverse effects.

Decentering a finished (stock lens) aspheric or atoric places the concentric area in front of the eye. The center of aspheric design will be somewhere else. This destroys the advantage of the aspheric design. It would be optically better not to use an aspheric or atoric design at all. A regular lens gives better optics than an aspheric lens that has the central zone of the lens moved away from where it should be.

Spotting with Autolensmeters

Autolensmeters perform in much the same manner as the manual variety. Their chief advantages are speed of operation when lenses of unknown power are measured and reduced training time for new operators.

Autolensmeters vary in the appearance of the screen and in operation. An example of an autolensmeter is shown in Figure 2-22. This particular instrument includes a measuring mode and a layout mode. In other words, for simple measurement of a lens power, the screen appears as shown in Figure 2-23. In the layout mode the target on the screen is similar to the view into a manual lensmeter (Figure 2-24). It is possible to spot the lens in the measuring mode but is more convenient using the layout mode.

Use of an autolensmeter for spotting lenses requires no presetting of the instrument. Power readings may

display in normal quarter-diopter increments or in

1 smaller increments—in some cases down to 100th of a

diopter.

How the spectacles are physically placed in the autolensmeter varies by manufacturer. For example, if the spectacles are placed in an autolensmeter with an upright design such as the Humphrey autolensmeter, the temples will hang downward and the top of the spectacle frame will be closest to the operator.

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

FIGURE 2-22 The Humphrey Lens Analyzer (Humphrey Instruments [division of Carl Zeiss, Inc.], San Leandro, Calif.) is an example of an automated lensmeter.

AUTOLENSMETER SPOTTING WITH NO PRESCRIBED PRISM

To lay out a single vision lens that has no prism in the prescription (Rx), the lens is held over the lens stop (Figure 2-25). When the lens is held in the instrument, the sphere, cylinder, and axis readings on the screen immediately show actual lens power. A gray circle surrounds the cross and represents the lens. The circle and cross move as the lens is moved.

If the lens is a sphere, having no cylinder component, it must now be centered in the lensmeter. The lens is positioned correctly when the gray lens circle surrounds the smallest of the concentric circles on the screen and the cross is exactly in the middle of the simulated lensmeter mires. The screen also reads the amount of horizontal and vertical prism. Both of these readings should be zero. Spherocylinder powers require that the lens be rotated until the correct axis appears.

Example 2-5

A prescription calls for a lens with a power of –5.50 –0.50 × 121. How might this lens be spotted using an autolensmeter?

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Solution

The lens is selected from inventory and held in position in the autolensmeter. The axis appears as a number, but the cross on the screen also turns to match the axis of the cylinder. The lens is rotated until the axis shown in the autolensmeter matches the axis called for in the prescription. Next the lens circle is moved on the screen to the central position (Figure 2-26).

The practitioner must verify that the cylinder axis is still correct and check whether the horizontal and vertical prism readings are at zero. If everything checks out correctly, the lens may be spotted with the spotting device (Figure 2-27). The spotting device places three dots on the lens in exactly the same way that the manual lensmeter does.

AUTOLENSMETER SPOTTING WITH PRESCRIBED PRISM

Using the autolensmeter to spot a lens with Rx prism is done much like with a conventional lensmeter. In the layout mode an autolensmeter is likely to have both a simulation of a conventional lensmeter target and a numerical prism reading that tells exactly how much vertical and horizontal prism is showing up in the lens at the measured position. As the lens position changes, the numerical prism readings also are changing.

Example 2-6

A right lens prescription calls for a power of –5.50 –0.50 × 121 with 2.50 of base out prism and 0.50 base up. How would the lens be positioned and spotted using the example autolensmeter?

Solution

The lensmeter is set for layout mode and for a right lens. The lens is held in the lensmeter and is positioned for the correct 121-degree cylinder axis as in the previous example. Next the lens is moved laterally. The target should move toward the base out and up side of the mires. The circles are used as a general guideline, but the numerical readout is monitored until it shows 2.50 of base out horizontal prism and 0.50 of base up vertical prism (Figure 2-28). The cylinder axis still must read correctly. The lens may now be spotted.

Story Told by the Numerical Readout

Reading prism with an autolensmeter relies on the numerical readout for prism amount as the final word, not the way the target appears prismatically in the image of the simulated lensmeter target. The simulated position of the target may not exactly replicate the accuracy of the manual lensmeter target, especially for