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

the dispenser use a remote site frame tracer. This option prevents many potential problems and saves time. It is explained with the discussion of frame tracers found later in this chapter.

EDGERS THAT MAKE PATTERNS

One type of frame for which traditional pattern makers are usually not capable of making a pattern is a rimless frame. Examples of frames without eyewires include rimless, semirimless, and nylon cord frames. A rimless frame does not have an eyewire that may be traced. However, if a frame tracer is available, then most tracers can be adapted to trace an old lens or the sample lens that comes with the new frame.

If a frame tracer that also can trace lenses is not available, some edgers are capable of making a pattern. To make a pattern with an edger, first the right lens is spotted with the lensmeter while it is still in the frame. This preserves the correct horizontal “180-line” orientation after the lens is out of the frame. Next, the coquille or old lens is centered on a grid to find the boxing center. Using an adapter kit, the lens is clamped and then mounted on the edger as if it were a pattern.

A pattern blank is mounted on a special “lens” block, placed in the grinding chamber of the edger, and edged as if it were a lens.

The head pressure of the edger must be set at its lightest so as not to chip a lens or bend or break a coquille being used as a pattern.

Consequences of Using a

Noncentered Pattern

Any calculations for decentering a lens are made on the assumption that the mechanical center of the pattern is at the boxing center of the pattern shape. So if the center of the pattern is off horizontally, the finished PD in the glasses will be off and will not match the wearer’s PD. This can cause unwanted prismatic effect and difficulty in comfortably wearing the prescription.

In the same way, when the mechanical center of a pattern is above its geometrical or boxing center, the MRP of the edged lens will be above its boxing center.

If the pattern is off, the difference must either be compensated for, or the pattern should not be used.

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USING A NONCENTERED PATTERN

Note: Some readers may wish to skip this section on noncentered patterns and return to it after familiarizing themselves with the centration of lenses presented in Chapters 4 through 6.

Mechanical Center

If the pattern’s mechanical center is 3 mm above its boxing center, then the MRP of the edged lens will be 3 mm above the boxing center. Normally both lenses will be edged using the same pattern. Because both MRPs are high by the same amount, for most lowpowered prescriptions this will not make much difference. Because both MRPs are elevated to the same level, no unwanted vertical prism will occur. Problems can occur if the MRP is specified, if the lenses are multifocals, or if just one lens is being edged.

Problems Resulting from the Replacement of Just One Lens

Having both lenses edged with a pattern in which the mechanical center is too high may result in future problems. For example, the wearer may return for an eye examination and needs only one lens replaced, or someone comes in with one lens badly scratched and needs a replacement. If the person who replaces the lens does not have the original pattern, a new one will be made according to standard centering methods. Unless a careful check is made, the replacement lens will be made with the MRP at the normal position. Now a difference exists in vertical height between left and right MRPs. This induces an unwanted vertical prismatic effect, which makes the prescription unacceptable.

This is why the MRP height of the nonreplaced lens always should be measured during the replacement process. By always checking vertical MRP height, the practitioner prevents any potential problem. If a difference does exist, the MRP of the new lens may be placed at the appropriate matching height.

If this careful check is not done and only the information given on the order is used, a discrepancy will exist in the vertical heights of the two lenses.

Immediate Multifocal Difficulties

When a pattern with the mechanical center vertically displaced is used to edge multifocals, failure to compensate can be disastrous. For instance, a pattern has a central hole (mechanical center) that is 3 mm higher than the boxing center. For bifocals whose desired height is specified as 18 mm, all calculations for segment drop are made on the assumption that this drop is occurring from the horizontal midline passing through the boxing center. Yet the blocked lens actually

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is centered 3 mm above the horizontal midline. Consequently, instead of having bifocals placed at 18 mm, the top of the bifocal line is at 21 mm.

Compensating for Vertically Displaced Pattern Centers

Compensation for either MRP positioning or multifocal height must be made at the time of lens centration and blocking. Once the lens is blocked, no more compensation is possible.

To compensate, the practitioner needs to know how much higher or lower the pattern’s mechanical center is than it should be. In other words, the distance between the pattern boxing and mechanical centers must be known. The following four steps may be used to calculate this distance:

1.The pattern’s B dimension is measured and divided by 2.

2.The vertical height of the pattern’s mechanical center is measured. This is the distance from the lowest part of the pattern (bottom of the enclosing rectangle) to the mounting line crossing the pattern hole.

3.The result in step 1 is subtracted from the result in step 2. This gives the difference between geometrical and mechanical centers in millimeters.

4.If the mechanical center is high, the MRP or segment height is lowered by the amount found in step 3.

To summarize, if a pattern is made with the central hole too high, the whole lens will be positioned higher in the frame than it should be. To compensate the lens is lowered during layout. If the central hole is too low, the practitioner must raise the lens to compensate.

Example 3-4

During layout the pattern is checked. The central hole is found to be 3 mm above the boxing center. Pertinent prescription data are as follows:

Segment height = 20 mm

Segment style = Flat top 28

Frame B = 42 mm

What is the correct amount of segment raise or drop and how would the correctly positioned lens appear before marking?

Solution

First the practitioner calculates the segment raise or drop in the conventional manner and then compensates for the pattern.

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Segment raise or drop = Height – B 2

= 20 – 42 2

= 20 – 21

= –1 (a minus direction denoting segment drop)

Without compensation the segment would be dropped 1 mm. However, because of the pattern the segment ends up being raised an additional 3 mm. Therefore the segment must be decentered 3 mm lower than it otherwise would be during layout. (A downward direction is minus. So this means an additional –3 mm of vertical segment decentration is required.)

Segment drop = –1 + –3 = –4

Thus the segment is positioned 4 mm below the horizontal reference line and is ready for marking, as shown in Figure 3-29.

Compensating for Horizontally Displaced Pattern Centers

Despite careful setup for cutting patterns, errors can occur that cause a displacement of the pattern’s central

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hole (mechanical center). Unfortunately, the amount of lateral displacement that occurs in the pattern will produce double the amount of error in the resulting PD. This occurs because the two lenses will be displaced in opposite directions. However, if the error is discovered before the lenses are blocked, an appropriate compensation may be made. This compensation is carried out in the same manner as in compensation for a vertically displaced pattern center.

Checking for Pattern Errors

To make any kind of compensation for central pattern hole displacement, the practitioner must first determine if a displacement has occurred. If a displacement has occurred, it is next determined how much that error is.

A simple system can even be made from a piece of centimeter/millimeter graph paper. A cross is drawn on the paper on the centimeter lines. Numbers may be added for ease of reference. Next a pattern is placed on the paper so that the central hole is in the middle of the cross. The other pattern holes should be lined up on the horizontal part of the cross (Figure 3-30). A pen or pencil is used to trace the circles on the paper. These circles must not be displaced and should be checked to ensure that they are absolutely centered.

To check a homemade pattern, the pattern is placed on the completed sheet so that the holes in the pattern line up exactly with the drawn circles on the page. The outerside edges of the pattern in each direction should be noted. For the pattern to pass as acceptable, top and bottom numbers must be identical. The numbers in the horizontal meridian to the far left and right also must match each other.

As may be remembered from an earlier section in this chapter, a homemade graph paper system is not the only available method for checking of pattern accuracy. A better system that uses identical methods is available on a Box-o-Graph. The Box-o-Graph was seen in Figure 3-13. The figure shows how to check a pattern for accuracy.

Determining Direction and Amount of Error

Once an error has been discovered, the amount of error and its direction must be determined. The easiest way to determine a horizontal error is to center the pattern’s central hole on a grid.

In the vertical dimension, the distance above the central hole, plus the distance below the central hole is equal to the B dimension of the pattern. If the pattern is correct, the distances from the central hole to the top and to the bottom will be equal. The distance to the top minus the distance to the bottom should equal zero. If these distances are not equal, the difference between

the two measures divided by two is the amount the central hole is off and equals the needed compensation. Expressed as a formula this would be written as follows:

Top – Bottom = Vertical compensation required 2

If the results are positive the lens must be raised to compensate for the pattern error; if negative, lowered. The same system applies to finding any needed horizontal compensation.

Temporal – Nasal = Horizontal compensation 2

Suppose the pattern is oriented as if it were a right lens. If the result is a positive number the mechanical center is displaced too far nasalward. This requires that the lens be decentered outward. However, if a negative number results, the mechanical center is too far temporalward. This requires that the lens be decentered inward. Box 3-1 provides a summary of compensation procedures, and Box 3-2 summarizes compensation direction.

Example 3-5

The pattern shown in Figure 3-31 is in error both vertically and horizontally. If this pattern is going to be used, what compensations would be necessary to ensure the accuracy of segment height and PD?

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BOX 3-1

Determining the Amount of Compensation for Pattern Center Displacement

Vertical Pattern Dimension

1.The pattern is placed on grid or Box-o-Graph and the pattern’s central hole is centered.

2.The grid is used to measure first the distance from the horizontal line to the highest part of the pattern and then the distance from the horizontal line to the lowest part of the pattern.

3.The difference is taken between these two numbers and divided by 2. This is the amount of compensation needed.

4.If the top is the largest measure, the compensating decentration must be upward. If the bottom is the largest measure, the compensation must be downward.

Horizontal Pattern Dimension

1.The pattern is placed on the grid or Box-o-Graph and the pattern’s central hole is centered.

2.The grid is used to measure first the distance from the vertical line to the most temporal part of the pattern and then the distance from the vertical line to the most nasal part of the pattern.

3.The difference is taken between these two numbers and divided by 2. This is the amount of compensation needed.

4.If the temporal side is the largest measure, the compensating decentration must be outward or temporally. If the nasal side is the largest measure, the compensation must be inward or nasally.

BOX 3-2

Pattern Center Displacement Compensation

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Solution

Horizontal and vertical components are considered separately. To determine the vertical displacement of the mechanical center, the practitioner must note that it is 21 mm to the top of the pattern and 23 mm to the bottom. The hole is too high in the pattern. To figure how much the MRP or segment height must be lowered, the following equation is used:

Top – Bottom = Vertical compensation required 2

which in this case is as follows:

21 – 23 = –1

2

This means that the lens MRP or segment height must be lowered 1 mm more than otherwise calculated.

For horizontal compensation, the temporal measurement is 25.5 mm and the nasal 26.5 mm. Therefore to find how far the pattern is off and how much compensation must be made for each lens, the following equation may be used:

25.5– 26.5 = –0.5 mm 2

This means that the MRP must be decentered in by 0.5 mm. This is in addition to the amount of decentration normally required.

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21

23

N

FIGURE 3-31 Were this pattern to be used for a multifocal prescription, the distance and near interpupillary distances (PDs) would both be 1 mm too wide. The segment height would also be a millimeter higher than anticipated.

Pattern Makers that Can Find the Center and Decenter

Pattern makers are similar to the frame tracers used with patternless edgers. (Frame tracers will be discussed shortly.) Much of frame tracing technology is based on principles used in previously existing pattern-making systems. Pattern makers can have some of the same smart features that frame tracers have. In fact, some pattern makers also double as frame tracers. These pattern makers may be hooked to a patternless edger and serve as a tracer or be used by themselves as a pattern maker.

A smart pattern maker can find the boxing center of the lens shape (Figure 3-32, A). If a pattern maker is able to determine the boxing center of a lens shape, it can just as easily determine the decentered point on the lens shape where the wearer’s eye will be positioned. All that is needed is an A, a DBL, and the wearer’s PD (see Figure 3-32, A). The A may be determined from the scan. So can the DBL if both lenses are scanned. (If only one lens is scanned, the DBL is entered manually.) The wearer’s PD is, of course, entered manually (Figure 3-32, B).

In choosing the decentered pattern option, the resulting pattern will be made with the central hole displaced by the needed decentration (Figure 3-33). If the pattern is made this way, the spotted lens does not have to be decentered. The lens block may be placed at the spotted major reference point of the lens without

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calculation for decentration.5 If this type of pattern is used for anyone other than this particular wearer, the lenses will come out incorrectly. (Note: If a frame has an especially small eyesize, decentering the pattern may displace the block too far nasally and cause it to strike the edger wheel.)

Pattern makers serving as frame tracers should have the capability of tracing a lens for rimless, semirimless, and nylon cord frames.

Placing the Pattern on the Edger

By convention most people begin the edging process with the right eye. When the pattern is snapped into placed on the edger, it will fit on the edger with either the front or the back of the pattern going on first. Going on one way will edge a right lens shape, whereas the other way will produce a left lens shape.

For most edgers, if the pattern is right-side up and has the nasal edge pointing toward the practitioner, the edger will cut a right lens shape. However, this is not always true. Therefore looking at the way the lens is positioned in the edger first is important, and the pattern is placed accordingly. For example, if the convex side of the upright lens faces left, then a right lens will require that the nasal side of the upright pattern be nearest the operator (Figure 3-34). If the convex side faces right, however, the pattern must be positioned in the opposite direction.

The pattern orientation for the left lens is exactly opposite from what it was for the right.

Using a Frame Tracer for Patternless Systems of Edging

Patternless edgers that do not use a physical pattern still need a shape to go by. This shape is given to the edger in digital form. Still, in order to get a digital version of the shape, that shape must sooner or later be physically traced and transferred to the edger digitally.

The most common method for generating a pattern shape is by using a frame tracer. A frame tracer is an apparatus that traces the shape of the frame’s lens area and converts it into digital form.

5Incidentally, this also allows high plus aspheric lenticular lenses to be blocked on the optical center, where the block has better adherence, instead of on the side of the aperture portion of the lens. It also holds the lens at a better angle in the edger when the lens is edged so that the bevel will not “drop off” the side of the lens.

A

B

FIGURE 3-33 If a pattern is made in the standard way, the mechanical center is at the boxing center as shown in A. If the pattern is custom made to avoid decentering the lens, it may appear more as shown in B.

ADVANTAGES OR DISADVANTAGES OF A FRAME TRACER

At first it would appear that a frame tracer is really no different from a pattern maker. It does everything that a pattern maker does, except for making a physical pattern. However, frame tracers are more than just digital pattern makers. Although not all frame tracers can do all of these extra functions, following are some of the things that frame tracers may be capable of doing:

•Trace both right and left lenses to determine size and shape consistency

•Trace the frame shape in three, instead of two, dimensions

•Work together with an edger to demonstrate what the placement of the bevel will look like ahead of time at any given point on the lens edge

•Trace a shape from an old lens or a coquille

•Transfer data to a surfacing laboratory to help determine how thin a lens may be surfaced

In spite of the many things frame tracers can do well, certain pitfalls with frame tracers exist. These pitfalls

are also present with pattern makers. However, with a pattern maker, the physical pattern shape is immediately visible and it is easier to detect a potential problem ahead of time. Following are some of the potential pitfalls:

•Frame tracers tell it like they trace it. A plastic frame may not retain the same shape it had with the demonstration lenses in place. If it flexes to a different shape, the tracer will trace the shape the frame has at that moment, not the shape it may have been designed with originally.

•Frame tracers may change the shape of the eyewire as they trace. If a frame has an especially thin plastic or metal eyewire, the tracer pin may exert enough outward force on the frame groove to cause the eyewire to give slightly. The tracer records this distortion as part of the intended lens shape.

•When a frame is traced from a remote site on one lens only, the new lens may not exactly match the old lens.

•Some frames have such narrow B dimensions that certain frame tracers are not able to trace the frame at all.

Tracing Both Eyes

Frame tracers usually trace both right and left sides of a frame. Some trace it with a single stylus doing first the right, then the left eye. (The stylus is the small pin that rides in the groove of the frame.) Other tracers, to increase speed, trace both eyes simultaneously.

The following list includes several reasons that both eyes are traced:

•The two sides of the frame may have slightly different sizes. Usually the lens is made for the larger eye so that the lens is not inadvertently small.

•If the tracer picks up a difference in shape between the two eyes, the differences may be averaged for consistency.

•By tracing both lenses the tracer is determining the size and shape of the lens and measuring the distance between lenses. Thus the tracer is capable of determining A, B, ED, angle of the ED, and DBL.

•In knowing all of the lens and bridge size measurements, with only the addition of the wearer’s PD, the tracer has furnished all the data to automatically calculate lens decentration. This can be used to simplify further lens layout.

Tracing the Frame Shape in Three Dimensions

The type of pattern used in the optical laboratory is a flat piece of plastic. It tells exactly what the shape of the frame’s eyewire looks like in two dimensions: up and down (the y axis) and left and right (the x axis), as it

would appear drawn on a piece of paper. A lens is not flat. It is arched as the result of a front surface that is usually convex and a concave back surface. Frame manufacturers make their frames to fit the average lens. This adds the third “in and out,” or z-axis, dimension. The frame front is not just flat. The eyewires are curved to accept the curve of the lens.

Unfortunately, because lenses come in a wide variety of lens powers, not all lenses are curved the same (Figure 3-35). When the curve of the lens does not match the curve of the frame, either the frame has to be reshaped or the bevel on the lens must be custom cut (Figure 3-36).

When a tracer traces the frame, the stylus does not move in a flat plane as it travels around the groove. If the frame is arched, the stylus moves up and down to keep from slipping out of the groove. Some tracers do not record up-and-down movement but instead report

FIGURE 3-35 Because not all lenses are curved the same, a lens bevel that follows the front surface of a minus lens will not fit into a frame in the same way that the bevel on a plus lens will.

the flat shape of the eyewire. These are called 2-axis frame tracers. Others record and use the third axis movement. These are called 3-axis frame tracers. If the edger is unable to use the third (z) axis information, the 3-axis tracer loses its advantage.

Recording the third dimension permits the following two things:

1.If the tracer or edger has a screen, it allows the operator to view what the bevel will look like in the frame. Some edgers even make suggestions as to how the bevel should be positioned as it tracks around the shape to make the glasses look the best.

2.It decreases the possibility of making a shape error on frames with a lot of face form. Measuring

in three dimensions helps get a tighter, more consistent lens fit and prevents possible gaps at outside corners.

Tracing a Shape from a Pattern, an Old Lens, or a Coquille

When a frame is rimless or of nylon cord, no rim exists to guide the stylus. A frame tracer may be adapted so that the stylus will trace the outer shape of an old lens or coquille.

To trace a lens from the old prescription, or to trace a dummy lens from a new frame, the frame is placed in a lensmeter. The lens is spotted with three dots along the 180-degree line (Figure 3-37). These three dots are necessary to be certain that the shape of the lens will