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

C H A P T E R 3 L E N S S H A P E S , PAT T E R N S , A N D F R A M E T R A C E R S

47

A

B

However, because of the way an edger works, each time the lens size is set higher by 1 millimeter, 0.5 millimeter is added to the original pattern shape. This 0.5 mm is added not just left and right in the horizontal direction but in every direction (Figure 3-15).

Example 3-1: The Way the System Works

A particular pattern is rectangular and has an A dimension of 36.5 mm and a B dimension of 26.5 mm. This pattern is put on an edger. If the edger is set to cut a lens having an eyesize of 46.5, what will be the vertical dimension of the edged lens?

48

FIGURE 3-13 A pattern may be measured on the Box-o- Graph (Kosh Manufacturing Co., Ft. Lauderdale, Fla.) by placing it over the centrally marked circles to assure horizontal and vertical alignment. The A dimension is the sum of the farthest left and right measures. If only A and B dimensions are needed (and not a check on the accuracy of central hole placement, as shown here), the measurements may be taken in the same manner as with lenses, placing the pattern in the upper left-hand corner of the Box-o-Graph. The pattern holes must nevertheless be perfectly horizontal.

Solution

The newly cut lens will be 10 mm larger in the A dimension than the pattern. Five millimeters have been added to each side of the lens. Because 5 mm also will be added above and 5 mm below the top and bottom edges of the pattern size, the new B1 lens dimension will be 10 mm larger than the pattern B dimension was. The new B1 lens dimension will be 36.5 mm. Although this may look more nearly proportional when the two are superimposed as shown in Figure 3-16, the two rectangles are not the same shape.

The phenomenon of nonproportionality occurs with use of a pattern that is not the same size as the lens to be edged. The difference is more obvious the larger the difference between pattern size and actual edged lens size. Therefore use of a pattern that is as close in size to the desired finished lens as possible is advantageous.

For this reason some manufacturers provide different patterns for frames of different eyesize ranges in the same frame design. Others may furnish one size pattern only, but they overcome the problem by manufacturing frames that differ slightly in proportionality from size to size. In this manner any sized lens that is edged with the pattern provided by the manufacturer still fits any eyesize frame.

C H A P T E R 3 L E N S S H A P E S , PAT T E R N S , A N D F R A M E T R A C E R S

50

50

Frame difference

30

of 20

FIGURE 3-14 The difference between the horizontal and vertical measurements of a frame is known as the frame difference.

FIGURE 3-15 In the figure, as the horizontal eye (A dimension) increases by 2 mm; the lens increases in size by 1 mm in every direction.

Calculations Based on Frame Difference

Any group of frames capable of having lenses edged from one pattern will, regardless of eyesize, all have the same frame difference. (Frame difference is equal to A

– B.) The frame difference number should be marked on the pattern. This marking is for the convenience of the operator during the multifocal lens layout process.

To calculate segment drop or raise, the B dimension of the frame must be known. If the B dimension of the frame to be used is not known, it must be either

FIGURE 3-16 In this drawing the outer square is created from the inner “pattern.” It is not the same shape even though the “frame difference” remains at 10 mm. However, this is the way shapes change when lens sizes increase or decrease using a lens pattern. To be the same shape, both dimensions of the rectangles would have to increase by equal proportions.

measured using the frame itself (if available) or calculated using the pattern as a basis. The B dimension of a lens is found by subtracting the frame difference stamped on the pattern, from the lens eyesize.

Example 3-2

A “lenses only” order must be completed for a frame having an eyesize of 48. The frame is not available at the laboratory. Bifocal height has been specified as 18 mm (Figure 3-17). The frame difference measurement on the pattern is stamped 4.5. (This means that the pattern is made for the boxing system and, according to that system, the difference of A – B is 4.5 mm.) How much segment drop or raise is required?

Solution

Segment drop is the distance from the horizontal midline of the edged lens shape to the top of the bifocal line. To calculate segment drop, the B dimension of the lens must be known. The B dimension is calculated as follows:

B = Eyesize – Frame difference

In this case frame difference will be as follows:

B= 48 – 4.5 = 43.5 mm

The distance from the horizontal midline of the lens shape to the bifocal line is a downward direction and is technically a negative number. Therefore it is calculated as follows:

FIGURE 3-17 If segment height, eyesize (A dimension), and frame difference are known, segment drop (or raise) may be found. This is also true in the location of the major reference point or progressive lens fitting cross raise.

Segment drop = Segment height – B 2

=18 – 43.5

2

=–3.75 mm

So the segment drop will be about 4 mm below the horizontal midline of the lens shape.

(If the frame difference had not been marked on the pattern, it could still have been found by measuring both A and B pattern dimensions with a ruler, a measuring gauge, or a Box-o-Graph.)

MECHANICAL CENTER OF THE PATTERN

No matter how carefully a lens is marked, unless the pattern is accurately made, the optical center of the edged lens will not end up where it should. For the optical center to come out right, the mechanical center of the pattern must be placed correctly. The mechanical center of a pattern is the point on the pattern around which the pattern rotates. (Other names for the mechanical center are rotational center, cutting center, or edging center.) The mechanical center is easy to find because it is found in the middle of the large hole in the pattern (see Figure 3-11). The large hole is used to snap the pattern on the edger.

The horizontal line that passes through the mechanical center of the pattern is sometimes referred to as the mounting line.3 When the mechanical center

3The mounting line of a pattern is not the same thing as the mounting line of a frame. The mounting line for a rimless frame is the line that passes through the points at which the pad arms are attached. The frame’s mounting line serves as a line of reference for horizontal alignment during standard alignment of a frame.

and boxing center of a pattern are at the same location on the pattern, then the pattern’s horizontal midline and the pattern’s mounting line are also the same.

Positioning the Mechanical

Center in the Pattern

The mechanical center is positioned in the pattern in the following two main ways:

1.The pattern is made so that the boxing center and mechanical center are at the same place on the pattern. This is the most common pattern construction. Patterns supplied by the frame manufacturer are made this way.

2.The pattern can be made so that the mechanical center (the large hole in the pattern) is moved intentionally away from the boxing center of the lens shape. This usually happens if the pattern is made for one specific person or one particular edging situation and will never be used for anyone else.

WHEN THE MECHANICAL CENTER IS AT THE BOXING CENTER

When a person wears a pair of glasses, the position of his or her eyes in relationship to the boxing center of the lens shape depends upon the distance between the person’s pupils compared with the size of the frame. If the frame is large and the wearer’s PD is small, the eyes are nasal to the boxing centers of the lens shape. No matter where the pupils are, the major reference point of the lens should be in front of the pupil.

To be more exact, when the wearer is looking into the distance, the pupil usually is slightly above the major reference point. In other words, the major reference point of the lens should be in the same vertical plane as the wearer’s pupil (Figure 3-18). If no prescribed prism is in the prescription, the optical center and MRP are exactly the same.

Centration Process

In the edging process, a lens is first spotted to mark the MRP. The three dots on the lens are reference points to help position the lens in the frame.

The lens must be attached to a block for edging (Figure 3-19, A). The block holds the lens in place while it is being edged. If the center dot (MRP) on the dotted lens is placed at the center of the lens block, after the lens is edged the MRP will correspond to the boxing center of the edged lens (Figure 3-19, B).

If MRP is at the boxing center of the edged lens, but

MRP

FIGURE 3-18 The standard horizontal major reference point (MRP) location of a single vision lens is on the horizontal midline. It should be in the same vertical plane as the wearer’s pupil.

the wearer’s eye is not, the position of the lens on the block was not correct. If the wearer’s eye is in the nasal half of the frame’s lens opening, the prespotted lens has to be moved away from the center of the block and toward the nose so that it will end up in front of the eye (Figure 3-20).

Centration and Decentration

The process of moving a lens so that it will be in front of the eye is called centration. To center the lens in front of the eye, the lens must be moved away from a given reference point. When a lens is moved away from a given point it is said to be decentered from that point. In this case, the lens is moved away from or decentered from the location of the mechanical and boxing centers. Taking great care during this process is important because the slightest error results in an inaccurate finished product.

CALCULATION OF HORIZONTAL DECENTRATION WITH THE BOXING SYSTEM

The first part of the centration process involves figuring out exactly where the eye will be in relationship to the boxing center of the edged lens. If the boxing center and the eye are not in the same location, the lens will have to be decentered. The distance and direction of this decentration must be calculated to ensure that the lens optics will be positioned properly before the eye.

Two measurements are required in the calculation of decentration. The first measurement is how far apart the person’s pupil centers are from each other. This is called the binocular4 interpupillary distance (PD). The second measurement is the distance between the geometrical (boxing) centers of the frame’s two lens

4Decentration may be based on how far the eyes are from one another (the binocular PD), or the distance may be figured for each eye separately. The distance of each eye from the center of the bridge measured separately is called the monocular PD. Monocular PDs will be covered later.

openings. This is called the distance between centers (DBC), or frame PD.

Determinating the Distance between Centers

For frames that conform to the boxing system of measurement, the DBC is equal to the eyesize (abbreviated A) plus the DBL. This is true because the geometrical center is located halfway across each lens opening.

Because the A dimensions of right and left lenses are equal the following equation is used:

52

DBC = Right lens A dimension + DBL + 2

Left lens A dimension

2

=DBC = A + DBL

This concept was shown in Figure 3-8.

If the frame is available, the DBC can be measured on the frame. The practitioner starts at the most nasal point on the right lens opening and measures on a straight horizontal to the vertical plane of the most temporal point on the left lens opening. This was shown in Figure 3-9. Because one of these two edge points may be higher or lower than the other, simply measuring in the middle of the frame may not be accurate.

DECENTRATION PER LENS USING BINOCULAR DISTANCE PDS

Most commonly the wearer’s PD will be less than the frame PD (DBC). This requires that the lenses be decentered inward (nasally) toward the center of the frame. The amount of decentration per lens can be determined by subtracting the wearer’s PD from the DBC (frame PD) and dividing by two.

Decentration per lens =

DBC – Wearer PD

2

Decentration (or decentration per lens) refers to the amount one lens is moved. Total decentration refers to the sum of both left and right decentrations. Therefore the following equation is used:

Total decentration = DBC – Wearer PD

Example 3-3

The order form indicates that the wearer’s PD is 64 mm. The frame size is 53 17. In other words, the eyesize is 53 mm and the DBL 17 mm. The symbol indicates that the dimensions are listed according to the boxing system. If the frame is known to conform to boxing measure standards, what is the decentration per lens required?

Solution

To determine initially the difference between the DBC and wearer’s PD, the practitioner must know the DBC. By definition, the following is true:

DBC = A + DBL

Because

A = 53 mm

C H A P T E R 3 L E N S S H A P E S , PAT T E R N S , A N D F R A M E T R A C E R S

and

DBL = 17 mm

then

DBC = 53 + 17 = 70 mm

The following equation is true:

Decentration per lens =

DBC – PD

2

In this instance the following applies:

70 – 64 = 6 = 3 mm per lens 2 2

(In the unusual case where the wearer’s PD was greater than the DBC, a negative decentration would result from the calculations. A negative number indicates that decentration outward is required and the MRP of the lens would be decentered temporally.)

REASONS THE MECHANICAL CENTER OF THE PATTERN AND THE BOXING CENTER MAY DIFFER

As stated previously, pattern boxing and mechanical centers normally coincide so that the pattern hole is in the middle of the pattern. The following list states the four main reasons why the pattern hole (mechanical center) may not be in the center of the pattern:

1.The pattern was intended to be made with the mechanical center at the boxing center, but it was made incorrectly. The hole is simply not where it should be.

2.The pattern is of the old “raised center” type.

3.The pattern was made using some other system besides the boxing system.

4.The pattern is made so decentration does not have to be calculated and is useful for one specific person and one specific frame. (This idea will be discussed later in this chapter.)

Pattern Making

Because of the vast number of available frame styles, having a complete library of patterns so that the correct pattern is available for every frame presented for lens fabrication is impossible. Ordering a pattern for every single frame that passes through the laboratory is impractical. The delays caused would not be acceptable to the wearer, not to mention the volume of paperwork that would be generated. For this reason, when running

FRAME SETUP FOR PATTERN MAKING

To make a pattern, either the frame must be on hand or, at the minimum, a labeled tracing from that frame. To make a pattern directly from a frame, all pattern makers require that the frame be positioned properly for tracing. The more advanced type of a pattern maker requires only that the frame be horizontally level. After the frame is traced, the pattern maker digitally determines the boxing center and places the mechanical center of the pattern there.

The more traditional pattern maker models require that one eye of the frame shape be centered carefully for tracing according to the boxing center. This is done so that the mechanical center of the pattern will end up corresponding to the boxing center of the lens shape.

The centering grid on a pattern maker may vary in its construction but basically consists of measured distances left, right, up, and down from an origin. This central origin represents the position that the mechanical (rotational) center of the pattern will occupy, as Figure 3-21 shows. The lens opening of the frame must then be centered so that the geometrical center of its

FIGURE 3-22 When a frame’s lens opening is centered for pattern making according to the boxing system, the left extremity must be equal to the right; and the upper and lower extremities must also be equal.

shape is precisely at the middle of the grid origin. This is done, as in Figure 3-22, by making sure the outermost points to the left and right fall at the same distance from the origin. The uppermost and lowermost points also must be the same distance from the origin.

To actually carry out this procedure, the frame must be oriented initially with its frame front face down on a centering grid. To ensure that the horizontal midline of

the pattern will correspond to the frame’s horizontal midline and not be tilted, the top of the frame is pressed against a horizontal bar (Figure 3-23). This horizontal bar slides up and down but does not tilt. Tilting the frame causes an off-axis pattern, which makes it impossible to cut a cylinder lens for the correct axis. When centering is achieved, the frame is clamped securely into place and is ready for tracing.

Tracing may be accomplished by rotation of the mounted frame on the pattern maker while a stylus rests inside the frame groove. The stylus is linked mechanically to a cutting mechanism that uses a pantographic system to cut the pattern from a pattern blank. The pattern blank is marked with an N, indicating the nasal side. This blank should be placed in the pattern maker so that it indeed corresponds to the nasal side of the frame when cut.

Most “homemade” patterns are cut precisely for the size of the frame. This prevents shape distortion caused by large differences in pattern and finished lens sizes, as was described previously. Maintaining a one-to-one eyesize/pattern size ratio understandably may require the use of larger pattern blanks for certain frames. (Pattern blanks are shown in Figure 3-24.) Once the pattern is cut, it may require some smoothing of the edges, depending on the sophistication of the cutting mechanism. When smoothing is necessary, it may be accomplished with a medium-fine file. The practitioner

should file only as much as necessary to remove roughness so that the basic shape or size is not changed.

MAKING A PATTERN FROM A LENS TRACING

Occasionally a special order lists no frame name but only a handmade tracing of the lens that is presently being worn in the frame. Such a request is difficult to fill properly. If the tracing is for a metal frame a bad fit is almost certain. With a plastic frame filling the request is possible but difficult. Even for a plastic frame such a request cannot be filled adequately unless certain specifications are given. These are the following:

•The size of the lens (The tracing is the same size as the lens. When a lens is traced, the tracing is naturally larger than the lens, or if the frame eyewire were traced, it is smaller.)

•The distance between lenses (DBL) to allow calculation for decentration

•The 180-degree line (If the lens is traced in an even slightly rotated position, the cylinder axis will be incorrect.)

Using a Similar Pattern

One possibility for filling such a request is to find a pattern that most closely approximates the shape and

will not cut any corners from the required shape. If the pattern is an extra or will be unneeded in the future, it may be reshaped to match the tracing. If not, the lens is edged using the pattern that is close in shape to the drawing, then both edged lenses are reshaped by hand until correct. Each lens is held up to the tracing for comparison.

Making a Pattern to Match

Another method is first to mark carefully the horizontal midline on the tracing. Next a vertical line is drawn through the boxing center. Some pattern blanks already are marked with horizontal and vertical lines as was seen in Figure 3-24.

The tracing is cut out and placed on the pattern blank so that horizontal and vertical markings align, then it is traced with a marking pen. The marked shape is cut from the blank in the manner shown in Figure 3-25 and then smoothed on the edges as shown in Figure 3-26. Such a pattern should be checked for

FIGURE 3-25 If a pattern is being cut by hand, curved cutting shears help to cut around corners.

center displacement to ensure accuracy of the PD and segment height. A method of checking for unwanted pattern center displacement is described later in the chapter in the section Checking for Pattern Errors.

FIGURE 3-26 Smoothing pattern roughness with a file gives better size and shape. This is true not only for hand-cut patterns but also for some patterns made by a pattern cutter.

Recommended Laboratory Requirements for a By-Hand Lens Tracing

If a dispenser sends in a drawing of a wearer’s lens traced around the actual lens, then the laboratory has a better chance of success by requesting the following:

•The A and B dimensions of the lens measured on the lens and not on the drawing

•The DBL, also measured using the frame

•A 180-degree line marked on the drawing

•An indication of the type of frame being used. Is the frame metal, plastic, grooved, other?

•An indication of whether the lens is a left or a right lens

•Preferably a circumference measure of the distance around the edged lens

The circumference of a lens is a much more sensitive measure of size than an A dimension. Lens circumference is sometimes called C-size. C-size is measured most easily with a circumference gauge (Figure 3-27). If both the dispenser and the laboratory use a circumference gauge, then size errors will be reduced. For those dispensers who send drawings of lenses, using millimeter graph paper will help considerably. A grid, with a form as shown in Figure 3-28, may be helpful to ensure all information is present.

For an electronically equipped laboratory, a much better alternative to lens drawings being sent is to have