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

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

17.If no prescribed prism is in the prescription, which one of the following points is not the same?

a.OC

b.MRP

c.PRP

d.NRP

18.For which of the following prescriptions is there a difference in the physical location of the OC and the MRP? (There may be more than one correct response.)

a.–4.00 D sphere

b.–4.00 –2.00 × 180

e.The OC and MRP are synonymous terms and therefore are always at the same point on a lens.

19.True or False? Use of an autolensmeter to spot lenses requires no presetting of the instrument.

20.True or False? With use of most autolensmeters to prepare a lens for edging, the lens is still spotted with three dots, as with a manual lensmeter.

21.A flat-top bifocal is spotted for the MRP. It is immediately evident that the three lensmeter dots are not parallel to the segment line. In which prescription is this instance of no consequence?

a.It is always of consequence.

b.–1.00 –1.00 × 180

c.pl –1.00 × 70 (pl denoting a “plano,” or zero power)

d.–2.25 D sphere

22.True or False? The spotting of blended bifocals is done in the same manner as the spotting of round-segment bifocals.

23.Which of the following is the standard vertical position of the lens MRP?

a.3 mm above the horizontal midline of the lens (The horizontal midline is in the middle of the frame B dimension.)

b.On the horizontal midline of the lens

c.3 mm below the horizontal midline of the lens

d.No standard vertical position exists.

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24.True or False? If the MRP “disappears” into the segment, the lensmeter power wheel is refocused so that the target lines come into view through the segment. The MRP, which “vanished,” can now be found and spotted.

25.True or False? Spotting a premarked progressive addition lens is done only for verification purposes. The centration process that follows can be accomplished by use of the marks already on the lens.

26.True or False? “Invisible” markings are found on progressive addition lenses. These markings allow the MRP and near portions of the lens to be located exactly.

27.The distance power for a progressive addition lens is verified at which of the following?

a.OC

b.MRP

c.NRP

d.PRP

e.DRP

28.Horizontal and vertical prismatic effect for a progressive addition lens is verified at which of the following?

a.NRP

b.PRP

c.DRP

Challenge Questions

29.How far from the MRP must the OC be moved to create the proper prismatic effect by decentration for the following lens?

+1.50 –1.50 × 90 0.5 base out

a.3.33 mm

b.30 mm

c.0 mm

d.There is no optical “center” for this lens.

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30.How far from the MRP must the OC be moved to create the proper prismatic effect by decentration for the following lens?

+1.50 +1.50 × 90 0.5 base out

(Notice the plus cylinder form of the prescription.)

a.1.67 mm

b.3.33 mm

c.0 mm

d.The distance cannot be figured from the measurements provided.

31. The prescription is R: –6.00 D sphere 1.0 base out. The lens is in the lensmeter (convex side facing the operator) with the lensmeter target exactly centered. In which of the following directions must the lens be moved before it may be correctly spotted?

a.It is correct as is and need not be moved.

b.Operator’s left

c.Operator’s right

d.Up

e.Down

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

The following lenses are clear, single vision lenses. Which of the statements apply to each prescription? (Note: More than one answer may be appropriate.)

32.–4.25 –1.00 × 035

a.After this lens has been spotted, it will be unaffected by any lens rotation around the center spot.

b.After spotting, this lens will be unaffected if the lens is rotated exactly 180 degrees around the center spot.

c.After spotting, this lens will be affected by any lens rotation.

33. –1.00 –2.00 × 018 2 base in

a.After this lens has been spotted, it will be unaffected by any lens rotation around the center spot.

b.After spotting, this lens will be unaffected if the lens is rotated exactly 180 degrees around the center spot.

c.After spotting, this lens will be affected by any lens rotation.

Geometrical or boxing center

FIGURE 3-2 The boxing system bases horizontal and vertical frame shape sizes on the smallest rectangle that completely encloses the shape. The center of that rectangle is the geometrical or boxing center.

HORIZONTAL LENS SIZE

The horizontal size as determined by the box is called the A dimension (Figure 3-3) or the eyesize. Unfortunately not all frames are marked with an eyesize equal to the A dimension. When calculations are done to determine

correct placement of the lens optical center for edging, the A dimension must be known. Using a marked eyesize different from the A dimension results in edging errors.

A common but inaccurate method for measurement of the eyesize of a frame is to measure the width of the frame’s lens shape across the center of the lens shape. In certain cases, such as with round or oval frames, this measurement may be equal to the A dimension of the frame. But often this measurement is less than the frame’s A dimension. This measurement of lens width at the center of the frame has its own name. It is called the C dimension.1 The C dimension is defined in the boxing system but is not used for lens fabrication purposes.

VERTICAL LENS SIZE

The vertical size as determined by the box is called the B dimension. Vertical lens size is needed for correct placement of the optical center for certain types of single vision lenses, bifocal or trifocal height for

1The C dimension should not be confused with the C size of a lens shape. The C size is the distance around the outside of the lens shape, similar to the circumference of a circle.

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

ED

(Same as DBC)

Distance between center (DBC) or

Geometrical center distance (GCD) or

Frame PD

FIGURE 3-3 In the boxing system, ED is the abbreviation for effective diameter. ED is twice the longest radius of the shape as measured from the boxing (geometrical) center. The angle from the 0-degree side of the 180-degree line to the effective diameter axis is X for the right lens. The ED is used in accurate calculation of the minimum lens blank size and lens thickness required to fabricate the prescription.

segmented multifocals, and fitting cross height for progressive addition lenses.

HORIZONTAL MIDLINE

The horizontal line passing through the middle of the boxed lens shape halfway between the top and the bottom is known variously as the 180 line, the datum line, or the horizontal midline.

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B

BOXING CENTER

The center of the boxed lens is called the boxing center. It is at the intersection of the horizontal and vertical midlines of the lens. An easier “on paper” way to find the boxing center is to draw corner-to-corner diagonals across the box (Figure 3-4). Another name for the boxing center is the geometrical center of the edged lens.

EFFECTIVE DIAMETER

Determining the boxing center of the lens shape makes it possible to find the smallest unedged round lens that could be edged successfully to this shape if the center of that round lens were at the boxing center. This diameter

FIGURE 3-4 The geometrical (boxing) center of a pattern or lens is always at the center of the enclosing box, regardless of where pattern holes are drilled or MRPs are placed.

is called the effective diameter and is abbreviated ED. The ED is found by location of the longest distance from the boxing center to the edge of the lens shape (Figure 3-5, A). Next this line is extended an equal amount in the opposite direction (doubled; Figure 3-5, B). In other words, the ED is twice the longest radius of the lens shape as measured from the boxing center.

Longest

radius

A

Effective

(ED) diameter

B

Angle of the ED

The ED is not the longest diagonal of the frame shape (Figure 3-5, C). This is the most common wrong method of measuring the ED and gives an answer that may be close or at times even equal to the ED but is not really the ED. In some cases it can be considerably different from the true effective diameter. This wrong method is commonly used because no easy way exists to directly measure the ED on a real frame.

ANGLE OF THE EFFECTIVE DIAMETER

As stated previously, when finding the effective diameter for a given lens shape, the practitioner may draw a line from the boxing center to the point on the lens edge farthest from the boxing center. That line is extended equally in the exact opposite direction. The ED line crosses the horizontal midline at an angle. That angle is measured using the right lens. The angle measurement begins from the right side of the horizontal midline of the lens and is called the angle of the ED (Figure 3-5, D). The angle of the ED is important during the surfacing

process. Knowing the angle of the ED enables the laboratory to grind the lens as thin as possible.2

DISTANCE BETWEEN LENSES

Each frame has a measurable distance between lenses (DBL). In the boxing system, the DBL is basically the shortest distance between those lenses. It can be visualized as the distance between the two boxes when those boxes are drawn around each lens in the pair (see Figure 3-3). The DBL also is called the bridge size. Like eyesize, the bridge size marked on the frame may not correspond to the bridge size needed in the fabrication of a pair of lenses.

The most common mistake made in measurement of the DBL is to measure the distance between the edges of the lenses along the horizontal midline of the lens (Figure 3-6).

2For additional information see Brooks CW: Understanding lens surfacing, Boston, 1992, Butterworth-Heinemann.

Measuring the A Dimension and Distance between Lenses on a Grooved Frame

When a practitioner measures the A dimension and DBL of a normal frame with a groove to hold the lenses in place, the starting and ending points are where the beveled lens edges are located. If the groove is a deep groove in a plastic frame, then where does the measurement begin? It does not begin at the inside edge of the plastic lens rim. It begins at the deepest part of the groove (Figure 3-7).

GEOMETRICAL CENTER DISTANCE

For a frame or pair of lenses, the distance between the two boxing centers commonly is known as the geometrical center distance, or GCD. It is called the geometrical center distance because the boxing center is also the geometrical center of the edged lens.

The GCD has several other names. One of the most common is distance between centers, or DBC. A third name is the frame PD. PD is short for pupil distance or

interpupillary distance. A PD technically refers only to a person’s pupils. Frames do not have pupils and cannot really have PDs. Yet the term frame PD is used extensively to mean the GCD. Unfortunately, adding still more to the confusion, this measurement has two other names. These synonyms are the frame center distance and boxing center distance. So the GCD also can be called the distance between centers, the frame PD, the frame center distance, and the boxing center distance.

The distance between centers may be found by adding the A dimension to the DBL (Figure 3-8). It also may be found by directly measuring the frame as shown in Figure 3-9.

MAJOR REFERENCE POINT, FITTING CROSS, AND SEGMENT HEIGHTS

The standard vertical position for the major reference point (MRP) of a spectacle lens is on the horizontal midline of the frame. However, the practitioner may request that the MRP be placed at a different vertical

FIGURE 3-8 (DBC) is the bridge size). in the upper

It can be seen from the figure that the distance between the two centers same as the A dimension (eyesize) plus the distance between lenses (DBL; Because the DBC cannot be directly measured, it may be measured as shown right-hand corner of the figure.

location. This request is based on the vertical location of the wearer’s eyes with reference to the frame. When a practitioner asks for a specific MRP height, that height will be equal to the distance from the lowest point on the lens (the bottom line of the box) up to the desired location of the MRP (Figure 3-10, A). MRP height is not the distance from the lens edge directly below the MRP to the desired location. This is also shown in Figure 3-10, A, and is the most common mistake made when measuring MRP height.

The vertical position of the fitting cross of a progressive addition lens (Figure 3-10, B) or the top of a segmented multifocal (Figure 3-10, C) is measured in the same way as the MRP is measured. Both are

measured from the level of the lowest point on the lens or the deepest position of the inside bevel of the frame’s eyewire.

Pattern Measurements and

Terminology

Patterns allow an edger to duplicate the desired lens shape for a specific frame. Their shape and size are critical for the correct duplication of lens shapes. Therefore to maintain accuracy, a standard method of measuring patterns is essential.

PATTERN SIZE

To determine pattern size, a pattern must be positioned exactly in the same orientation as the frame shape when the frame is held horizontally; in other words, the pattern may not be rotated at an angle. The pattern shape then is enclosed in a box by four tangent lines according to the same boxing system as used for frames and lenses. The perpendicular lines used for boxing in a pattern must be perfectly horizontal and vertical, and each side of the box must touch the pattern, as shown in Figure 3-11. As with frames and lenses, the pattern size for the boxing system is the distance between the two vertical sides of the box. It is not the width of the pattern along the center 180 line. Boxing pattern size corresponds to the frame eyesize or A dimension. The vertical dimension of the pattern is measured vertically between the top and bottom of the box and corresponds to the B dimension of the frame. A or B dimensions may not be measured with the ruler held at an angle. Neither may a pattern be measured if it is in a rotated position.

The use of some sort of eyesize gauge may result in pattern measurements (Figure 3-12, A) if the sides of the measuring portions are high enough to enclose the outermost portions on left and right sides. This same gauge can be used to measure lenses (Figure 3-12, B) and other objects. Even with such a handheld gauge, the possibility of a slight rotation of the pattern occurs. Many practitioners choose to use a Box-o-Graph (Kosh Manufacturing Co., Ft. Lauderdale, Fla.) for pattern

and lens size measurements. The Box-o-Graph is shown in Figure 3-13. A Box-o-Graph helps to be certain that no rotation has occurred. It does not give enough accuracy to ensure first-cut sizing when edging but is considerably more accurate than a ruler.

MEASURING FRAME DIFFERENCE

The difference between the horizontal and vertical dimensions of the pattern is the pattern difference. Pattern difference has the same numerical value as frame difference. As would be expected, the frame difference is the difference between frame A and B dimensions (Figure 3-14). Therefore the term frame difference is used synonymously with pattern difference.

Expressed as a simple equation, this takes the following form:

A – B = Frame difference

The frame difference number is often printed on the pattern (see Figure 3-11) and does not change, even when the pattern is used to cut out lenses of different sizes.

EFFECT OF EYESIZE CHANGES ON LENS SHAPE

Presumably when eyesize increases or decreases for a given frame style, frame difference should change proportionally. If the lens shape were to stay exactly the same, eyesize changes would result in a proportional change in frame difference.