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

FIGURE 3-38 The lens is mounted on a lens block in an adapter. The tracer needs to know if the lens being traced is a right or a left lens.

not be rotated off axis, causing the cylinder axis to be wrong. Once the lens is spotted, it is blocked on an adhesive padded block that has been placed in an adapter in preparation for tracing (Figure 3-38). The lens and adapter is placed in the tracer (Figure 3-39) and traced.

Most patternless edgers cannot directly use a pattern. Therefore being able to trace an existing pattern becomes an important frame tracer feature. The same mechanism used to trace lens shape from an existing lens is also used to trace a lens pattern (Figure 3-40).

Transferring Data to a Surfacing Laboratory

For the surfacing laboratory to grind a lens to the optimum thickness, the laboratory needs accurate data. This is especially true for plus lenses. The size and shape the lens will have when edged are essential for calculating plus lens thickness. The more exact the data, the more precisely the thickness may be controlled. If the lens is traced, those tracing values may be sent to more places than just the edger. Values can be sent to a surfacing program that calculates lens curves and thickness then controls the lens generator.

USING A FRAME TRACER IN A VARIETY OF LOCATIONS

A frame tracer can be used in a variety of locations and situations. Each has certain advantages and disadvantages.

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Adjacent to the Edger

When a tracer is situated right next to the edger, the person doing the edging has the frame in front of them (Figure 3-41). The advantage to this set up is ease in visualizing what bevel placement will look best. This works best if the tracer is interactive with the edger. If the tracer is not interactive, it simply feeds the shape into the edger. Any further adjustments are dependent upon the edger’s stand-alone capabilities. Feeding information to more than one edger from the same tracer still may be possible.

Part of the Edger

A tracer that is part of the edger has the advantage of requiring less working space (Figure 3-42). When laboratory space is limited, this may be very helpful. The amount of interactivity and capabilities of the combination of tracer and edger depends solely upon the number of functions built into the system. It can be very basic or quite advanced. The disadvantage is that such a tracing system is possible to use only with that one edger.

In the Order Entry Area of a Laboratory

When the tracer is placed in the order entry area of the laboratory, information is entered only once. The laboratory that has a tracer at “order entry” will be wired with a central laboratory computer. That computer tracks the job and is capable of downloading needed information into any piece of surfacing or edging equipment that is hooked into the system when the tray number of the job is keyed in. The information on shape and eye and bridge sizes are used to calculate lens thickness for surfacing and decentration for edging.

Placing a Tracer in a Remote-Site Dispensary

One of the biggest headaches for dispensers is the situation in which a wearer wants to keep his or her old frame but cannot or will not give it up long enough to send it to the laboratory. This is especially troublesome if the frame is a metal frame and/or a frame that cannot be identified. If a frame tracer is on site, the dispenser can remove one or both lenses, trace the shape, reinsert the lens or lenses, and give the spectacles back to the wearer (Figure 3-43).

The information is then sent to the laboratory electronically. It enters the computer system just as if it had been entered in the laboratory order entry area.

Once the lenses are completed, just the lenses are sent to the dispensary. There, with the wearer present, the dispenser removes the old lenses from the wearer’s frame and replaces them with the new ones.

Such a system is capable of speeding up the process for all prescriptions. Because most dispensaries acquire

frames directly from frame manufacturers and distributors, they will be using the frame selected from the display board. In a dispensary without an in-house optical laboratory, the frame must be sent to the laboratory, fabricated, and sent back. In cases in which the shape of the frame is unknown, the laboratory will be reluctant to start the job until all information is present. When the dispensary uses a frame tracer to send that information, the laboratory can get a head

start on the prescription before the new frame arrives. To save time, dispensary personnel may choose to not send the new frame but insert the lenses themselves.

The disadvantages of a frame tracer are also present in a remote setting. The frame may be distorted before tracing, or flex during tracing and not retain the same shape it had with the old lens or demonstration lens in place. This can be a problem when ordering either one or both lenses.

1.According to the boxing system, which of the following is the measurement for the horizontal size of the edged lens?

a.ED

b.GCD

c.A dimension

d.B dimension

e.C dimension

2.Which of the following describes the effective diameter of a lens shape?

a.The longest horizontal dimension of a lens shape that passes through the boxing center

b.The longest vertical dimension of a lens shape that passes through the boxing center

c.The longest diagonal of the lens shape that passes through the boxing center

d.Twice the longest radius of a lens shape as measured from the boxing center

e.The smallest lens size that will be possible to use after allowing for lens decentration

3.The angle of the effective diameter is measured as which of the following?

a.The angle from the 90-degree line to the effective diameter line, measured clockwise for the right eye

b.The angle from the 90-degree line to the effective diameter line, measured counterclockwise for the left eye

c.The angle from the 180-degree line to the effective diameter line, measured clockwise for the right eye

d.The angle from the 180-degree line to the effective diameter line, measured counterclockwise for the right eye

e.The angle from the 180-degree line to the effective diameter line, measured counterclockwise for the left eye

4.To measure the DBL on a metal or plastic grooved frame made to hold beveled lenses in place, measurements are made in which of the following ways?

a.From the deepest part of the groove on one nasal eyewire to the deepest part of the other nasal eyewire on the horizontal midline of the frame

b.From the two nasal-most edges of the eyewire rims along the horizontal midline of the frame

c.From the deepest part of the groove on one nasal eyewire to the deepest part of the other nasal eyewire at the location where the distance between those lenses is smallest

d.From the two nasalmost edges of the eyewire rims at the place where the distance between those lenses is smallest

5.Which of the following is not a synonym for geometrical center distance?

a.Distance between centers

b.Frame PD

c.Frame center distance

d.Boxing center distance

e.All the above are synonyms.

6.When using a metal or plastic frame, the practitioner makes the correct measures for MRP height, segment height, and fitting cross height beginning at which of the following?

a.The point on the inside of the eyewire groove directly below the center of the pupil

b.The level of the lowest point on the inside of the eyewire groove. This will not necessarily be at the same level as the point on the eyewire groove directly below the pupil

7.True or False? Horizontal pattern size is measured horizontally through the central hole in the pattern.

8.If a pattern is made for use with the boxing system, at which of the following will it have its central hole?

a.Boxing center

b.Datum center

c.Optical center

d.Major reference point

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9.Which of the following is the point on a pattern around which it always rotates?

a.Boxing center

b.Datum center

c.Mechanical center

d.Optical center

10.Which of the following terms does not mean the same thing as the other three terms?

a.Cutting center

b.Mechanical center

c.Boxing center

d.Edging center

11.Frame difference is calculated by which of the following equations?

a.A + DBL

b.A – DBL

c.A – C

d.B – C

e.A – B

12.True or False? Using the same pattern for frames having two different sizes means that the frame size and the frame’s lens shape may be altered somewhat.

13.True or False? One frame style is available in several different eyesizes. All those eyesizes are able to have lenses edged from the same pattern. Therefore all of these frames, regardless of eyesize, will have the same “frame difference.”

14.A pattern has a frame difference of 10. A 50-mm eyesize frame of the same shape has a B dimension of 40. Which of the following B dimensions does an identically shaped frame with a 55-mm eyesize have?

a.44 mm

b.45 mm

c.46 mm

d.40 mm

e.None of the above

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

15.A pattern has an A dimension of 46 and a pattern difference of 5. If the A dimension of the lens to be cut is 50, which of the following will be the B dimension of the lens?

a.45

b.55

c.46

d.54

e.41

16.A frame has a B dimension of 40 mm. If the segment height of the lens is 17 mm, what is the segment drop? (Segment drop will be listed as a negative number because it is below the midline of the lens.)

a.–2

b.–3

c.–4

d.–17

e.–23

17.A pattern is marked with a frame difference of

7 mm. If the lens eyesize is 50 mm and the segment height is to be 19 mm, which of the following indicates the segment drop?

a.–2.5 mm

b.–6 mm

c.–9.5 mm

d.–1 mm

e.None of the above

18.A pattern has a pattern difference of 7. Which of the following indicates the amount of segment drop for a frame with an A dimension of 52 and a segment height of 17 mm?

a.–12.5 mm

b.–9 mm

c.–7.5 mm

d.–5.5 mm

e.None of the above

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

19.A lens is to be edged for a frame with an A dimension equal to 48 mm and a DBL of

18 mm. If the wearer has a PD of 60 mm, which of the following indicates the decentration per lens?

a.6 mm

b.5 mm

c.4 mm

d.3 mm

e.2 mm

20.A lens is to be edged for a frame with an A dimension of 47 mm and a DBL of 19 mm. If the wearer has a PD of 60 mm, which of the following is the total decentration?

a.6 mm

b.5 mm

c.4 mm

d.3 mm

e.2 mm

21.A pattern is made with the hole intentionally displaced nasally in order to avoid having to decenter the lens. If the pattern is made for a frame having an eyesize of 46 mm and a DBL of 18 mm—and the wearer’s PD is 60 mm—which of the following distances from the boxing center of the pattern would you expect the hole to be placed?

a.4 mm nasally

b.3 mm nasally

c.2 mm nasally

d.0 mm (because the pattern should never be made this way)

22.A pattern with its mechanical center 3 mm above the geometrical center of the pattern is used. To make sure that the optical center of the lens ends up as ordered, how much horizontal and vertical decentration per lens is needed?

+4.75D sphere

+4.50D sphere

Height of OCs = 25 mm A = 49

B = 47

ED = 54

DBL = 18

PD = 63

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23.If a lens is to be made from a tracing that was done from an old lens, which of the following need not be measured?

a.Eyesize

b.180-degree line

c.DBL

d.C-size

e.ED

24.If a pattern center is displaced too far nasally, in which of the following directions must the lens be decentered to compensate for the error?

a.Nasally

b.Temporally

c.Decentration dependent on lens power

25.A homemade pattern is placed on a Box-o-Graph to check it for accuracy. From the mechanical center, the outermost tangents in each direction are as follows:

Top: 23 mm

Bottom: 23 mm

Nasal: 26 mm

Temporal: 24 mm

Which of the following choices indicates how far the mechanical center of the pattern is located from the boxing center of the pattern? (Remember, the mechanical center is where the hole is located.)

a.The mechanical center is displaced 1 mm nasally from the location of the boxing center of the pattern.

b.The mechanical center is displaced 1 mm temporally from the location of the boxing center of the pattern.

c.The mechanical center is displaced 2 mm nasally from the location of the boxing center of the pattern.

d.The mechanical center is displaced 2 mm temporally from the location of the boxing center of the pattern.

26.For the pattern described in the question above, which of the following decentration compensations must be made during layout so that the PD comes out correctly?

a.Decenter each lens in an additional 1 mm.

b.Decenter each lens 1 mm less in than would be otherwise indicated.

c.Decenter each lens in an additional 2 mm.

d.Decenter each lens 2 mm less in than would otherwise be indicated.

27.A frame has the following dimensions:

A = 46

B = 42

DBL = 18

The pattern to be used has its mechanical center 2.5 mm above the boxing center. The ordered MRP height for the prescription is 25 mm. Which of the following choices indicates how much the MRP should be raised to be sure it ends up in the frame where it belongs?

a.5.5 mm

b.3.0 mm

c.2.5 mm

d.1.5 mm

e.None of the above

28.A pattern made on a pattern maker has the central hole displaced 1 mm too high and 1 mm too far nasally. For a single vision lens, which of the following choices indicates how much vertical and horizontal decentration is required to obtain a properly centered lens when the lens with the following dimensions is placed in a frame?

A = 46

B = 43

C = 45

ED = 48

DBL = 18

Wearer’s PD = 60

a.2 in, 1 up

b.3 in, 1 up

c.3 in, 1 down

d.1 in, 1 down

e.None of the above

29.True or False? Although pattern makers sometimes make a pattern with the center displaced too far nasally or temporally through human error, pattern makers cannot and should not make a pattern with the center hole laterally displaced on purpose.

30.Which of the following is not a valid function for a frame tracer?

a.To gather shape data to transfer to a patternless edger directly wired to that edger

b.To gather shape data to transfer to a patternless edger to a remote location using phone lines

c.To allow the collection of three-dimensional frame information to better locate the bevel on the edge of the lens

d.To determine the DBL of the frame

e.All the above are possible functions.

31.True or False? Frame tracers are capable of tracing a shape from a pattern, an old lens, or a frame’s demonstration lens.

Challenge Question

32.Which choice indicates how far away from the boxing center of the edged lens the optical center would be for the following lens? (Because the prismatic effect was achieved by surfacing the lenses, rather than by decentering a standard, uncut lens blank, the distance between these two points may not be directly measurable.)

–0.50 D sphere 3.5 base out A = 48

B = 43

ED = 53

DBL = 18

PD = 62

a.1.75 mm

b.17.5 mm

c.14.3 mm

d.72 mm

e.None of the above

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on the frame being worn. The first measurement is how far apart the person’s pupil centers are from each other. This is known as the interpupillary distance (PD). The second measurement is the distance between the geometrical (boxing) centers of the frame’s two lens openings. This is known as the distance between centers (DBC).

DETERMINATION OF DISTANCE BETWEEN CENTERS

For frames that conform to the boxing system of measurement, the DBC is equal to the eyesize (abbreviated

A) plus the distance between lenses (DBL).

DBC = A + DBL

DECENTRATION PER LENS

Most commonly the wearer’s PD will be less than the distance between centers. This will require 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 2.

DBC – Wearer PD = Decentration per lens 2

Example 4-1

A wearer’s PD is 62 mm. The frame size has an A dimension of 48 mm and a DBL of 20 mm. What is the decentration per lens required?

Solution

The following formula is used to find decentration per lens:

Decentration per lens = DBC – PD 2

and because

DBC = A + DBL

Decentration per lens = (A + DBL) – PD 2

then

Decentration per lens = (48 + 20) – 62 2

= 68 – 62 2

= 6 2

= 3 mm

C H A P T E R 4 C E N T R AT I O N O F S I N G L E V I S I O N L E N S E S

DETERMINING DECENTRATION FROM MONOCULAR INTERPUPILLARY DISTANCES

When a prescription specifies the wearer’s PD in reference to each eye individually, PDs were taken one eye at a time. This measurement is referred to as the monocular PD. For a monocular PD the reference is basically from the center of the bridge of the nose to the center of the pupil. A more conventionally measured binocular PD of 64, for example, may result in a right monocular PD of 31 and a left monocular PD of 33. This difference between left and right PDs is not unusual considering the asymmetry of facial features of many healthy individuals.

For a monocular PD, decentration is determined first by dividing the DBC of the frame by 2, then subtracting the monocular PD; thus the following equation is true:

Decentration = DBC – Monocular PD

2

or

Decentration = A + DBL – Monocular PD

2

DECENTRATION FOR READING GLASSES

Laboratory personnel may not know whether the prescription is being fabricated is for distance vision or near vision. A single number given for PD could be either the distance PD or the “near PD.” Near PD designates a smaller separation of the MRPs of the finished lenses to allow for convergence. Convergence is the inward turning of people’s eyes that occurs when they do close work.

Occasionally an order for single vision lenses is received that lists distance and near PDs. This would be written as 65/62, for example. Unless otherwise noted, it must be assumed that the distance PD (65) is desired, even if the prescription is for low plus lenses. When the order form also contains the instructions “for reading glasses,” the smaller measure is chosen (62). This is the near PD.

When in doubt as to which measurement to use, the practitioner should check with whomever ordered the glasses.

Historical Background

So for the example, the decentration needed per lens is 3 mm inward.

Most lens centration is done using an instrument specially designed for this purpose. However, in the past the centration of lenses was carried out with only