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

S E R I E S 1 0

Flat-Top Bifocals

RELATING MAJOR REFERENCE POINT AND SEGMENT POSITIONS

Reference: Pages 117 through 121.

For each of the following exercises, perform the following steps:

•Find the total inset.

•Find the segment drop or raise.

•Lay out the lens front side up for blocking.

•Dot the location of the MRP as it should appear on the lens.

•Mark the lens with a cross to show where the center of the block should be. When drawing the cross, skip over the MRP dot so as not to hide it from view.

Example

Right lens

A = 48 mm

B = 38 mm

DBL = 20 mm

PD = 62/59

Segment height = 16 mm

The following exercises describe how the different parts of the problem are solved.

A.Total inset

= [A + DBL] – Near PD

2 = [48 + 20] – 59

2

= 68 – 59 2

=9

2

=4.5

B.Segment drop or raise

=Segment height – B2

=16 – 382

=16 – 19

=–3 mm drop

C.Layout for the lens is shown in Figure CSS-7 on p. 443.

D.Location of the MRP is at the distance decentration.

=[A + DBL] – Distance PD

2 = [48 + 20] – 62

2

= 68 – 62 2

=62

=3 mm

The center lensmeter dot (MRP) is 3 to the right of the background grid center. Its location also may be figured backward from the location of the segment, as segment inset:

Segment inset = Distance PD – Near PD 2

=62 – 59

2

=32

=1.5 mm

So the center lensmeter dot is 1.5 mm to the left of the segment center (Figure CSS-8, p. 443).

E.When the lens is marked with a cross to indicate the location of the center of the block, it will appear as shown in Figure CSS-9 on p. 443.

1.Right lens

A = 52 mm B = 38 mm

DBL = 22 mm Wearer’s PD = 66/62

Segment height = 18 mm

2.Left lens

Lens power is –0.75 –0.25 × 65

A = 56 mm B = 50 mm

DBL = 19 mm Wearer’s PD = 67/63

Segment height = 22 mm

3.Right lens

Lens power is +1.25 –0.50 × 110

A = 48 mm B = 42 mm

DBL = 24 mm Wearer’s PD = 64/61

Segment height = 21 mm

4.Right lens

A = 52 mm B = 44 mm

DBL = 19 mm Wearer’s PD = 61/59

Segment height = 18 mm

5.Left lens

A = 53 mm B = 51 mm

DBL = 15 mm Wearer’s PD = 62/59

Segment height = 20 mm

S E R I E S 1 2

Franklin-Style Bifocals

Reference: Page 131.

Franklin-style lenses* can be surfaced with a variety of methods. Two methods are as follows:

1.Drop, but no segment inset

With this method the lens is surfaced such that the optical center will be above the segment line by an amount equal to the segment drop. The optical center is directly above the geometrical center of the segment. It is the same as if a flat-top bifocal were ground with equal distance and near PDs.

2.Adherence to both drop and segment inset amounts

In this method the optical center is ground above the segment geometrical center by an amount equal to the drop and is outset by an amount equal to the segment inset.† When ground in this manner, the optics of the Franklin-style lens duplicate those of a large segment flat-top bifocal.

Regardless of how Franklin-style lenses are surfaced, the layout for edging process is the same.

For purposes of these exercises, place three lensmeter dots on the lens such that the center dot is exactly above the center of the segment line by an amount equal to the calculated drop. The other two dots must parallel the segment line. (In practice, the MRP may not be always in the center of the lens.)

1.Right lens

A = 56 mm B = 46 mm

DBL = 20 mm PD = 69/66

Segment height = 21 mm

2.Right lens

A = 48 mm B = 39 mm

DBL = 14 mm PD = 49/47

Segment height = 23 mm

3.Left lens

A = 52 mm B = 50 mm

DBL = 16 mm PD = 60/57

Segment height = 22 mm

*Franklin-style lenses are known more commonly as Executive bifocals. Executive is AO Sola’s (Petaluma, Calif.) trade name for the Franklin-style lens.

†Some Franklin-style lenses are produced in semifinished form with the near optics already inset by a given amount. These semifinished lenses are marked for right and left eyes like flat-top lenses are.

11.Right lens

A = 54 mm B = 51 mm ED = 63 mm

DBL = 14 mm PD = 68/65

Segment height = 17 mm

S E R I E S 1 3

Centration Using Irregular Patterns

Reference: Pages 57 through 61.

Some “homemade” patterns have the central hole (mechanical center) slightly off center. Unless compensation is made in the centration process, the major reference point also will be displaced away from its intended location in the edged lens.

WHEN THE PATTERN CENTER IS OFF VERTICALLY

Unless otherwise indicated, for the following problems, assume that the MRP should be placed on the horizontal midline. For each of the following exercises, perform these steps:

•“Spot” the lens MRP.

•Calculate distance decentration.

•Calculate vertical compensation (how far the MRP must be dropped or raised to have it come out right for the nonstandard pattern).

•Lay out the lens for the calculated horizontal decentration and vertical compensation.

•Mark the lens with a cross to indicate where the center of the block will be.

1.The central pattern hole is 3 mm above the pattern boxing center. Right lens

A = 56 mm B = 47 mm ED = 64 mm

DBL = 16 mm PD = 62 mm

2.The central pattern hole is 1 mm above the pattern boxing center. Left lens

A = 48 mm B = 42 mm ED = 50 mm

DBL = 20 mm PD = 64 mm

3.The central pattern hole is 0.5 mm above the pattern boxing center. Left lens

A = 55 mm B = 46.5 mm

ED = 63.5 mm DBL = 16 mm PD = 62 mm

4.The central pattern hole is 1 mm above the pattern boxing center. Right lens

A = 53 mm B = 45 mm ED = 62 mm

DBL = 14 mm

PD = 60 mm (Requested MRP height is 25 mm.)

5.The central pattern hole is 0.5 mm above the pattern boxing center. Right lens

A = 58 mm B = 47 mm ED = 61 mm

DBL = 20 mm

PD = 62 mm (Requested MRP height is 26 mm.)

WHEN THE PATTERN IS OFF HORIZONTALLY

When patterns are made in the finishing laboratory, they should be checked upon completion to ensure that the hole is exactly in the center. If it is not, it can nevertheless be used if compensation is made. For each of the prescriptions listed below, mark the lens for blocking. For each of the following exercises, perform these steps:

•Spot the lens with the three parallel lensmeter dots.

•Determine how much inset or outset is required to simply compensate for the pattern error.

•Calculate normal inset per lens (or total inset, in the case of bifocals) as is normally done.

•Compensate the normal inset from step 3 by the amount of compensation you found in step 2.

•Mark the lens with a cross to indicate where the center of the block will be.

6.The central pattern hole is displaced 1 mm nasally. Left lens

A = 44 mm B = 23 mm ED = 42 mm

DBL = 22 mm PD = 60 mm

7.The central pattern hole is displaced 0.5 mm nasally. Right lens

A = 50 mm B = 40 mm

ED = 55.5 mm DBL = 20 mm PD = 73 mm

8.The central pattern hole is displaced 0.5 mm nasally. Right lens

A = 54 mm B = 48 mm ED = 60 mm

DBL = 14 mm PD = 61/58 mm

Segment height = 20 mm