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

Iwould like to thank Ginger Long, Antonio Turner, and Glenn Herringshaw, who have helped considerably in the preparation of the second edition of this book. They also have been a valuable part of

both the operation of and education at the optical laboratory at Indiana University.

Ginger Long reviewed the manuscript while it was used in the course on lense finishing. She was helpful in pointing out mistakes and ambiguities in the questions at the back of each chapter. Students were assigned problems and Ginger graded them. (They did not have the answers now found in the back of the book!) Ginger also helped with many of the photos and made suggestions on how to convey certain ideas.

Thanks to Antonio Turner for the help he provided with the photographs in Chapter 13, Drilled, Slotted, and Notched Mountings. He supplied a number of beneficial ideas and his hands are the hands seen in a majority of the photos in that chapter.

I am especially grateful to Glenn Herringshaw, Indiana University’s Optical Laboratory Manager, for the valuable help he has given. Glenn carefully reviewed

each chapter. When he discovered deficiencies, we worked together to figure out how to overcome them. A number of procedures outlined in the text were a result of his suggestions. Glenn helped in photography and also carefully examined each drawing and pointed out needed improvements. When something did not seem right, Glenn was the first one I would call to discuss the issue. He is optically knowledgeable and equally skilled in the practicalities of optical lens finishing.

Dan Torgersen, OLA (Optical Laboratories Association) Technical Director, was a valuable resource for questions on safety and impact resistance issues. His responses were always thorough and well thought out.

I also appreciate Joe Bruneni’s help in answering a spectrum of questions on a number of topics. Joe is a valuable resource for the whole optical industry and much appreciated by all.

And again, as with the first edition, I would like to thank my wife, Vickie, and my children, Debbie, Cliff, Abigail, and Kenneth, for their support and encouragement.

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frame. The area where this occurs is known as the finishing laboratory. A finishing laboratory is also referred to as an edging laboratory because here the lenses are “edged” to the proper shape to fit the spectacle frame.

A great deal more happens in the lens finishing process than just lens edging. This text focuses primarily on the finishing aspect of lens fabrication.

An edging laboratory does not require a surfacing laboratory to function. Most facilities that have a surfacing laboratory also have a finishing laboratory. However, the finishing laboratory has versatility. It may be associated closely with a surfacing laboratory, or it may function independently. Figure 1-1 is an overview of how lenses are processed in the optical laboratory.

Finished and Semifinished Lens

Terminology

Ophthalmic lenses may be divided into the following three broad categories:

•Single vision lenses

•Segmented multifocal lenses

•Progressive addition lenses

SINGLE VISION LENSES

Single vision lenses are the most basic type of lens. These lenses have the same power over the entire surface of the lens. Single vision lenses are used when the same optical power is needed for both distance and near vision. They also are used when a person requires no prescription for distance but needs reading glasses. Whenever possible, single vision lenses are edged from lenses kept in stock at the finishing laboratory. Because these lenses are finished optically to the correct power on both the front and back surfaces, they are called finished lenses. Finished lenses are also referred to as uncuts because they have not yet been “cut” to the correct shape and size (Figure 1-2, A). When single vision lenses are in uncut form and do not require surfacing, they are called stock single vision lenses.

The finishing laboratory personnel would much prefer to use a stock single vision uncut lens because it is less expensive than a custom surfaced lens. However, if the stock lens is too small for the frame, then a stock single vision lens will not work. Instead the lens must be produced in the surfacing laboratory. The surfacing laboratory starts with a lens having only one surface that is ready to use, or “finished.” This is usually the front surface. The laboratory must grind and polish the

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second surface to the required power. A lens with only one of the two surfaces finished is called a semifinished lens because it is only half finished. The prefix semi- means half (Figure 1-2, B).

Finished uncut and semifinished lenses have not been edged. Before a lens has been edged it is called a lens blank.

SEGMENTED MULTIFOCAL LENSES

Segmented multifocal lenses have more than one power. Each power is located in a distinct area of the lens bordered clearly by a visible demarcation line. When two different areas exist, the lens is called a bifocal (Figure 1-3, A). When three areas exist, the lens is called a trifocal (Figure 1-3, B).

Multifocal lenses may be made ready for the finishing laboratory in one of several ways:

•Multifocals may be individually ground and polished to power by a surfacing laboratory from a semifinished lens blank.

•Multifocals may be individually cast molded to the prescribed power, instead of being surfaced from a semifinished lens blank. Cast molding creates the lens from a liquid resin material. It is the same process used to make both plastic semifinished lenses and stock single vision plastic lenses. Cast molding multifocal lenses to power skips the semifinished lens stage. Cast molding to power may be done by a larger wholesale facility or, if equipment is available, in conjunction with a finishing laboratory (Figure 1-4).

•Multifocals may be made by laminating front and back lens halves together. In simple terms, the front half of the lens contains the multifocal segment and the back half contains the power. The two are glued together to make one complete lens. This lamination process also skips the semifinished lens stage. It does not require a lot of equipment and, like cast molding, may be carried out at the finishing laboratory (Figure 1-5).

•Multifocals may be kept as stock, finished bifocals in the finishing laboratory. This option is possible only if the lens power is spherical. At present, stock, finished bifocals are not often used.1

1Instead of being custom made, stock, finished bifocals may be mass produced with both inside and outside surfaces already ground and polished. Stock, finished bifocals are normally used only when both left and right eyes are spherical in power. Attempting to match one spherocylinder lens from a surfacing laboratory with a prefinished bifocal from another source may create unnecessary problems and mismatches. To prevent optical errors, the exact vertical position of the optical center of the stock, finished bifocal in relationship to the near segment must be known. This position must be duplicated accurately in the custom-surfaced, paired spherocylinder lens.

SURFACING

Determine correct base curve

Determine needed center or edge thickness

Determine lap tools required

Lay out lenses for surfacing

Block for surfacing

Crib lenses—i.e., reduce blank diameter (optional)

Perform the following:

Surface generating

Fining

Polishing

Deblocking

Inspection for correct power, surface quality, and other imperfections

EDGING

Spot lenses

Trace the frame for patternless edging, or, for patterned edging, pull pattern from pattern stock or make a pattern

Perform centration for blocking

Block the lenses

Edge the lens

Hand-edge (unless already done in the edger)

PROGRESSIVE ADDITION LENSES

Progressive addition lenses are used as an alternative to a segmented multifocal lens. They have distance power in the upper half of the lens. Lens power gradually increases as the wearer looks down and inward to view near objects.

Progressive addition lenses are prepared for the finishing laboratory in the same way as segmented lenses. These are listed in the previous section.

Overview of the Lens Finishing

Process

Edging often is used to denote the entire lens finishing process. In actuality, many steps come before and after the actual edging process. These are outlined in the following section and described in more detail in later chapters.

SPOTTING THE LENS

A lens-measuring device is needed to determine lens power, optical center location, and other optical characteristics of a lens. It may be referred to as a lensometer, lensmeter, focimeter, vertometer, or lens analyzer, depending upon the manufacturer. In this text the author uses the more generic term lensmeter. The lensmeter precisely determines lens power and exactly locates optical points within the lens. The process of finding these optical points, orienting the lens properly to meet the needs of the prescription, and then placing dots on the lens is referred to as spotting. It takes its name from the three horizontally aligned ink “spots”

FIGURE 1-5 Another alternative to the lens surfacing process is to create the needed power using a front and a back half. The front half contains the near add power, when needed. The back half completes the distance power and contains any needed cylinder power. The two halves are glued together to create the lens. (From Brooks CW: Understanding lens surfacing, Boston, 1992, Butterworth-Heinemann, p 299.)

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placed on the lens surface by the lensmeter. See Chapter 2 for a complete discussion of spotting.

CENTRATION AND BLOCKING OF THE LENS

Because the pupil of the eye is seldom found to be directly on line with the middle of the frame’s lens opening, the lens must be moved to correspond to the location of the eye. Because the lens is to be centered in front of the eye, this next process is called centration. Centration is done by use of the three reference dots that were placed on the lens during spotting. Once the lens has been positioned, a small block is secured to the lens so that it may be edged. Securing the block to the lens is called blocking.

Taken together, spotting and centration make up lens layout. The instrument used to center and block the lens is called a layout blocker or simply a blocker.

DETERMINATION OF LENS SHAPE

Before a lens may be cut to the proper shape for the frame, the needed shape must be quantified. Two types of edgers exist—one that uses an actual plastic pattern to guide it in shaping the lens and another that uses an electronic method to supply lens shape. Following are more detailed explanations of both types:

•The first is called a patterned edger. A patterned edger operates using a small, flat piece of plastic that matches the lens shape needed for the frame. This pattern itself is either supplied by the frame manufacturer or is made by the laboratory using a pattern maker.

•The second type of edger is called a patternless edger. It does not use a physical pattern but uses an electronic shape generated from the frame itself by way of a frame tracer. The tracer makes an electronic version of the shape needed and downloads it to the edger.

Some edgers operate in both patterned and patternless modes.

EDGING THE LENS

The blocked lens is now placed in the edger and the lens is edged to shape.

DEBLOCKING, TINTING, COATING, ENGRAVING, AND HARDENING

Once the lens is edged to the proper shape and size for the frame, it is taken off the block. The process of block removal is called deblocking. Before the lens is placed in

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the frame, several other possible procedures may be carried out. If the lens is ordered with a specific color and is plastic, it may be tinted. Tinting may be performed with hot dyes or by use of a vacuum coating process. Depending on ever-changing fashion, a lens could be decorated with an engraving.

If the lens is glass, it must be treated to increase impact resistance. This hardening process may be done by heat treating or chemical tempering the lens. Both glass and plastic lenses may be antireflection (AR) coated to reduce surface reflections.

LENS INSERTION OR MOUNTING

The edge of the lens may be angled to a point to allow insertion into a frame with a groove. Lens edges that are angled to a point are called beveled lenses. The process of putting beveled lenses in a frame is lens insertion.

Alternatively a lens may be held in the frame with a nylon cord, with screws, or by methods closely related to these. If a nylon cord is used, the edge of the lens must be grooved to accept the nylon cord. Some edgers groove the lens during the edging process. If the lens is not grooved in the edger, the edge is made flat. Then a groove is cut into the edge on a separate lens groover.

If a lens is to be held in place with screws, the edge is made flat without a bevel. Holes then are drilled in the lens to accept the screws. When lenses are grooved or drilled, the process of placing them in the frame is called mounting. (Frames that secure the lenses in place in this manner often are called mountings instead of frames.)

STANDARD ALIGNMENT

After the lenses have been placed in the frame, the glasses may not be aligned properly. The process by which the glasses are bent or readjusted to conform to a proper alignment is known as standard alignment or truing.

VERIFICATION AND CLEANUP

Before a prescription is released to the dispensary, it needs to be verified for accuracy. This is done using the same instrument as was used during lens spotting—the lensmeter. The prescription must optically conform to accepted standards of tolerance. Once judged acceptable, the frames and lenses are cleaned and passed on to the dispensary.