OPTOM FASLU MUHAMMED

History doesn't show when people first began to manufacture glass or even who discovered how to turn sand into glass.

What we do know is that the oldest pieces of manufactured glass are beads that date to around 3500 BC.

Glass was primarily used as jewelry until the 16th century BC when artisans discovered how to fashion glass into vases and other art objects.

The first written manual containing instructions on how to make and shape glass dates to 650 BC.

Glass was so precious and hard to come by that glass objects were used as currency by ancient Romans, Egyptians and Greeks.

When sand is heated to 1700 degrees Celsius it melts. The molecular structure of sand changes during the heating and subsequent cooling and becomes glass.

Glass is prized for being chemically inert and tolerant of high temperatures.

When someone needs glasses, the first requirement is an eye examination in the doctor’s office to determine the correct lens prescription. The next step takes place in the optical dispensary, where a frame is chosen.

Traditionally the optical laboratory consists of two main areas, a surfacing laboratory and a finishing laboratory.

One area creates the needed lens power, usually by a process called lens surfacing, which is performed at a facility referred to as a surfacing laboratory.

The second area takes the correctly powered lens and finishes it.

Finishing is accomplished through optical positioning of the lens and grinding of the edges so that the lens fits the shape of the chosen frame. The area where this occurs is known as the finishing laboratory.

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

• Single vision lenses

• Segmented multifocal lenses

• Progressive addition lenses

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.

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.

When single vision lenses are in uncut form and do not require surfacing, they are called stock single vision lenses.

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 second surface to the required power.

A lens with only one of the two surfaces finished is called a semi finished lens because it is only half finished. The prefix semi means half.

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

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 .

When three areas exist, the lens is called a trifocal.

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.

All the ophthalmic lenses currently available in the ophthalmic industry are:

• Glass material• Resin material

Process 1 Mix silica with a small amount of sodium

bicarbonate or potash and limestone.

The silica comes from sand or pulverized sandstone.

The sodium bicarbonate is baking soda and helps lower the melting point of the silica.

Potash is sodium containing potassium in a water-soluble form and also lowers the melting point of silica.

The limestone stabilizes the mixture and makes the glass stronger.

Heat the mixture to approximately 2,500 degrees F in a melting furnace for 24 hours. This allows the mixture to melt and mix thoroughly and allows any bubbles in the molten glass to rise to the surface.

Allow the mixture to cool several hundred degrees.

This brings the glass to a point where it's workable.

At this point it can be blown or drawn into whatever shape is desired. In manufacturing, glass is typically pulled into flat sheets.

Allow the molten glass to run into a wide, shallow channel of liquid tin, over which three separate rollers simultaneously flatten, smooth and draw out the glass.

Glass floats even as it's cooling. The liquid tin bath gives it a semi-solid surface on which to rest without sticking to that surface as it dries. This also keeps the glass from wrinkling on contact with a cooler surface.

Roll the glass via a series of rollers into a cooling oven or kiln. This oven lets the glass cool in a controlled manner from the approximately 1,000 degrees F it has currently reached.

The glass rolls along a series of rollers until it cools enough to cut, about 350 degrees F.

Place the sheets into a large, flat area and continue cooling via high-speed fans. Glass reaches its final rigid state here.

Score the glass with a carbide tipped scoring tool. Snap it by rolling it over a slightly higher roller, which acts as a fulcrum. Place the glass in storage until needed.

Oxides :-are used in the lens compositions primarily into two major categories:

(a) Network Formative Oxides :– Network formative oxides are used at the base of any composition which could almost form a lens on their own. For example, SiO2, B2O3, P2O5.

(b) MOdifier Oxides : – They are used to modify the basic properties contributed by formative oxides. These may range from viscosity to electric properties or from chemical resistance to coefficient of expansion. For example, K2O, Na2O, BaO.

According to composition, some oxides behave either as formatives or as modifiers. They are called intermediary oxides. For example, AI2O3, ZnO, TiO2

Have the basic function of eliminating gaseous inclusions (bubbles) in molten glass. Examples are Antimony Oxide, Alkaline Nitrates.

Complete absence of colorants is essential to obtain clear ‘white’ glass.

When added deliberately, they lead to selective absorption for each wavelength in the spectrum and so determine the tints. Example: Cobalt oxide gives rise to blue, Nickel oxides for brown.

The most common glasses currently popular in the industry for ophthalmic lenses

Crown is the most commonly used glasses for spectacle lenses.

Their composition is characterized by high silica, lime and sodium.

Some crown glasses through the addition of metal oxides like nickel and cobalt show specific tints and absorbent properties.

Borosilicate with a high boron content.

Microcrystals of silver halide which are responsible for the photochromic phenomenon

Lead oxide has been traditionally included in the glass composition to produce high index glasses. This produced “flint” glass.

Lead oxide is replaced by titanium oxides, thus preserving high index while reducing glass density.

Niobium, zirconium and strontium are also included for adjusting optical properties.

resiN MATeriAL

Resin lenses are made up of small molecular units called monomers which link together to form long chain known as polymers and the process of linking the monomers together is known as polymerization.

There are two processes to make resin lenses:1. Thermosetting2. Thermoplastic

Thermosetting and thermoplastic differ basically in the lining up of the molecules in their structure.

Thermosetting are cross linked molecular structure, resembles a ladder with extra rungs.

They do not melt or flow when heat is applied and makes the material less flexible.

CR39,Most hi-index resin lenses are made by thermosetting process

Thermoplastic are not cross-linked molecular structure. Molecular chains are independent of each other, looks like a ladder without rungs.

Optically they are not so stable during process. They soften under heat and therefore, good for injection molding process.

They are very sensitive to abrasion. Polycarbonates are the good examples.

(A) Thermoplastic (B) Thermosetting

Lens surfacing

Spectacle lens manufacturing involving many steps ,collectively called as Lens surfacing. Marking Blocking Grinding Smoothing Polishing De –blocking & cleaning.

To correctly locate the lens for different stages of surfacing ,certain markings are essential.

Optical center position ,cylinder axis, base apex direction in case of prism.

Done by marking with suitable ink or jugging at blocking stage.

The block holds the lens in place through entire surfacing process.

Alloy or wax is used to hold the lens blank to the surfacing block.

Block assures proper alignment in grinder

Alloy creates bond with the lens & tape

Alloy is melted and reused

Note the various openings that hold block in the correct position

Optical surfaces on glass are produced by the process of grinding and polishing.

Roughing First stage is removing the unwanted

materials from the lens surface. This is followed by trueing and smoothing.

Roughing is performed by machines known as surface generators and the process is termed as generating.

The grinding agent is usually a diamond –bonded disc or wheel, termed as lap.

Presently the polishing tools are lined with pads cut from polyurethane sheets.

The process of polishing is completed ,the lens is separated from its holder ( De- blocking ) and cleaned properly.

Lacquer can be used for cleaning.

GLAZING

Glazing refer to the process fitting lenses to a spectacle frame or mount.

◦ Laying off◦ Lens cutting◦ Lens edging◦ Springing in and rimless fitting.

Marking of cylinder axis ,optical center in the correct position relative to the lens shape.

Can be use focimeter ,marking devices and a specially designed protractor.

Optic center Marking

Axis Marking

Earth line Marking

Frame shape Marking

Scoring a slightly oversize outline of the desired shape on one surface of the uncut lens and removing the waste.

Lens cutting can be performed entirely by hand or with the aid of a wheel cutter.

With automatic edging machines.

Mini bevel edge Groove edge Flat edgeBevel edge

Manual edging

Auto edging

Pattern edging (semi auto)

Pattern less edging (fully auto)

Pattern size (A)(B

)

Location of nasal side

180* Cutting line

Frame Difference

Mechanical center

Physical pattern

Physical Pattern

Steps

Lens marking Pattern cuttingLens selection

Pattern fixing Lens blocking

Lens chucking

Pattern less edger Frame tracer

The final step of fitting well edged lenses into rimmed plastic frame is known as springing In .

Fitting of lenses to rimless mounts is called Rimless fitting.

Lens Insertion

Thank u……