Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009
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Coatings and Tints of Lenses 191
IMPORTANT FACTS REGARDING HARDCOAT
•The hard coating of resin lens is applied on the outer surface only by some lens manufacturer and on both surfaces by others. However, all polycarbonate lenses possess scratch resistant coating on both surfaces.
•These hard coatings are usually very long lasting.
•Bleaching of tint or colour change in a resin lens without hard coating is a simple process. However, it is not possible if the resin lens is already having hardcoating.
•Scratch resistant coatings additionally increase antireflection properties and incident light transmission. 92.06% light transmission by uncoated resin lens increases to 96% after addition of hardcoating.
ULTRAVIOLET INHIBITORS
All persons who are exposed to sun for longer period should wear UV protective lenses, since the ozone layer which is responsible for earth’s protection against UV radiational effect is being continuously depleting. An ideal UV protective lens should absorb close to 100% of UV rays. The ozone layer in the earth’s atmosphere absorbs UVC (200–280 nm) from sunlight (Fig. 13-4). Crown glass absorbs UV radiation upto 300 nm and resin lens absorbs UV radiation upto 350 nm (Fig. 13-4). The harmful effect of UV radiation on eyes can still occur upto 380 nm radiations. Polycarbonate lenses even when uncoated absorb all harmful radiations of UV rays. UV inhibitor coating applied on resin lens can block UV radiation upto 400 nm. UV radiation causes pterygium, pinguecula, snow blindness, cancers of the eye and ocular adnexa. UV coating imparts a very light yellow tinge/hue to the lens.
Infrared rays of wavelengths between 700–1400 nm (IR-A) are partly transmitted to retina and filters to these rays are usually incorporated in protective goggles.
WATER RESISTANT/HYDROPHOBIC COATING
Hydrophobic coating is applied over lenses for ease of cleaning. Antireflection coating treated Lenses, tend to show smudges and
192 Textbook of Visual Science and Clinical Optometry
Fig. 13-4: IR rays and UV rays wavelengths and their absorption by ozone layer, cornea, lens, different lens material and UV inhibitor coating. UV-A = 315–
400 nm, UV-B = 280–315 nm, UV-C = 200–280 nm, IR-A = 700–1400 nm, IR-B = 1400–3000 nm and IR-C = 3000–104 nm
marks more apparent due to the soft nature of the coating. To reduce this, lenses may be coated with hydrophobic coating. These coatings are even thinner than A.R. coating. They make the surfacewetting angle very low. In simple terms, they reduce adhesion of water or oil droplets and allow them to run off easily. It is applied over the A.R. coated lenses and increases hardness also. Hydrophobic coating is essentially a special layer of silicon deposit.
POLAROID LENSES
Polaroid lenses are meant to eliminate glare, reflected from flat surfaces such as water, snow, highways, etc. at certain angles. The polarising materials consist of nitrocellulose packed with ultramicroscopic crystals of herapathite. Their optic axes are aligned parallel to one another. Only 37% of incident light is
Coatings and Tints of Lenses 193
transmitted through polaroid lens. Polaroid lenses are available in glass, resin and polycarbonate lenses.
FUNCTIONS OF POLAROID LENS
•Protection of eyes from direct and reflected UV rays.
•Improved visibility due to reduced glare from flat surfaces.
LENS CARE AND CLEANING
•Lenses should be cleaned by holding them under running water, preferably warm to wash away dust and debris, which may act as an abrasive. The lens is dried by wiping with a soft cloth or tissue paper.
•Since the lenses are susceptible to scratching on hard surfaces, patients must be cautioned against placing them with the lenses in contact with very hard surface.
•Resin lenses and polycarbonate lenses must be kept in cases which have a soft inner lining to avoid damage to the lens surfaces.
C H A P T E R Frames and Lenses:
14 Dimensions,
Measurements
and Styles
SPECTACLE FRAME DIMENSIONS
The international method of spectacle frame dimension is known as “Box system”. Earlier “Datum system” was practised. Basic frame dimensions in the “Box system” are:
1.Horizontal centre line—It is a line passing through the geometrical centres of the rectangular boxes that enclose the two lenses (1 in Fig. 14-1).
2.Vertical centre line—It is the line passing vertically through the centres of the rectangular boxes which enclose the two lenses (2 in Fig. 14-1).
3.Boxed centre—It is the point of intersection of the horizontal and vertical centre line (3 in Fig. 14-1).
4.Horizontal lens size—It is the distance between the vertical sides of the rectangular box that encompasses the lens (4 in Fig. 14-1).
5.Vertical lens size—It is the distance between the horizontal sides of the rectangular box that encompasses the lens (5 in Fig. 14-1).
6.Boxed lens size—The horizontal and vertical lens sizes of the box that encompasses the lens constitute the boxed lens size (boxed lens size = 4 × 5 in Fig. 14-1).
7.Distance between the lenses (DBL) measurement—It is also called bridge size. It is measured from the inside nasal point of one to the inside nasal point of the other (6 in Fig. 14-1).
8.Distancebetweentherims/eyewires(DBR)measurement—Itis thedistancebetweentherimsatacertainspecifiedlevel(a)below middle of the lower margin of the bridge (7 in Fig. 14-1).
Frames and Lenses 195
Fig. 14-1: Box systems–different frame dimensions. 1 = Horizontal centre line, 2 = Vertical centre line, 3 = Boxed centre, 4 = Horizontal lens size, 5 = Vertical lens size, 6 = DBL/bridge size and 7 = DBR
PARTS OF A FRAME
1.Eyewire/Rim/Frame front—It is the part of the frame which holds the lenses.
2.Temple—Temple is the long handle of the frame that hooks around the ear for support. It is of the following basic shapes:
•Curl sides/cable (Fig. 14-2A)
•Drop end sides/Standard (Fig. 14-2B)
•Straight side/Library (Fig. 14-2C)
3.Temple tip—It is the plastic end piece on the temple for protection of the ear and skin.
4.Bridge—It is the part in the middle of the frame joining the two eyewires.
5.Nose pads—These are synthetic pads which rest directly on the nose for support. They either screw onto or snap into metal pieces.
6.Hinges—It join the temples with the eyewires.
196 Textbook of Visual Science and Clinical Optometry
Figs 14-2A to C: (A) Curl sides/cable, (B) Drop end sides/Standard and (C) Straight side/Library
COMMON TERMINOLOGIES ASSOCIATED WITH FRAME
Temple length—It is the length of the temple in millimetres (mm). Mount—To fix the lenses in the eyewires.
Dismount—To take the lenses out of the eyewires
Dowel Point—It is the point where the hinge is located for connecting the temple with the eyewire.
SHAPES OF FRAMES
Usually following basic shapes are available for frame (Fig. 14-3):
1.Oval/Elliptical
2.Square
3.Round
4.Octagon
5.Aviator/Pilot–It is triangular in shape with a lower nasal cut away and lower temporal extension outwards.
6.Cat eye
Frames and Lenses 197
7.Pantoscopic round oval (PRO)–It consists of a lower half circle and an upper half of ellipse.
Fig. 14-3: Shape of frames. 1= Oval/Elliptical, 2= Square, 3= Round,
4= Octagon, 5= Aviator/Pilot, 6= Cat eye and 7= Pantoscopic round oval (PRO)
COMMON TYPES OF FRAMES
1.Full rim–The metallic or synthetic plastics hold the lens on all sides.
2.Semi rimless/Supra–Here the lens is surrounded by the frame material in the upper half and by a thin nylon thread in the lower half. The lenses are held in position not by screws, but by the thin nylon cord which is attached to the rims. The nylon cord passes through a groove on the edge of the lens.
198 Textbook of Visual Science and Clinical Optometry
3.True rimless/Drill mount–Here the lenses are held in position by screws to the bridge and the temples. The true rimless frames are essentially consist of three pieces, two temples and a bridge. The lenses are mounted at four points, two temporal and two nasal by screws.
SPECIAL TYPES OF SPECTACLE FRAMES
I. Rarer/Antique Variety
•Lorgon (or Quizzer)–It contains only one lens with a handle to hold the lens before the eye.
•Lorgnette–It contains two lenses mounted on a handle into which the lenses may fold (when not in use).
•Monocle–It is a single lens with or without a frame which is worn by holding it between the brow and the cheek.
II. Medical Variety
•Ptosis spectacles
•Hearing aid spectacles
•Low visual aid spectacles
•Welding spectacles.
III. Sports Variety
•Swimming goggles
•Squash goggles
•Ski goggles
•Snooker spectacles
•Divers spectacles.
IV. Half Eyes–They are usually worn for near vision, so that the person can look over the top of the frame for distant vision. Here the lenses cover only the lower half of the field of vision.
INFORMATIONS AVAILABLE FROM THE FRAME
Usually following information are obtained from the frame (Fig. 14- 4):
•Temple length–Temple length is usually inscribed on the inner surface of the temple in millimetres (135 mm and 138 mm in Fig. 14-4).
Frames and Lenses 199
•DBL (Distance between the lenses)–DBL/Bridge size is usually inscribed on the inner surface of the temple in millimetres (18 mm and 20 mm in Fig. 14-4).
•Horizontal boxed lens size–Size of the frame is usually inscribed on the inner surface of the temple in millimetres (52 mm and 50 mm in Fig. 14-4).
•Model No., name of the manufacturer and colour code of the frame are also usually inscribed on the inner surface of the temple (Fig. 14-4).
The DBL/bridge size usually ranges from 18 to 20 mm in Indian adult people, whereas in Chinese people it is usually 22 mm. In European people it is usually 16 mm. The horizontal boxed lens size is usually available in the order of 46, 48, 50, 52, 54 and 56 (millimetre).
Fig. 14-4: Inner surface of temples with information inscribed
034 |
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= Model No.= |
C06009 |
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C3 |
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= Colour of Frame = |
COL04 |
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52 |
= Horizontal Boxed Lens Size (in mm) = |
50 |
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= Symbol of box system |
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18 |
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= Bridge size/DBL (in mm) = |
20 |
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135 |
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= Temple length (in mm) = |
138 |
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200 Textbook of Visual Science and Clinical Optometry
LENS STYLES
Fig. 14-5: Classification of lens styles
UNIFOCAL (OR SINGLE VISION) LENS
This is a lens with single dioptric prescription for either distance or near vision (Fig. 14-5).
MULTIFOCAL LENS
This is a lens with more than one prescription of dioptric strength (Fig. 14-5).
Bifocal Lens
This is a lens with two prescriptions for distance and near vision. Usually the upper segment is larger and is used for distant vision while the lower segment is smaller and is used for near vision. Types of bifocal lenses are:
Depending on shape
•Executive (E)/Franklin style bifocal–It is thicker than other types (Fig. 14-6A).
•D-bifocal/Flat-top bifocal (Fig. 14-6B).
•Kryptok/Curved bifocal (Fig. 14-6C).
•Round bifocal (Fig. 14-6D).
•Pantoscopic (P)/Pantobifocal (Fig. 14-6E).
•B bifocal (Fig. 14-6F).
Depending on manufacturing technique
•Split bifocal (or two piece bifocal)–Here two separate glass segments are held together in a frame (Fig. 14-7). It is the original type of bifocal invented by Benjamin Franklin. It is obsolete now.