International Standards

13.1 International Standards

The Glossary of Terms and Symbols is a dual publication of International  Standard ISO 8320 – 1986. ‘Rigid’ lens specifications are given in ISO 8321–  1:1991.

ISO 9000 covers manufacturers of assessed capability and implies that verification, production, tolerances, quality assurance, and sampling conform to this standard.

13.1.1 International Standard terms

The terminology for a standard tricurve lens in ISO 8320 – 1986 symbols is:

Example:

7.90:7.80/8.70:8.60/10.75:9.20 tc 0.15 BVP −3.00 D Tint light blue

7.90= back optic zone radius (BOZR) r0

7.80= back optic zone diameter (BOZD)Ø0

8.70= first back peripheral radius r1

8.60= first back peripheral zone diameter Ø1

10.75= second back peripheral radius r2

9.20= total diameter ØT

0.15= geometric centre thickness tc −3.00 = back vertex power (BVP)

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X

Section TWO Rigid gas-permeable lens fitting

For a lenticular lens (reduced optic):

7.90:7.80/8.70:8.60/10.75:9.20 tc 0.45 te 0.16 BVP +10.00 D FOZD 8.30 Tint light blue

0.45= geometric centre thickness tc

0.16= edge thickness te

+10.00 = back vertex power (BVP)

8.30= front optic zone diameter Øa0

The subscript ‘a’ indicates an anterior surface component and the format is the same for both plus and minus lenses.

13.2 Examples of rigid lens types and fittings

(Assuming a spherical ‘K’ reading of 7.75 mm and low minus power)

Individual designs

•TD 8.60 mm CAEL 0.12 mm 7.80:7.00/9.00:7.80/10.90:8.60

•TD 9 00 mm CAEL 0.15 mm 7.80:7.00/8.75:7.80/10.10:8.60/11.30:9.00

•TD 9.20 mm CAEL 0.12 mm 7.80:8.00/8.80:8.60/12.30:9.20

•TLT 13  m, EC 60  m 7.80:7.70/8.30:8.20/9.20:9.20

•TD 9.50 mm CAEL 0.175 mm 7.80:7.50/8.90:8.50/10.00:9.00/11.15:9.50

•TLT 12  m, EC 60  m 7.80:8.30/8.20:8.80/9.00:9.80

Proprietary designs

•Series II (No. 7 Laboratory) 7.80:7.80/8.65:8.70/9.90:9.50

•Quantum (Bausch & Lomb): 7.70/9.60

•Aquila (CIBAVision): 7.80/9.30 or 7.80/9.80

•Standard Polycon II (CIBAVision): 7.80:7.80/AEL 0.10 at 9.00

•Asphericon (CIBAVision): 7.80/9.80 standard e

13.3 Rigid lens verification

All lenses should ideally be checked before use:

•To ensure the accuracy of diagnostic lenses.

•To ensure prescription lenses are suitable for dispensing.

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•To establish the specification of the patient’s existing lenses.

•Where lens parameters are thought to have altered.

•Where lenses are thought to have distorted.

•To confirm that lenses are being worn in the correct eyes.

•To confirm that current and old lenses have not been confused.

•To ensure records contain full details of lens specification.

13.3.1 Back optic zone radius (BOZR)

Radiuscope

Based on Drysdale’s method which measures the distance between the lens surface and the centre of curvature.

•To obtain a good image, ensure the lens is well dried before being placed on a drop of distilled water in the concave holder.

•To help location of the images, ensure the instrument light is at the centre of the lens.

•Travelling from the zero to the second position, the image of the bulb filament is seen. The quality of this image gives an indication of any lens distortion.

•Always take two or three readings and average the results.

•The image at the lens surface is usually much larger and brighter than that at the centre of curvature, as well as showing any surface scratches.

•The zero reading with unstable rigid lenses can ‘creep’ and may require several attempts before giving a reliable result. If the creeping does not stop, average the first three readings.

Other methods

Keratometer

Uses a lens holder with a front surface silvered mirror. 0.03 mm is added to correct for the concave surface.1 Autokeratometers can also be used.

Radius-checking device

The radius is derived from the focimeter front vertex power (FVP), using refractive index and thick lens formula.2

Toposcope

Uses moire fringes.1

13.3.2 Peripheral radius (BPZR)

Peripheral radii can be measured with the radiuscope if the lens is tilted and the band width is at least 1 mm. A qualitative assessment can be made with a Burton lamp by observing the reflection of the white light tube in the lens surface.

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13.3.3 Total diameter (TD) and zone diameters (BPZD)

Measuring magnifier (band magnifier)

Consists of an engraved graticule plus an adjustable eyepiece with ×7 magnification. The lens is repositioned on the scale for different zones and measurement is easier with sharp transitions.

V gauge

Consists of a V-shaped channel graduated between 6.0 mm and 12.5 mm. Only measures TD.

Projection magnifier

Projects a magnified image of the entire lens onto a calibrated screen.

PRACTICAL ADVICE

•Ensure that the lens is dry or it can be difficult to remove from a smooth surface because of capillary attraction.

•Avoid wet cell instruments because of difficulty in lens manipulation.

13.3.4 Back and front vertex power (BVP and FVP)

Focimeter

•Place focimeter in a vertical position or use a V-slot holder.

•For BVP, place the concave surface towards the focimeter.

•The lens must be placed as close as possible to the focimeter, either by using a very small stop or by removing the stop cover. The reading may still give more plus or less minus than the true BVP because of the steep lens radius.

•For FVP, place the convex surface towards the focimeter stop. FVP reads less than the BVP, with a greater difference in plus powers than minus.

PRACTICAL ADVICE

•Note the quality of the image on the focimeter. Distortion indicates a poor optic.

•A good image does not necessarily guarantee a distortion-free optic because a small stop is used and only the centre of the lens is measured.

Power profile mapping (e.g. VC 2000)

An extremely accurate computerized method, mainly used by manufacturers, for the bi-dimensional measurement of the contact lens power map. Rigid contact lenses are evaluated in air and soft lenses in a saline wet cell. The power profile of both spherical and toric lenses can be made by computation from thousands of

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measurement points. Information is obtained on tolerances for all lens designs and provides automatic axis measurement relative to the true optical parameters for toric lenses. It is also possible to achieve accurate results for multifocal lenses.

13.3.5 Centre and edge thickness (tc and te)

Thickness gauge

Consists of a spring-loaded, ball-ended probe geared to a direct reading scale.

PRACTICAL ADVICE

•Take several readings of the lens edge, as the thickness may vary around the circumference.

•Take care not to damage a thin edge.

Radiuscope

The lens holder is left dry and the target focused on each lens surface in turn. The distance between the two images multiplied by the refractive index of the material gives the central lens thickness.

13.3.6 Edge form

Edge form is best examined with about ×20 magnification using either a hand loupe or the slit lamp.

13.3.7 Surface quality

Surface scratches and defects can be assessed in a variety of ways:

•Projection magnifier with a clean dry lens.

•Slit lamp, using transillumination.

•Band magnifier.

•Radiuscope by examining the first image.

13.3.8 Material

Confirming lens material is difficult, although comparison of specific gravity measurements can give an approximate guide. The most reliable indication can sometimes be colour, since certain rigid gas-permeable lenses are available in only one distinctive tint (e.g. Equalens, very pale lilac, Conflex, very pale blue; Polycon II, medium blue).

13.3.9 Other features

•Engravings (e.g. ‘R’ or a dot for the right lens).

•Laboratory codes.

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