Lens Checking: Soft and Rigid

Introduction

Successful contact lens fitting requires the manufacture of high-precision contact lenses and the establishment of meaningful tolerances. Such tolerances are established by national and international standards organisations.

Poor-quality contact lenses can compromise wearing comfort. Contact lenses therefore have to be verified and/or inspected by the contact lens practitioner during the fitting sessions and follow-ups.

Depending on the classification, contact lens type and design, the inspection and verification of a lens may include any of the parameters listed in Tables 18.1 and 18.2 . Tolerances for dimensional and optical parameters of rigid lenses are given in Tables 18.3 and 18.4 ( ).

Table 18.1

Contact Lens Parameters Which May Need Verifying

* Where there is more than one such zone.

Table 18.2

Possible Methods of Test for Rigid Contact Lenses (for methods see later in chapter)

Back central optic zone radius	(1) Optical radiuscope microspherometer (2) Keratometer (calibrated for concave surfaces) (3) Interferometry techniques
Back (central) optic zone diameter	×10 measuring magnifier
Back scleral zone radius (of preformed lenses)	(1) Radiuscope (2) Keratometer (calibrated for concave surfaces)
Basic or primary optic diameter	Sagitta method
Back peripheral (optic) radius	(1) Radiuscope (2) Topographical keratometer (3) Interferometry techniques
Back peripheral (optic) diameter	×10 measuring magnifier
Axial edge lift, radial edge lift	(1) Radiuscope (2) Sagitta method
Total diameter	(1) V-channel gauge (2) ×10 measuring magnifier (3) Projection magnifier with scale
Front (central) and/or peripheral optic zone diameter	×10 measuring magnifier
Bifocal segment height	×10 measuring magnifier
Centre thickness	(1) Measuring dial gauge (2) Radiuscope (3) Projection magnifier with scale
Edge thickness	Measuring dial gauge
Scleral vertex clearance (from cast)	Measuring dial gauge
Back vertex power, prism, optical centration	Focimeter with contact lens support
Surface quality, edge form	(1) Binocular microscope (2) Projection magnifier (3) Measuring magnifier

Table 18.3

Dimensional Tolerances for Rigid Contact Lenses

Permission to reproduce extracts of ISO 18369-2 ( ) Table 3 is granted by BSI (see Glossary ).

	Tolerance Limit
	Corneal Contact Lens
Property	PMMA (mm)	Gas Permeable (mm)	Scleral Contact Lens (mm)	Relevant Method
Back optic zone radius	±0.025	±0.05	±0.10	ISO 18369-3, Clause 4.2
Back optic zone radii of toroidal surfaces ^a,b				ISO 18369-3, Clause 4.2
Where 0 < Δ r ≤0.2	± 0.025	± 0.05	±0.12
Where 0.2 < Δ r ≤0.4	±0.035	±0.06	±0.13
Where 0.4 < Δ r ≤0.6	±0.055	±0.07	±0.15
Where Δ r >0.6	±0.075	±0.09	±0.17
Back optic zone diameter ^c	±0.20	±0.20	±0.20	ISO 18369-3, Clause 4.4
Back scleral radius (of preformed lens)	–	–	±0.10	ISO 18369-3, Clause 4.2
Basic or primary optic diameter	–	–	±0.20	ISO 18369-3, Clause 4.4
Back or front peripheral radius (where measurable) ^c	±0.10	±0.10	±0.10	ISO 18369-3, Clause 4.2
Back peripheral diameter ^c	±0.20	±0.20	±0.20 (for preformed lenses)	ISO 18369-3, Clause 4.4
Total diameter ^b	±0.10	±0.10	±0.25	ISO 18369-3, Clause 4.4
Front optic zone diameter ^c	±0.20	±0.20	±0.20	ISO 18369-3, Clause 4.4
Bifocal segment height	–0.10 to +0.20	–0.10 to +0.20	–0.10 to +0.20	ISO 18369-3, Clause 4.4
Centre thickness	±0.02	±0.02	±0.10	ISO 18369-3, Clause 4.5

a Δ r is the difference between the radii of the two principal meridians.

b The tolerance applies to each meridian.

c These tolerances apply only to contact lenses with spherical surfaces and distinct curves; they are for a finished contact lens and any blending may make measurement difficult.

Table 18.4

Optical Tolerances for Rigid Contact Lenses

Permission to reproduce extracts of ISO 18369-2 ( ) Table 4 is granted by BSI (see Glossary ).

Dimension	Tolerance Limit	Relevant Method
Label back vertex power in the weaker meridian		ISO 18369-3, Clause 4.3
0 to ±5.00 D	±0.12 D
over ±5.00 D to ±10.0 D	±0.18 D
over ±10.00 D to ±15.0 D	±0.25 D
over ±15.00 D to ±20.0 D	±0.37 D
over ±20.00 D	±0.50 D
Optical centration for scleral lenses only (maximal error)	0,5 mm	ISO 18369-3 Clause 4.4
Cylinder power		ISO 18369-3 Clause 4.3
to 2.00 D	±0.25 D
over 2.00 D to 4.00 D	±0.37 D
over 4.00 D	±0.50 D
Cylinder axis	±5°	ISO 18369-3, Clause 4.3

Soft lenses are more difficult to verify than rigid lenses because of greater flexibility and dehydration effects. The dimensional properties are influenced by dehydration and also by factors such as pH and tonicity of the storage solution ( , , , ). Table 18.5 outlines the dimensional and optical tolerances for soft contact lenses ( ).

Table 18.5

Parameter Tolerances for Soft Contact Lenses

Permission to reproduce extracts of ISO 18369-2 ( ) Table 5 is granted by BSI (see Glossary ).

Dimension	Tolerance Limits	Relevant Method
Back optic zone radius/equivalent posterior radius of curvatur/base curve equivalent	±0.20 mm	ISO 18369-3, Clause 4.2
Sagitta (see Note 1)	±0.05 mm	ISO 18369-3, Clause 4.2
Total diameter	±0.20 mm	ISO 18369-3, Clause 4.4
Centre thickness (see Note 2)		ISO 18369-3, Clause 4.5
≤0.10 mm	± [0.010 mm + 0.10 t _c ]
>0.10 mm	± [0.015 mm + 0.05 t _c ]
Label back vertex power		ISO 18369-3, Clause 4.3
F ′ _v \| ≤10 D	±0.25 D
10 D < \| F ′ _v \| ≤20 D	±0.50 D
F ′ _v \| >20 D	±1.00 D
Cylinder power		ISO 18369-3, Clause 4.3
F ′ _c \| ≤2 D	±0.25 D
2 D < \| F ′ _c \| ≤4 D	±0.37 D
F ′ _c \| >4 D	±0.50 D
Cylinder axis	±5°	ISO 18369-3, Clause 4.3

NOTE 1: Tolerance is applicable when this parameter is the one used to describe the posterior surface of the lens.

NOTE 2: Examples of tolerance calculations:

Nominal thickness	Tolerance
0.035 mm	± [0.010 + 0.004] = ±0.014 mm
0.070 mm	± [0.010 + 0.007] = ±0.017 mm
0.150 mm	± [0.015 + 0.008] = ±0.023 mm
0.300 mm	± [0.015 + 0.015] = ±0.03 mm

Terminology

Unification of terms and definitions in the field of contact lenses is important in a globalised world. ISO 18369-1 ( ) identifies and defines the terms applicable to the physical, chemical and optical properties of contact lenses, their manufacture and uses. It also contains an alphabetical list of terms with the relevant international symbols and abbreviations. The terms relating to contact lens care products and the classification and specification of soft and rigid contact lenses are also listed.

Lens Specification

The recommendation of the ISO 18369-1 ( ) is that all linear dimensions are made in millimetres (mm) and that all additional specific requirements, such as edge form, material, tint or power and orientation of specified prism, may be included as ‘Additional notes’.

Examples

Tri-curve corneal contact lens

\begin{matrix} r_{0} : \emptyset_{0} / r_{1} : \emptyset_{1} / r_{2} : \emptyset_{T} / F'_{v} / t_{c} \\ 7.60 : 7.00 / 8.30 : 8.80 / 12.25 : 9.60 / - 5.00 / 0.10 \end{matrix}

$\begin{matrix} r_{0} : \emptyset_{0} / r_{1} : \emptyset_{1} / r_{2} : \emptyset_{T} / F'_{v} / t_{c} \\ 7.60 : 7.00 / 8.30 : 8.80 / 12.25 : 9.60 / - 5.00 / 0.10 \end{matrix}$

where:

r ₀ = back optic zone radius
Ø ₀ = back optic zone diameter
r ₁ = first back peripheral radius
Ø ₁ = first back peripheral diameter
r ₂ = second back peripheral radius
Ø _t = total diameter
F′ _v = back vertex power in air
t _c = specified value of centre thickness

Bicurve hydrogel contact lens

\begin{matrix} r_{0} : \emptyset_{0} / r_{1} : \emptyset_{T} / F'_{v} \\ 8.80 : 12.00 / 9.50 : 14.50 / - 2.00 \end{matrix}

$\begin{matrix} r_{0} : \emptyset_{0} / r_{1} : \emptyset_{T} / F'_{v} \\ 8.80 : 12.00 / 9.50 : 14.50 / - 2.00 \end{matrix}$

where:

r ₀ = back optic zone radius
Ø ₀ = back optic zone diameter
r ₁ = first back peripheral radius
Ø _T = total diameter
F′ _v = back vertex power in air

Radii, Eccentricity and Edge Lift

There are different methods available for measuring rigid and soft contact lenses, but they are not all suitable for checking both types of lenses.

Keratometers

In clinical practice, keratometers are used primarily to measure the central curvature of the cornea, but they can also be used to verify front and back surface radii of contact lenses. A convex surface, such as the anterior cornea, produces a reduced, virtual and upright image of the keratometer mires, whilst the concave back surface produces a reduced, real and inverted image (see Chapters 7 and 8 ).

Rigid Contact Lenses

The keratometer is used with a special contact lens holder and a special holding attachment ( Fig. 18.1 and 18.2 ). According to ISO 18369-3 ( ), rigid contact lenses should generally be measured in air, but a measurement in a wet cell is also possible.

Fig. 18.1, Contact lens holder for use with a keratometer.

Fig. 18.2, Contact lens holding attachment according to ISO 18369-3 ( International Standard 2006 ) for the measurement of rigid contact lens radii with the keratometer.

In some keratometers, the mires are reflected from regions outside the paraxial zone and an allowance is made during calibration for the aberrations thus introduced.

Aberrations from concave and convex surfaces are different. Keratometers are calibrated for convex surfaces, and so errors occur when they are used to measure concave back optic zone radii (BOZR). Some manufacturers have produced tables for converting convex radii to concave. The correction factors for radii for concave surfaces are greater than for convex surfaces and range from 0.02 mm for steep radii to 0.04 mm for flat. Thus, for lenses used in practice, it is sufficient to add 0.03 to the radius reading given by the keratometer. The Zeiss 30 SL/M and the Rodenstock CMES keratometer do not need a conversion factor added.

An autorefractor/keratometer or computer-assisted videokeratography equipment with a rigid contact lens holder ( Fig. 18.3 ) can also be used to determine BOZR ( Fig. 18.4 ). Depending on the type of videokeratograph, a special software module for determining the exact eccentricity of aspheric contact lenses may be necessary ( ).

Fig. 18.3, Nidek contact lens holder for use with an autokeratometer.

Fig. 18.4, A computer-assisted videokeratographer detailing the geometry of a rigid contact lens. The lens being shown has a back toric design. toric lens design.

A correction factor must also be added when verifying the radii of aspheric lenses, as the mire images of the keratometer are reflected from a zone significantly flatter than the contact lens centre. The correction factor is the difference between the nominal vertex radius and the nominal sagittal radius at a semi-chord that is half the keratometer mire image separation on the surface ( ).

When measuring the eccentricity of aspheric contact lenses, a special lens holder is used. This needs to pivot around its vertical axis ( ) so that the central vertical radius and the sagittal radius at 30° can be measured (see p. 359). The eccentricity of the contact lens can be quantified with the equation

e = \sqrt{\frac{r_{s}^{2} - r_{0}^{2}}{\sin^{2} φ \times r_{s}^{2}}}

$e = \sqrt{\frac{r_{s}^{2} - r_{0}^{2}}{\sin^{2} φ \times r_{s}^{2}}}$

where e is the eccentricity, r _s is the sagittal radius, r ₀ is the vertex radius and φ is the angle at which the sagittal radius is measured ( ).

Steep and Flat Radii

The range of the keratometer needs to be extended when measuring back scleral radii. This is done by recalibrating the keratometer using a −1.00 or −2.00 D trial case lens taped in front of the objective. Three steel balls, of known radii (e.g. 9.00, 12.00 and 15.00 mm) are used for recalibration and entered onto a graph with the measured radius on the x -axis and actual radius on the y -axis. Provided the same trial case lens is always used, the same graph can determine longer radii than the keratometer was designed to measure.

Steep radii can be determined (useful in keratoconic corneae) by taping a positive trial case lens of about +1.50 D in front of the keratometer objective and recalibrating with steep steel balls.

Soft Contact Lenses

Measuring radii of soft lenses with a keratometer is complicated by lens flexibility and dehydration effects. The lens must be placed in a wet cell containing normal saline ( n = 1.336) and the wet cell separated from the keratometer by air. The wet cell is mounted on an inverting prism/mirror which is in front of the keratometer ( Fig. 18.5 ).

There are various difficulties that must be taken into account when measuring soft lenses in liquid using a keratometer:

■
Keratometer mires appear smaller in solution (between 5.40 and 7.10 mm) than in air, and the refractive index (RI) of the storage solution must be considered. Both measurements must be converted or else compensating lenses need to be used. The Rodenstock CMES keratometer incorporates an additional objective lens mounted on a turret on the telescope to directly measure the radii of soft contact lenses in saline.
■
If reflected light is used to measure the radii of a submerged lens, then a double image is formed by reflections at the front and back surfaces. The surface proximal to the mires will produce a brighter image, which is smaller for negative and larger for positive lenses ( ). Fig. 18.6 shows the classification of the Zeiss mires when measuring the radius of a soft lens. Less light is reflected from each surface in liquid than in air.
■
Fresnel's Law for light of normal incidence is:

R = {[\frac{n^{'} - n}{n^{'} + n}]}^{2} \times 100 %

$R = {[\frac{n^{'} - n}{n^{'} + n}]}^{2} \times 100 %$

where R = percentage of light, n = RI of surrounding medium (1.336 for saline solution and 1.00 for air), n ′ = RI of the second medium (≈1.430 for hydrogel material). In air, therefore, R = 3.13%; in saline, R = 0.115%.

■
The luminosity of the mires must be increased significantly to compensate for the reduction in intensity of reflected light when a contact lens is measured in a saline cell. Many modern keratometers are illuminated with brightness-controlled halogen bulbs and provide scale intervals of 0.01 mm, which read to radii of 5.00 mm or less.

As the water content (WC) of the contact lens increases, keratometer measurement becomes more difficult. studied the precision repeatability of keratometry in measuring BOZR of soft lenses. Thirty-two lenses (+5.00 to −5.00 D and 38% and 55% WC) were measured. For 38% WC, radii were ±0.058 to ±0.107 mm, and for 55% WC, radii were ±0.140 to ±0.198 mm.

Silicone Rubber Contact Lenses

Silicone rubber lenses should be floated on the surface of a liquid without tension ( ). As measurements are carried out in air, the values measured correspond to real radii.

Axial Edge Lift and Eccentricity

The pillar and collar technique

Axial edge lift of multicurve rigid lenses ( ) and flattening gradient ( p -value or eccentricity) of aspheric rigid lenses ( ) can be derived from measurements of the sagittal depth across single or multiple chord diameters. ¹

1 The clinical significance of axial and radial edge lift is dicussed in Chapter 9 , ‘Rigid gas-permeable corneal lens fitting’, under ‘Corneal shape’ and ‘Selection of the edge curve’.

■
A pillar, whose diameter is smaller than the total lens diameter, serves as a contact lens holder, and the collar, whose diameter is slightly larger than the contact lens diameter (0.1−0.2 mm), ensures proper centration of the lens on the pillar ( Fig. 18.7 ).
■
A travelling microscope is focused on the front surface of the lens, the lens is removed and the microscope refocused on the surface of the pillar.
■
The distance the microscope has travelled between the two foci minus the central thickness of the lens gives the sagittal depth across a chord corresponding to the pillar diameter.
■
Using the sag formula , where r = radius of curvature and s = sagittal at semi-aperture y , the required sag of the lens under test ( b ) at the total diameter (TD) can be calculated. The axial edge lift ( e ) then is given by the formula e = b − s .
■
For determining the axial edge lift of multicurve rigid lenses, this method is accurate and reproducible ( ).
■
Rearranging Baker's equation for ellipsoids as gives the p -value, p , of an elliptical lens surface, where r ₀ is the vertex radius and y is half the pillar diameter over which the sagitta, s , has been measured. The p -value and the eccentricity, e , of a surface are related by .
■
Deriving the eccentricity or the p -value of aspherical lenses with increasing eccentricity is far more complex and requires the mathematical description of the surface to be known.

For determining the eccentricity of an aspheric lens, this method is less precise than a keratometer-based method using central and peripheral radii ( ).

Optical microspherometers (radiuscopes)

The optical microspherometer is commonly known as a radiuscope (the trade name of an American Optical instrument).

The measurement of small radii with a radiuscope was originally described by and is based on the principle that, for a curved mirror, an image is formed in the same plane as the object when the object is at the centre of the curvature because reflected light returns along its incident path. An image is also formed on the surface, and the distance between the two images is equal to the radius of the surface ( Fig. 18.8 ).

■
A radiuscope essentially consists of a microscope with a linear scale (calibrated to 0.01 mm), which reads the position of the microscope body or microscope stage.
■
Light from an illuminated target (consisting of a ring of dots or radial lines) attached to the microscope is imaged by the microscope objective after being reflected through a right angle by a semi-transparent mirror.
■
The radius is determined by shifting the contact lens or the mire coupled with the microscope objective from one focus to the next.

The radiuscope can be used to measure the concave and convex surfaces of both rigid and soft lenses.

Fig. 18.8, Diagram to show Drysdale's principle. (1) First position of lens, image focused on lens surface; (2) second position of lens, image is now at centre of curvature of surface. r = lens radius. T = illuminated target, T 2 = image of T, T 3 = image of T 2 at the eyepiece.

Examples of radiuscopes are shown in Fig. 18.9 . The CG Auto Microspherometer MS/T by Neitz Instruments ( Fig. 18.9b ) eliminates the operator's subjective judgement by automatically focusing the mires using a microgrid. A number of horizontal and vertical points are focused and analysed by a computer ( ).

Fig. 18.9, (a) Nidek Radiusgauge RG-200.

Rigid Contact Lenses

The BOZR of a rigid lens is determined as follows:

■
A concave lens holder on the microscope stage is filled with saline or sterile water.
■
The lens is centred on the holder convex side down; this prevents distortion of the contact lens and eliminates reflections from the front surface.
■
The microscope eyepiece is then focused on the target on the surface of the lens by moving either the microscope and target or the microscope stage.
■
The measurement is recorded or the gauge set to zero.
■
A second focus, at the centre of curvature of the surface, is obtained by racking the microscope stage up or down.
■
The difference between the two dial gauge readings gives the radius of curvature of the surface.

To measure back peripheral radii, the holder must be tilted and the microscope stage moved across so that the target is focused in the peripheral band of the contact lens. Provided that the band is 1 mm wide and incident light is still normal to the surface being measured, it is possible to measure its radius. Where the peripheral curve is too narrow, the image at the centre of curvature is similar to that of a toroidal surface. A line image, formed by the surface along the circumference of the band, is closest to the correct radius and can be focused parallel to the direction of the tilt.

described a method to measure axial edge lift of rigid lenses:

■
The lens is placed on the radiuscope holder in the usual manner, and a microscope cover slide is placed on the top.
■
Central readings are taken successively on the underside of the cover slide and then on the back surface of the lens.
■
The difference between these readings gives the lens primary sag, i.e. from the sag formula , where r = radius of curvature, s = sagitta at a semi-aperture y .
■
The sag b , corresponding to the BOZR of the lens under test at a diameter corresponding to the TD of the lens, is found. The axial edge lift e = b − s .

also described a method to determine radial edge lift using a radiuscope.

investigated two methods of clinically checking and verifying the aspherical back surfaces of rigid contact lenses. In the first method, a radiuscope was modified with a tilting table. By measuring radii at off-axis points, the eccentricity was calculated for each aspherical surface tested. The conformity of the surface to a true conicoid could then be evaluated.

published correction factors for aspheric contact lenses to be subtracted from the radiuscope reading for various lens designs and diameters.

found no differences between the uncorrected BOZR measurements of polymethylmethacrylate (PMMA) lenses with a keratometer and those with radiuscope.

Soft Contact Lenses

There are different methods available for measuring the BOZR of soft contact lenses with a radiuscope. The method used for rigid lenses is not usually effective, as capillary attraction between the lens and the holder distorts the lens.

Soft lenses are usually measured in a wet cell, designed so that there is no air layer between the liquid and the cover. The radius of the lens measured in air is calculated from the radius of the lens measured in liquid, taking into account the RI of the liquid in the wet cell:

r = r^{'} \times n,

$r = r^{'} \times n,$

where r = radius of curvature in air, r ′ = radius of curvature in liquid and n = RI of saline solution (1.336).

The reflected mires produce a poor image in the wet cell ( ).

Spherometers (mechanical and electronic)

These determine the radii of spherical surfaces using a mechanical or electronic spherometer and then calculate the radius from the sagittal depth.

The contact lens is placed on a support ring with a fixed chord diameter, and the thumbscrew is turned until contact is made between the spherometer stylus and the contact lens apex.

In an electronic device, the vertical stylus is moved by a small motor, which produces electrical contact, and the radius is measured digitally directly from the scale.

With either type, to maintain the shape and avoid dehydration, the lens is placed in a wet cell with normal saline solution and illuminated by an external light source.

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