EN ISO 18369-3:2017

SLOVENSKI STANDARD
01-november-2017
1DGRPHãþD
SIST EN ISO 18369-3:2006
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Ophthalmic optics - Contact lenses - Part 3: Measurement methods (ISO 18369-3:2017)
Augenoptik - Kontaktlinsen - Teil 3: Messverfahren (ISO 18369-3:2017)
Optique ophtalmique - Lentilles de contact - Partie 3: Méthodes de mesure (ISO 18369-
3:2017)
Ta slovenski standard je istoveten z: EN ISO 18369-3:2017
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18369-3
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2017
EUROPÄISCHE NORM
ICS 11.040.70 Supersedes EN ISO 18369-3:2006
English Version
Ophthalmic optics - Contact lenses - Part 3: Measurement
methods (ISO 18369-3:2017)
Optique ophtalmique - Lentilles de contact - Partie 3: Augenoptik - Kontaktlinsen - Teil 3: Messverfahren
Méthodes de mesure (ISO 18369-3:2017) (ISO 18369-3:2017)
This European Standard was approved by CEN on 1 July 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18369-3:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 18369-3:2017) has been prepared by Technical Committee ISO/TC 172 “Optics
and photonics” in collaboration with Technical Committee CEN/TC 170 “Ophthalmic optics” the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by March 2018, and conflicting national standards shall
be withdrawn at the latest by March 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 18369-3:2006.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18369-3:2017 has been approved by CEN as EN ISO 18369-3:2017 without any
modification.
INTERNATIONAL ISO
STANDARD 18369-3
Second edition
2017-08
Ophthalmic optics — Contact lenses —
Part 3:
Measurement methods
Optique ophtalmique — Lentilles de contact —
Partie 3: Méthodes de mesure
Reference number
ISO 18369-3:2017(E)
©
ISO 2017
ISO 18369-3:2017(E)
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
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copyright@iso.org
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ii © ISO 2017 – All rights reserved

ISO 18369-3:2017(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Methods of measurement for contact lenses . 1
4.1 General . 1
4.2 Radius of curvature . 2
4.2.1 General. 2
4.2.2 Optical spherometry (rigid contact lenses) . 3
4.2.3 Sagittal height method . 6
4.3 Label back vertex power .11
4.3.1 General.11
4.3.2 Focimeter specification .11
4.3.3 Calibration .12
4.3.4 Focimeter measurement of rigid lenses .13
4.3.5 Focimeter measurement of hydrogel lenses .13
4.3.6 Measurement of hydrogel contact lenses by immersion in saline .13
4.3.7 Addition power measurement .14
4.4 Diameters and widths .14
4.4.1 Total diameter .14
4.4.2 Zone diameters and widths .19
4.5 Thickness .20
4.5.1 General.20
4.5.2 Dial gauge method .20
4.5.3 Low-force mechanical gauge method .21
4.6 Edge inspection .22
4.7 Determination of inclusions and surface imperfections .22
4.8 Spectral transmittance .23
4.8.1 General.23
4.8.2 Instrument specification, test conditions and procedure .23
4.9 Saline solution for testing .24
4.9.1 General.24
4.9.2 Formulation .24
4.9.3 Preparation procedure .25
4.9.4 Packaging and labelling .25
5 Test report .26
Annex A (informative) Measurement of rigid contact lens curvature using interferometry .27
Annex B (informative) Measurement of label back vertex power of soft contact lenses
immersed in saline using the Moiré deflectometer or Hartmann methods .29
Annex C (informative) Measurement of the radius of curvature of contact lenses using
the ophthalmometer .33
Annex D (informative) Paddle support for focimeters used for power measurements of
contact lenses .38
Bibliography .40
ISO 18369-3:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 7, Ophthalmic optics and instruments.
This second edition cancels and replaces the first edition (ISO 18369-3:2006), which has been
technically revised.
A list of all parts in the ISO 18369 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

INTERNATIONAL STANDARD ISO 18369-3:2017(E)
Ophthalmic optics — Contact lenses —
Part 3:
Measurement methods
1 Scope
This document specifies the methods for measuring the physical and optical properties of contact
lenses specified in ISO 18369-2, i.e. radius of curvature, label back vertex power, diameter, thickness,
inspection of edges, inclusions and surface imperfections and determination of spectral transmittance.
This document also specifies the equilibrating solution and standard saline solution for testing of
contact lenses.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3696:1987, Water for analytical laboratory use — Specification and test methods
ISO 9342-1, Optics and optical instruments — Test lenses for calibration of focimeters — Part 1: Test lenses
for focimeters used for measuring spectacle lenses
ISO 18369-1:2017, Ophthalmic optics — Contact lenses — Part 1: Vocabulary, classification system and
recommendations for labelling specifications
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18369-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
4 Methods of measurement for contact lenses
4.1 General
Clause 4 specifies methods for measuring finished contact lens parameters.
Clause 4 is applicable to testing laboratories, suppliers and users of contact lens products or services, in
which measurement results are used to demonstrate compliance to specified requirements.
Alternative test methods and equipment may be used provided the accuracy and precision are
equivalent to or more capable than the test methods described.
Each method should be capable of measurement with a precision [repeatability and
[8]
reproducibility (R&R)] of ≤30 % of the allowed tolerance range .
ISO 18369-3:2017(E)
Lenses should be equilibrated by soaking in standard saline or packaging solution for sufficient time
that the parameter to be measured remain constant within the ability of the method to measure the
parameter.
NOTE The process might be influenced by the nature of the lens material, volume of the solution used for
equilibration and the nature of the solution used to hydrate the lens (if any).
The nature of the equilibration solution (i.e. standard saline solution or packaging solution) and the
equilibration process should be identified in the test report.
Many methods require use of specific temperature ranges and this should be considered when
equilibrating the lenses for testing.
4.2 Radius of curvature
4.2.1 General
There are two generally accepted instruments for determining the radius of curvature of rigid contact
lens surfaces. These are the optical microspherometer (see 4.2.2) and the ophthalmometer with contact
lens attachment.
The ophthalmometer method measures the reflected image size of a target placed at a known distance
in front of a rigid or soft lens surface, and the relationship between curvature and magnification of the
reflected image is then used to determine the back optic zone radius (see Annex C).
For hydrogel contact lenses, sagittal depth can be measured using ultrasonic, mechanical and optical
methods that are available and are applicable to hydrogel contact lens surfaces as indicated in 4.2.3 and
Table 1. Sagittal depth can also be used to determine equivalent radius of curvature.
The sagittal methods are generally not recommended instead of radius measurement for rigid spherical
surfaces because aberration, toricity and other errors are masked during sagitta measurement. Sagittal
depth of rigid aspheric surfaces can be useful.
In addition to these measurement methods, a method using interferometry and applicable to rigid
contact lenses is given in Annex A for information.
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ISO 18369-3:2017(E)
Table 1 — Reproducibility values for different test methods
a
Refer to Test method/application Reproducibility, R
4.2.2 Optical spherometry
Spherical rigid lenses ±0,015 mm in air
Annex C Ophthalmometry
Spherical rigid lenses ±0,015 mm in air
Spherical rigid lenses ±0,025 mm in saline solution
Spherical hydrogel lenses
(38 % water content, t > 0,1 mm) ±0,050 mm in saline solution
c
4.2.3 Sagittal height method
b
Hydrogel contact lenses
38 % water content, t > 0,1 mm ±0,05 mm in saline solution
c
55 % water content, t > 0,1 mm ±0,10 mm in saline solution
c
c
70 % water content, t > 0,1 mm ±0,20 mm in saline solution
c
NOTE This table provides reproducibility for spherical rigid lenses because this type of lens was included in the ring test
carried out. However, in general, the values equally apply to aspheric and toric rigid lenses.
a
R is the reproducibility as defined in ISO 18369-1:2017, 3.1.12.9.3.
b
The three water contents given in this table were the ones used to conduct the ring test. For other water content lenses,
extrapolation can be used.
c
The reproducibility is equal to the tolerance and, therefore, the sagittal height method is not relevant for water
contents of 70 % and above.
4.2.2 Optical spherometry (rigid contact lenses)
4.2.2.1 Principle
The microspherometer locates the surface vertex and the aerial image (centre of curvature) with
the Drysdale principle, as described below. The distance between these two points is the radius of
curvature for a spherical surface and is known as the apical radius of curvature for an aspheric surface
derived from a conic section. The microspherometer can be used to measure radii of the two primary
meridians of a rigid toric surface and with a special tilting attachment, eccentric radii can be measured
as found in the toric periphery of a rigid aspheric surface. When the posterior surface is measured, the
back optic zone radius is that which is verified.
The optical microspherometer consists essentially of a microscope fitted with a vertical illuminator. See
Figure 1. Light from the target, T, is reflected down the microscope tube by the semi-silvered mirror, M,
and passes through the microscope objective to form an image of the target at T′. If the focus coincides
with the lens surface, then light is reflected back along the diametrically opposite path to form images
at T and T′′. The image at T′′coincides with the first principle focus of the eyepiece when a sharp image
is seen by the observer [Figure 1 a)]. This is referred to as the “surface image”.
The distance between the microscope and the lens surface is increased by either raising the microscope
or lowering the lens on the microscope stage until the image (T′) formed by the objective coincides
with C (the centre of curvature of the surface). Light from the target T strikes the lens’ surface normally
and is reflected back along its own path to form images at T and T′ as before [Figure 1 b)]. A sharp
image of the target is again seen by the observer. This is referred to as the “aerial image”. The distance
through which the microscope or stage has been moved is equal to the radius (r) of curvature of the
surface. The distance of travel is measured with an analogue or digital distance gauge incorporated in
the instrument.
In the case of a toric test surface, there is a radius of curvature determined in each of two primary
meridians aligned with lines within the illuminated microspherometer target.
ISO 18369-3:2017(E)
It is also possible to measure the front surface radius of curvature by orienting the lens such that its
front surface is presented to the microscope. In this instance, the aerial image is below the lens, such
that the microscope focus at T′ need be moved down from its initial position at the front surface vertex
in order to make T′ coincide with C.
Key
C centre of curvature of the surface to be measured
T target
T′ image of T at a self-conjugate point
T′′ image of T′ located at the first principal focus of the eyepiece, TM = MT′′
M semi-silvered mirror
r radius of curvature of the surface
Figure 1 — Optical system of a microspherometer
4.2.2.2 Instrument specification
The optical microspherometer shall have an optical microscope fitted with a vertical illuminator and
a target and have a fine focus adjustment. The adjustment control shall allow fine movement of the
microscope or of its stage. The adjustment gauge shall have a linear scale.
The objective lens shall have a minimum magnification of ×6,5 with a numerical aperture of not less
than 0,25. The total magnification shall not be less than ×30. The real image of the target formed by the
microscope shall not be greater than 1,2 mm in diameter.
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ISO 18369-3:2017(E)
The scale interval for the gauge shall not be more than 0,02 mm. The accuracy of the gauge shall be
±0,010 mm for readings for 2,00 mm or more at a temperature of 20 °C to 25 °C. The repeatability of the
gauge (see Note 1 and Note 2) shall be ±0,003 mm.
The gauge mechanism should incorporate some means for eliminating backlash (retrace). If readings
are taken in one direction, this source of error need not be considered.
The illuminated target is typically composed of four lines intersecting radially at the centre, separated
from each other by 45°.
The microspherometer shall include a contact lens holder that is capable of holding the contact lens
surface in a reference plane that is normal to the optic axis of the instrument. The holder shall be
adjustable laterally, such that the vertex of the contact lens surface may be centred with respect to the
axis. The contact lens holder shall allow neutralization of unwanted reflections from the contact lens
surface not being measured.
NOTE 1 The term “gauge” refers to both analogue and digital gauges.
NOTE 2 “Repeatability” means the closeness of agreement between mutually independent test results
obtained under the same conditions.
4.2.2.3 Calibration
Calibration (determining the measuring accuracy) shall be carried out using at least three concave
spherical radius test plates over the range to be tested.
EXAMPLE Three concave spherical radius test plates made from crown glass:
—  Plate 1: 6,30 mm to 6,70 mm;
—  Plate 2: 7,80 mm to 8,20 mm;
—  Plate 3: 9,30 mm to 9,70 mm.
The test plates have radii accurately known to ±0,007 5 mm.
Calibration shall take place at a temperature of 20 °C to 25 °C and after the instrument has had sufficient
time to stabilize.
Mount the first test plate so that the optical axis of the microscope is normal to the test surface. Adjust
the separation of the microscope and stage so that the image of the target is focused on the surface
and a clear image of the target is seen through the microscope. Set the gauge to read zero. Increase the
separation between the microscope and the stage until a second clear image of the target is seen in the
microscope. The microscope and surface now occupy the position seen in Figure 1 b).
Both images shall have appeared in the centre of the field of view. If this does not occur, move the test
surface laterally and/or tilted until this does occur. Record the distance shown on the gauge when the
second image is in focus as the radius of curvature.
Take at least 10 independent measurements (see Note) and calculate the arithmetic mean for each set.
Repeat this procedure for the other two test plates. Plot the results on a calibration curve and use this
to correct the results obtained in 4.2.2.4.
NOTE The term “independent” means that the test plate or lens is to be removed from the instrument, the
instrument zeroed and item remounted between each reading.
4.2.2.4 Measurement method
Carry out the measurements on the test lens in air at 20 °C to 25 °C.
Mount the lens so that the optical axis of the microscope is normal to that part of the lens surface of
which the radius is to be measured. Three independent measurements shall be made. Correct the
ISO 18369-3:2017(E)
arithmetic mean of this set of measurements using the calibration curve obtained in 4.2.2.3 and record
the result to the nearest 0,01 mm.
In the case of a toric surface, the contact lens shall not only be centred, but also rotated such that the two
primary meridians are parallel to lines of the target within the microspherometer. The measurement
procedure described shall be carried out for each of the two primary meridians.
In the case of an aspheric surface, where the apical radius of curvature shall be measured, the procedure
is the same as for a spherical surface with the exception that placement of the surface vertex at the
focus of the microscope has to be more precise. At this point, there shall be no toricity noticeable in the
aerial image.
NOTE 1 The equivalent spherical radius of curvature of an aspheric surface can be determined by
measurement of the sagittal depth (s) of the surface over the optic zone (y) using the methods employed in 4.2.3.
The sagittal depth is converted to an equivalent spherical radius using Formula (1):
sy
r=+ (1)
28s
where s is the sagittal depth, in millimetres, and y is the chord distance, in millimetres.
NOTE 2 This method is independent of eccentricity (e) and can be used to verify those equivalent radii
calculated using eccentricity values. In addition, this method of determining the equivalent radius is applicable
to aspheric surfaces that are not based on conic sections.
NOTE 3 Eccentricity of a conoidal aspheric surface can be computed from the sagittal and apical radii
of curvature measured at chord diameters ( y) away from the apex of the surface. Although the apex of these
surfaces appears spherical when centred in the microspherometer, the surfaces become progressively toric as
the point of measurement is brought away from the apex (as the chord diameter, y, is increased). As there is
a known relationship between apical radius, eccentricity, chord diameter and sagittal radius for any conoidal
surface, eccentricity and its consistency over the surface can be evaluated.
4.2.3 Sagittal height method
4.2.3.1 Principle
Sagittal depth is the distance from the vertex of the contact lens surface to a chord drawn across the
surface at a known diameter. For the determination of the sagittal depth of the back optic zone, the
contact lens is positioned concave side down against a circular contact lens support of fixed outside
(chord) diameter (see Figure 2).
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ISO 18369-3:2017(E)
a) b) c)
Key
r radius of curvature of lens
s sagittal depth
y outside (chord) diameter of lens support
D total diameter
Figure 2 — Measurement of sagittal depth of a soft contact lens
A soft contact lens shall be equilibrated in standard saline solution (see 4.9) before measurement. The
equivalent posterior radius of curvature can also be determined using sagittal depth measurement.
The following three types of method may be used for posterior sagittal depth measurement of soft lenses.
a) Optical comparator
The vertical distance between the back vertex of the lens and the chord is measured visually under
magnification. It can be difficult to accurately detect the back vertex of the contact lens using an
optical comparator. An alternative method to measuring the posterior sagitta is to measure the
total sagitta of the contact lens and subtract the centre thickness.
b) Mechanical or optical sensor
This method introduces a central vertical probe that is extended so that it just touches the back
surface vertex, its length from the chord equals the sagittal depth [see Figure 2 b) and Figure 3].
An optical sensor can also be used to measure the distance from the lens support plane to the lens
back surface vertex.
c) Ultrasound
Sagitta can also be ultrasonically assessed by measuring the time of travel through standard saline
of an ultrasonic pulse from an ultrasonic transducer to the back vertex and by reflection back to
the transducer. The resultant measured sagittal depth is, therefore, half of the distance calculated
by multiplication of the time by the velocity of sound in saline at the temperature involved and then
subtraction of the vertical height from the transducer to the top of the lens support.
Radius of curvature for a spherical surface (e = 0), or apical radius of curvature for a conicoidal surface
with specified eccentricity (e > 0), can be calculated from the sagittal depth using the appropriate
formula (see Table 2).
ISO 18369-3:2017(E)
Table 2 — Summary of radius of curvature formulae in terms of sagittal depth (S),
eccentricity (e), chord diameter (y) and lens total diameter (D)
Sphere
Sy
Figure 2 a)
r =+
28S
Ellipsoid
pS + y /4
()
where the shape factor p = 1 − e
r =
a
2S
Sphere (EPC method)
SD
Figure 2 c)
r =+
28S
4.2.3.2 Instrument specification
4.2.3.2.1 The optical comparator. This shall have a minimum magnification of 10× and shall have
incorporated a soft lens wet cell with a lens support appropriate to the radius being measured. For back
optic zone radius, a hollow cylindrical contact lens support sized for the back optic zone radius should
be used. In the measurement of equivalent posterior radius of curvature, a flat stage sized to allow slight
overhang of the contact lens is optimal.
In order to measure total posterior sagitta, the contact lens shall rest horizontally with its concave
(posterior) surface against the circular outside edge of the flat-rimmed support. The cylindrical support
shall be constructed in such a way as to provide a chord diameter (y) appropriate to the posterior
surface lens design when a soft lens is centred on the support. The flat stage support shall be sized to
allow the contact lens to overhang approximately 0,100 mm when centred on the stage. This overhang
will allow more accurate measurement of lens total diameter.
4.2.3.2.2 Mechanical analyser. The instrument shall allow the contact lens, lens support and probe
to be focused together. It shall allow the operator to see that the contact lens is centred on the support
so that the probe approaches along the lens axis and, finally, just touches the back vertex of the lens
(see Figure 3 and Figure 4). This is the end point required to obtain a measurement value. The distance
travelled by a solid mechanical probe from the plane of the lens support to the lens back surface vertex is
the sagittal depth (S). An optical sensor can also be used to measure the distance from the lens support
plane to the lens back surface vertex.
A reticule or digital readout should display minimal increments of ≤10 % of the sagittal tolerance and
should be capable of measuring sagittal depth with a precision (R&R) of ≤30 % of the allowed tolerance.
Resolution greater than 10 % can be used but will affect determination of accuracy, precision, process
capability and gauge capability.
The temperature of the wet cell and contact lens shall be maintained at 20 °C ± 1,0 °C.
8 © ISO 2017 – All rights reserved

ISO 18369-3:2017(E)
Key
1 illumination system
2 wet cell with test sample
3 imaging lens
4 projection screen
Figure 3 — Principle of the mechanical analyser
Key
s sagittal depth
1 contact lens
2 lens support
3 probe
Figure 4 — Detail of the mechanical analyser showing the lens support and probe
4.2.3.2.3 Ultrasound method. In the case of ultrasonic measurement of sagittal depth, the
requirements for the wet cell and support are shown in Figure 5. An ultrasonic transducer shall be fitted
ISO 18369-3:2017(E)
under the centre of the contact lens support. It should have a frequency greater than 18 MHz, beam width
of 2,0 mm or less at focus and a focal length of 15 mm to 50 mm.
The temperature of the wet cell and contact lens shall be maintained at 20 °C ± 0,5 °C because ultrasound
method is temperature dependent as well as material dependent.
Key
1 contact lens
2 container
3 saline solution
4 transducer
5 ancillary equipment
Figure 5 — Ultrasonic measurement of sagittal depth in a wet cell
4.2.3.3 Calibration
Instrument calibration shall be carried out to assure measuring accuracy to known standards and
multiple instrument equivalence using three height test plates. The test plates shall be chosen so as to
determine the measuring accuracy over the desired range of contact lens sagittal heights. The actual
heights shall be known to within 0,002 mm.
Calibration can also be carried out using rigid single curve test pieces of known accuracy.
EXAMPLE Three concave spherical radius test plates made from crown glass:
—  Plate 1: 6,30 mm to 6,70 mm;
—  Plate 2: 7,80 mm to 8,20 mm;
—  Plate 3: 9,30 mm to 9,70 mm.
The test plates have radii accurately known to ±0,007 5 mm.
Calibration shall take place in a wet cell with a temperature of 20 °C ± 1,0 °C after the instrument has
had sufficient time to stabilize and after the calibration pieces have been equilibrated in standard
saline solution within the wet cell.
Each test plate shall be measured from the same direction three times and the arithmetic mean
calculated. Differences between calculated and actual radius shall be used to construct a correction
calibration curve.
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ISO 18369-3:2017(E)
4.2.3.4 Measurement method
The lens shall be allowed to equilibrate in standard saline (see 4.9).
The temperature of the lens and surrounding saline after stabilization shall be 20 °C ± 1,0 °C except for
the ultrasound method which shall be 20 °C ± 0,5 °C.
Allow the contact lens to float down by gravity over the contact lens support in saline solution, taking
care to centre the lens over the support. When measuring the sagittal depth of aspheric surfaces,
particular attention should be paid to accurate centration of the lens on the support.
The measurement shall be recorded to the nearest 0,01 mm and shall be converted to radius of
curvature or apical radius of curvature using the formulae shown in Table 2.
A minimum of three independent measurements are used to obtain an arithmetic mean value.
4.3 Label back vertex power
4.3.1 General
Label back vertex power of a single power contact lens is the dioptric power as measured with a focimeter
as specified in 4.3.2, calibrated as specified in 4.3.3 using the measurement method given in 4.3.4 or
4.3.5. Proper design of the contact lens stop, calibration and measurement apertures and calibration
standards are all necessary to properly measure the label back vertex power of contact lenses.
Label back vertex power can also be determined by immersing the lens in saline solution. Examples of
techniques that can be employed are given in Annex B.
4.3.2 Focimeter specification
4.3.2.1 Focimeter, having a minimum range of −20,00 D to +20,00 D with a minimum measuring
accuracy of ±0,06 D, and capable of manual focusing. Other focimeters may be used provided the readings
derived are shown to be equivalent to those of a manually focusing focimeter. A focimeter conforming to
ISO 8598-1 can be used.
4.3.2.2 Lens supports allowing centration of the contact lens optic zone around the optical axis of the
focimeter.
Two interchangeable lens supports are used. These are
1) a support that allows calibration with spherical aberration-free (spectacle-type) lenses in the
vertex plane of the focimeter, and
2) an alternative smaller and shorter support of 4,0 mm to 5,0 mm in diameter so that the back vertex
of a contact lens can also be positioned in the vertex plane of the focimeter. See the example in
Figure 6. This will provide the appropriate vertex power by compensating for the sagittal depth
change due to a back optic zone radius of 8,00 mm in a contact lens (see Figure 7 for illustration).
Contact lenses with a back optic zone radius value substantially different from this can require
further distance correction. An example of paddle support is shown in Annex D.
NOTE Although the contact lens stop will reduce sagittal errors, it will have little effect on reducing spherical
aberration.
ISO 18369-3:2017(E)
Dimensions in millimetres
h − h = 0,55 mm
s c
Key
h height of contact lens support
c
h height of spectacle lens support
s
r radius of contact lens support
c
1 contact lens support (4,0 mm to 5,0 mm in diameter)
2 spectacle lens support
Figure 6 — Example of focimeter stops for calibration and power measurement
Key
s sagittal depth of back central optic zone
y/2 semichord length (chord diameter = y)
r back optic zone radius (base curve radius)
r radius of contact lens stop
c
Figure 7 — Contact lens resting on a contact lens stop
4.3.3 Calibration
Instrument calibration shall be carried out to assure measuring accuracy to known standards using
spherical test lenses with minimal spherical aberration as specified in ISO 9342-1. The label back vertex
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ISO 18369-3:2017(E)
powers of the calibration lenses shall be spaced so as to determine the measuring accuracy over a
broad range of label back vertex powers. Minimum requirements for this purpose include four plus
lenses and four minus lenses to cover the power range of the focimeter, e.g. −5,00 D; −10,00 D; −15,00 D;
−20,00 D; +5,00 D; +10,00 D; +15,00 D; +20,00 D. The actual label back vertex power of these lenses shall
be traceable to a national or International Standard and known to within 0,03 D.
At a temperature of 20 °C to 25 °C and using the spherical test lenses, calibrate the focimeter fitted with
the appropriate spectacle lens stop.
Place each test lens centrally with its back surface against the appropriate lens stop and focus the
focimeter to obtain the clearest possible image. Record the focimeter reading to the nearest 0,06 D or
less. Take three independent readings and record the arithmetic mean. Differences between calculated
and actual label back vertex power shall be used to construct a correction calibration curve, if the
results fail to meet the calibration accuracy criteria.
NOTE The term “independent” means that the test lens is removed from the instrument and remounted
between each reading.
4.3.4 Focimeter measurement of rigid lenses
Before making the measurement, rigid lenses shall be maintained at a temperature of 20 °C to 25 °C
for at least 30 min. During the measurement, maintain the focimeter and contact lens support at a
temperature of 20 °C to 25 °C.
Place the contact lens with its posterior surface against the contact lens support to properly position
the back vertex as the reference point for measurement. It is important that the back vertex of the
contact lens is centred on the optical axis of the focimeter and lens surfaces be clean and free of debris
or solution. Take four independent readings of the label back vertex power. Calculate the arithmetic
mean and, using the calibration curve, determine the corrected arithmetic mean.
4.3.5 Focimeter measurement of hydrogel lenses
Equilibrate the hydrogel lenses. See ISO 18369-1:2017, 3.1.1.21.
Blot the lens with a lint-free absorbent cloth or filter paper to remove surface liquid, and place it upon
the contact lens support within 10 s. The process shall be the same as that in 4.3.4. Place the posterior
surface of the lens appropriately on the lens support.
Multiple independent readings may be taken to minimize measurement error. Calculate th
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