ISO/TS 6838:2024

Technical
Specification
ISO/TS 6838
First edition
Ophthalmic optics — Contact lenses
2024-07
— Tolerances and methods for
measurement of multifocal contact
lens addition power
Optique ophtalmique — Lentilles de contact — Tolérances et
méthodes de mesure de la puissance additionnelle des lentilles de
contact multifocales
Reference number
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ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Tolerances . 2
4.1 Tolerance limits .2
4.2 Conditioning of contact lenses prior to testing .2
4.3 Tolerances for rigid and soft contact lenses .3
5 Methods of measurement for multifocal add power. 3
5.1 General .3
5.2 Wavefront sensor .3
5.2.1 Wavefront instrument specification .3
5.2.2 Wavefront calibration .3
5.2.3 Wavefront method of measurement .4
5.2.4 Power profile examples .4
5.2.5 Bifocal contact lens .6
5.3 Focimeter .7
5.3.1 Principle .7
5.3.2 Focimeter specification .8
5.3.3 Focimeter calibration .9
5.3.4 Measurement of most plus and least plus power .9
6 Ring test results . 10
6.1 Ring test objectives .10
6.2 Ring test background.10
6.3 Ring test executive summary .11
6.3.1 WFS calibration verification using (13) certified low-power glass lenses .11
6.3.2 Focimeter calibration verification using (10) standard B+L calibration lenses . 12
6.3.3 WFS gauge repeatability and reproducibility (GR&R) using PV2 MF HA lenses . 12
6.3.4 Focimeter gauge repeatability and reproducibility (GR&R) using PV2 MF HA
lenses . 12
6.4 Calibration lens accuracy requirements. 12
6.4.1 Focimeters. 13
6.4.2 Wavefront sensor low-power lenses . 13
7 Discussion .15
7.1 Ring test conclusions . 15
7.2 Ring test recommendations . 15
7.3 Multifocal add-power measurement challenges. 15
7.3.1 Immersed wavefront sensor measurement challenges . 15
7.3.2 In-air wavefront sensor measurement challenges .18
7.4 Questions to answer before the TS moves to become a standard .18
Bibliography .20

iii
Foreword
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Ophthalmic optics and instruments.
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iv
Technical Specification ISO/TS 6838:2024(en)
Ophthalmic optics — Contact lenses — Tolerances and methods
for measurement of multifocal contact lens addition power
1 Scope
This document specifies proposals for the tolerances and methods for measuring the base and add power
of refractive/symmetric multifocal contact lenses. This document is not intended to measure current
production lenses (or similar) and does not include measurement of diffractive multifocal contact lenses nor
the measurement of distance (or label) power.
This document is intended to obtain additional feedback from clinicians, manufacturers, and health
authorities on the proposals for tolerances and test methods. This document is not intended to be used in
any quality system or by any governing body to control the manufacturing or acceptance 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 9342-1, Optics and optical instruments — Test lenses for calibration of focimeters — Part 1: Reference lenses
for focimeters used for measuring spectacle lenses
ISO 18369-3:2017, Ophthalmic optics — Contact lenses — Part 3: Measurement methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
power map
localized radial back vertex power (in-air) as a function of two- dimensional coordinates from the centre of
the lens
3.2
power profile
one-dimensional localized optical power as a function of radial distance from the centre of the lens as
derived from the power map, thereby creating the power-map equivalent of a power profile
3.3
average most plus power
area-weighted average power across an annular zone corresponding to the most plus region of a progressive
optical zone
3.4
average least plus power
base power
area-weighted average power across an annular zone corresponding to the least plus region of a progressive
optical zone
3.5
distance power
label power
power, determined clinically, to produce the best distance vision
Note 1 to entry: The distance power may not equal the measured base power. In this case, a regression will be required
to determine the distance power from the base power. Clinicians should contact the manufacturer’s Professional
Services for the regression(s).
3.6
measure add power
difference between the average most plus and average least plus power of the contact lens
Note 1 to entry: The average most plus and least plus powers are measured over optical zones provided by the
manufacturer.
3.7
label add power
the clinically determined add power of the multifocal contact lens
Note 1 to entry: The label add power may not equal the measured add power. A regression may be required to
determine the label add power from the measured add power.
3.8
localized radial back vertex power

1 ∂Wr(),θ
Pr =
()
r ∂r
where W(r, θ) is the measured wavefront
Note 1 to entry: The definition of associated terms, such as bifocal or progressive contact lens, can be found in
ISO 18369-1.
4 Tolerances
4.1 Tolerance limits
When tested as specified in ISO 18369-3, the dimensional and optical properties for multifocal contact
lenses addition power (or add power) shall be as specified within the appropriate tolerance lines given in
Table 1 and Table 2.
The proposed add power tolerance is based on a variety of sources, including the results of the ring testing
discussed in this specification, guidance found in the national standard ANSI Z80.20-2016, the combined
statistical nature of how add power is computed as a difference and the 0,25 D tolerance on spherical power,
and feedback from clinicians and manufacturers.
4.2 Conditioning of contact lenses prior to testing
Contact lenses shall be equilibrated in standard saline or packing solution, unless otherwise specified in the
relevant test methods specified in ISO 18369-3.

4.3 Tolerances for rigid and soft contact lenses
Optical tolerances for rigid and soft contact lenses are given in Table 1 and Table 2.
Table 1 — Optical tolerances for rigid contact lenses
Dimension Tolerance limit Relevant method
Multifocal add power ±0,38 D 5.2 or 5.3
Table 2 — Optical tolerances for soft contact lenses
Dimension Tolerance limit Relevant method
Multifocal add power ±0,38 D 5.2 or 5.3
5 Methods of measurement for multifocal add power
5.1 General
Prescribing multifocal contact lenses requires clinicians to consider the user’s pupil diameter, how the lens
conforms to the user’s cornea, how the lens centres on the cornea, the inherent aberrations of the user’s eye,
how the user processes the variable optical power across their pupil, excess visual function, and the unique
optical design of the multifocal lens as reflected in the manufacturers’ fitting guides. Accounting for these
design and manufacturing specificities and clinical practices, the exact labelled distance and add powers
may exist over different regions of the contact lens for different designs. A regression equation, provided
by the manufacturer, may be required to determine the label distance and label add power. The process of
determining these regressions will vary with manufacturers. Regressions created using a focimeter or a
wavefront sensor may be different.
5.2 Wavefront sensor
5.2.1 Wavefront instrument specification
The optical wavefront over the relevant optical zone shall be measured using an accurate and precise
wavefront sensor (e.g. Shack Hartmann, Moiré deflectometer, phase-shifting Schlieren deflectometer, or
other types of interferometers). The wavefront sensor (WFS) shall produce power profiles of the contact
lens under test. In addition, the wavefront sensor shall be capable of outputting the average optical power
over user-defined regions.
A key specification for any wavefront method is the converting of immersed effective power to in-air back
vertex power. As described in 7.3.1.3, there are three recognized methods for this conversion.
5.2.2 Wavefront calibration
5.2.2.1 Wavefront calibration in-air
Calibration should be completed with calibration test pieces, known as test standards, with known
nominal values. Calibration should occur with the test standards in-air. Minimum requirements for this
purpose include four plus lenses and four minus lenses to cover the power ranges of the contact lenses to be
measured. Consideration should be given to matching the range of test standards to the range of lenses and
the intended test condition (in-air or immersed). Test standards shall be placed in calibrated cuvettes, as
shown in Figure 1. Refer to 6.4.2 for additional information.
Calibration shall follow the steps outlined in ISO 18369-3:2017, Annex B, including following the
manufacturer’s instructions to calibrate the instrument.
NOTE Be careful to check with the manufacturer for the correct contact lens orientation for back vertex power
measurement.
It is recommended to plot the power deviation versus the nominal standard power to better understand the
shape (or form) of any bias.
5.2.2.2 Wavefront verification in-solution
5.2.3 Wavefront method of measurement
The following steps, given in Table 3, shall be followed to determine the base power and measured add
power of refractive multifocal contact lenses.
Table 3 — Steps to determine the base power and measured add power of refractive multifocal
contact lenses
Step Action
1 Measure the wavefront over the relevant optics zone using an accurate and precise
wavefront sensor (e.g. Shack Hartmann, Moiré deflectometer, phase-shifting Schlier-
en deflectometer, other types of interferometers).
2 The manufacturer specifies two or more annular regions that will be used to deter-
mine the
a)  average most plus power, and
b)  average least plus power of the multifocal contact lens.
Generally, the zones shall be as wide as possible to improve averaging but narrow
enough to cover only the region of interest. The manufacturer shall provide, via pro-
fessional services, the positions of the most plus and least plus annular zones.
3 Determine the average power over each region using the power profile or power map
data derived from the measured optical wavefront.
4 The add power is the absolute value of the difference between the average most plus
and average least plus powers. If necessary, a regression may be applied to correlate
the measured add power with the label add power. If required, the manufacturer
shall provide the regression formula, via Professional Services, to determine the
label add power.
NOTE 1 The measured add power is a metric derived the power profile; and the measured add power may be different from the
labeled add or the value listed in the fitting guide.
NOTE 2 Additional independent average powers may need to be taken to meet ISO 18369-3 guidance for precision.
5.2.4 Power profile examples
In the following examples, the location of the label power relative to the power profile does not equal the
average Most Plus Power. The shape of the power profile is a degree of freedom for the manufacturer.
5.2.4.1 Centre-near progressive
Figure 1 shows the power profile of a centre-near, progressive multifocal contact lens. In this case the most
and least plus zones are chosen to ensure the power does not change appreciably over each zone, but still
contains the profile region of interest.

Key
1 most plus
2 label power
3 least plus
Figure 1 — Example 1 power profile
Figure 2 shows a second example of a centre-near progressive contact lens.
Key
1 most plus
2 label power
3 least plus
Figure 2 — Example 2 power profile
5.2.4.2 Centre-distance progressive
Figure 3 shows the power profile of a centre-distance contact lens.

Figure 3 — Example 3 power profile
5.2.5 Bifocal contact lens
Figure 4 shows the measured power profiles for a centre-near, bifocal contact lens.
Key
1 most plus
2 label power
3 least plus
Figure 4 — Centre-near, bifocal contact lens
5.2.5.1 Multi-Zone, bifocal contact lens
Figures 5 and 6 show the power profiles for a multi-zone, bifocal contact lens. In Figure 5, single most-plus
and least-plus zones are defined, whereas in Figure 6, three most-plus and least-plus zones are defined. For
Figure 6, the least-plus power would be the average of the power in the three bottom zones. The most-plus
power would be the average of the three top zones.

Key
1 most plus
2 label power
3 least plus
Figure 5 — Multi-zone, bifocal power profile with two defined regions
Key
1 most plus 1
2 least plus 1
3 most plus 2
4 least plus 2
5 most plus 3
6 least plus 3
7 label power
Figure 6 — Multi-zone, bifocal power profile with six defined regions
5.3 Focimeter
5.3.1 Principle
A manually-focusing focimeter may be used to measure the most plus power and least plus power of a
multifocal contact lens via the use of lens supports with various aperture geometries. The determination of
such lens supports requires knowledge of the optical zones provided by the manufacturer. The manufacturer
shall provide, via professional services, the positions of the most plus and least plus annular zones. The
location and width of the focimeter aperture shall be chosen to correspond with the location of the most

plus and least plus powers for each lens design. Multiple apertures may be required to measure add power
across the desired power range.
5.3.2 Focimeter specification
5.3.2.1 Focimeter
The focimeter shall have 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.
5.3.2.2 Focimeter lens supports/apertures
Focimeter supports allow for centration of the contact lens optic zone around the optical axis of the focimeter,
correct placement of the contact lens back vertex relative to the measurement plane of the focimeter and
transmission of light through a defined aperture geometry.
The supports and fixtures detailed in Figures 7 and 8 show one design type for such lens supports. Other
support configurations can also be utilized (such as supports designed like ISO 18369-3:2017, 4.3.2.2).
Figure 7 shows three lens paddles with different aperture geometries. Figure 8 shows an adjustable paddle
fixture t
...