ISO/TR 28980:2007

TECHNICAL ISO/TR
REPORT 28980
First edition
2007-01-15

Ophthalmic optics — Spectacle lenses —
Parameters affecting lens power
measurement
Optique ophtalmique — Verres de lunettes — Paramètres affectant le
mesurage de la puissance de la lentille




Reference number
ISO/TR 28980:2007(E)
©
ISO 2007

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ISO/TR 28980:2007(E)
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ISO/TR 28980:2007(E)
Contents Page
Foreword. iv
1 Scope . 1
2 Discrepancies due to different focimeter designs and measurement methods . 1
2.1 Focimeter design . 1
2.2 Wavelength dependence. 3
2.3 Target pattern. 4
2.4 Front vertex versus back vertex measurements. 5
2.5 Choice of the focus. 7
3 Systematic errors. 8
3.1 Non-linear errors. 8
3.2 Lens sag error . 8
3.3 Temperature dependence . 9
3.4 Calibration . 9
3.5 Accuracy versus display step . 9
4 Random errors . 9
4.1 General. 9
4.2 Focusing issues: Theoretical limit. 9
4.3 Positioning issues . 11
5 Conclusion . 13
Bibliography . 15

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ISO/TR 28980:2007(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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.
ISO/TR 28980 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 7,
Ophthalmic optics and instruments.

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TECHNICAL REPORT ISO/TR 28980:2007(E)

Ophthalmic optics — Spectacle lenses — Parameters affecting
lens power measurement
1 Scope
The purpose of this Technical Report is to explain the changes relating to power measurements in the revised
[6] [7]
editions of ISO 8980-1 and ISO 8980-2 .
In order to illustrate the issues raised, an inter-laboratory power measurement study was conducted on ten
different lenses measured by nine organizations worldwide. Twenty-five focimeters of different types were
1)
used. The test lenses were spherically powered allyl diglycol carbonate (ADC) hard resin lenses surfaced to
–4,00 D, –2,00 D, 0,00 D, +2,00 D, and +4,00 D, each with an addition power of 2,50 D. Five were D28
bifocals and five were progressive power lenses. The measurements were front distance power, front near
power, back distance power and back near power, for each lens and for each focimeter used. Each lens was
measured nine times: five measurements taken without repositioning the lens and four measurements taken
with the lens repositioned each time.
The assessed parameters were divided into three categories:
⎯ discrepancies due to focimeter design and measurement methods;
⎯ systematic errors;
⎯ random errors.
For each parameter, experimental results are given, as well as theoretical ones when needed. Measurement
data include front distance portion power, front near portion power, back distance portion power and back
near portion power.
[8]
NOTE The results of all measurements are available on the ISOTC Server at the address given in the Bibliography .
Unless stated otherwise, in order to show relevant information, the results shown are for the D28 bifocals
when no different behaviour was found for the progressive power lenses.
2 Discrepancies due to different focimeter designs and measurement methods
2.1 Focimeter design
Traditional manual focimeters have relied on the user adjusting the instrument to obtain the clearest focus of
the test mire. All these instruments are based on the “Focus on Axis” (FOA) design, in which the focal point of
the focimeter remains on the axis of the focimeter when the lens under test is measured at a point of the lens
where the prism is not zero.
Some automated focimeters also use this optical design, but many use the “Infinity on Axis” (IOA) design, in
which the collimated beam coincides with the focimeter axis and the focal point of the focimeter goes off the

1) ADC is often referred to by its original trade name, CR-39®.
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ISO/TR 28980:2007(E)
axis of the focimeter when the lens under test is measured at a point of the lens where the prism is not zero
(see Figures 1 and 2). As automated focimeters can have either a FOA or IOA design, users should ask the
manufacturer what design is employed.

Key Key
1 focimeter's optical axis 1 focimeter's optical axis
2 lens 2 lens
3 focimeter lens support 3 focimeter lens support
4 focal point on the optical axis 4 infinite on the optical axis
Figure 1 — FOA focimeter Figure 2 — IOA focimeter
When there is no prismatic effect, the two designs will give the same results. On the other hand, the results
will differ if the prism is not zero, as shown by Coddington's equations for oblique astigmatism (for further
details, see textbooks on optics).
Figure 3 is a graph of the results from the inter-laboratory measurements study.
As the lenses used had moderate power and prism, the average FOA-IOA difference remains within the
power tolerance for focimeters. Nonetheless, even for these lenses, for near power the discrepancy reaches
nearly 2/3 of the lens power tolerance allowed by the ISO standards.
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ISO/TR 28980:2007(E)

Key
X nominal distance power, in dioptres
Y power difference, in dioptres
1 D28 – differences in distance power
2 D28 – differences in near power
Figure 3 — Graph of the inter-laboratory measurements showing the difference in power
between the FOA and the IOA focimeter design in the presence of prism
(average value of measurements of the back vertex power made at the sagittal focus, addition power 2,50 D)
2.2 Wavelength dependence
The refractive index of the lens material varies with the wavelength. This variation is given by the Abbe
number (for further information, see textbooks on optics).
The reference wavelength differs depending on the country. In some countries, the helium d-line (587,56 nm)
is used, while in others, it is the mercury e-line (546,07 nm).
Figure 4 shows the effect of the wavelength dependence on the lens power for two materials:
⎯ a material with an Abbe number, ν, of 57,8 and a nominal index, n, of 1,5 (n = 1,4977, n = 1,4996), and
d e
⎯ a material with an Abbe number, ν, of 30,0 and a nominal index, n, of 1,59 (n = 1,5855, n = 1,5899).
d e
NOTE n is the nominal index at the wavelength of the helium d-line; n is the nominal index at the wavelength of the
d e
mercury e-line.

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ISO/TR 28980:2007(E)

Key
X lens power, in dioptres
Y power difference, in dioptres
1 V = 57,8 and n = 1,5
2 V = 30,0 and n = 1,59
Figure 4 — Wavelength dependence: Calculated power difference between 546,07 nm
and 587,56 nm for two different materials

Hence, the theoretical power discrepancy between the helium d-line and the mercury e-line for a +4,00 D ADC
lens with a 2,50 addition would be approximately 0,03 D for a total near power of +6,50 D. It should be noted
that reference wavelength dependence is also important in the calibration of focimeters (see 3.4).
Because automated focimeters usually work with red light or infrared radiation, the instrument has a built-in
correction for the readings for low dispersion or high Abbe number materials such as ADC. Lenses, especially
high powered ones, made from higher index materials with lower Abbe numbers, require the instrument to be
adjusted/reset in order to correct for the different Abbe number.
2.3 Target pattern
2.3.1 Manual focimeters
For manual focimeters, the target is a graticule.
2.3.2 Automated focimeters
Automated focimeters have several approaches for the pattern their target uses to determine the focus.
Among the most common ones are the “4-rays target pattern” and the “annular pattern”. Note that an annular
pattern becomes elliptical when the lens under test is an astigmatic power lens (see Figure 5).
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ISO/TR 28980:2007(E)

Key

1 measured lens
2 screen
3 aperture with 4 holes
4 annular aperture

a) 4-rays target pattern b) Annular target pattern
Figure 5 — Examples of target patterns commonly used in automated focimeters
Figure 6 shows the results from the inter-laboratory studies comparing focimeters using the “4-rays target
pattern” and the “annular target pattern”.
It is easy to note that there is very little difference between the two patterns. This can be explained by the
accuracy and precision of the focimeters.
In these focimeters, the position of the focus is computed. Therefore, when measuring areas of progressive
power lenses with built-in aberrations (such as coma), different target patterns can yield significantly different
results, and in certain complex cases, some instruments cannot make a measurement and therefore produce
an error message.
2.4 Front vertex versus back vertex measurements
ISO 8980-1 and ISO 8980-2 specify for the addition determination that the measurement for near portion
power and distance portion power can be made either at the back or the front vertex. This distinction can lead
to major discrepancies in power measurements. Indeed, the more the lens differs from the symmetrical
biconcave or biconvex shape, the greater this difference will be.
Figure 7 shows the average differences between the back vertex power minus the front vertex power for the
distance and near portions of the lenses.
The differences result from the relative surface curvatures and thicknesses and can be significant, especially
for positive powered lenses. Consequently, identification of the measurement method is essential.
The distance power of a lens is that given by the back vertex measurement method, while for most multifocal
and progressive power lenses, the addition is given by the difference between the two front vertex powers. If
the addition power for such a lens is measured as the difference in back surface powers, the error for the
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