TECHNOLOGY Lens Inspection
Lens Mapper for Intraocular and
They provide qualitative means of evaluating
Contact Lenses
lens performance
Power, asphericity and modulation transfer function (MTF) are major parameters to be measured at the end of intraocular and contact lenses
manufacturing production. This is mandatory to guarantee a consistent product quality and to insure the patient will get the best visual results with
the implanted or worn lens. By Luc Joannes
Intraocular lens (IOL) and contact lens (CL) industries have
the power profiles and progressive channel data made available
quite different histories in term of quality control. The main
by the lens manufacturer to the optometrist, fitting a contact
reason is probably that CLs are considered as disposable and
lens on the other hand remains in most cases a “trial and
can be replaced at any time the vision is not satisfactory. The
error” or iterative process, potentially frustrating to both
surgical intervention necessary to implement the IOL does
patient and dispenser. The message of Dr Susan Bowers was
not provide this f lexibility for frequent, multiple lens
really a call to the contact lens manufacturers for support.
replacements. The metrology is thus more standardized and
applied to 100% of the production output.
It is thus clear that lens power mapping is valuable aid for both
IOL and CL industry, ophthalmologists and dispensers.
Historically, a heritage from the spectacle glass industry, CLs
have been and are still predominantly measured with lens meters
Lambda-X proposes a Phase-Shifting Schlieren (PSS)
which provide a power measurement as well as a qualitative and
technique embodied in its NIMO mapper, that supersedes
subjective evaluation of the lens quality. ISO 18369-3 standard
other mapping techniques in term of spatial resolution and
only mentions back vertex power measurements even though
easiness of positioning the lens under test, making the
it introduces deflectometry and Hartmann methods.
instrument an ideal tool for both R&D and production
operations. Moreover the high dynamic range of the NIMO
On the other hand, if IOLs are still sometimes measured with
allows to measure lenses in saline as well as dry using the
lens meters, the ISO 11979-2 standard imposes to quantify
same instrument.
optical quality of IOLs. More or less evolved MTF benches
or with wavefront and power analyzers are best used to test
In addition to the NIMO dedicated to spectacle lenses and
and ascertain IOL properties as lens meters cannot provide
IOLs, Lambda-X has launched a third NIMO especially
the type of data as listed in the ISO standard.
designed to measure both soft and RGP contact lenses.
Multifocal or progressive contact lenses are becoming
The instrument is based on a patented quantitative deflectometry
increasingly available in the market. These complex lenses
technique 1.By combining the Schlieren principle with a
provide a variable focal over the optical zone. Power variations
Phase-Shift ing method, the NIMO instrument is capable of
can be ‘continuous’ or discrete. Conventional lens meters,
measuring light beam deviations, which are used to calculate
providing single point measurements, are by design not
the power characteristics of optical lenses.
capable to provide data on the lens optical quality or
conformity to the intended lens design. CL manufacturers
A schematic layout of this instrument is shown in figure 1.
have no other solutions then to use power analyzers or must
A backlight source emits green light at 546nm. A liquid
rely on the accuracy of the manufacturing equipment.
crystal display LCD is placed at the focal length of lens L1.
Lenses L2 and L3 form an image on camera C through a
42
42_44_LambdaX.indd 42
The optometrists, on the other hand, have a only very limited
telecentric arrangement. The lens to be measured will be
information on multifocal and progressive lenses, making
placed in the object plane between lens systems L1 and L2.
it more difficult to fit those lenses to a their patients. During
The lenses to be measured are placed in the object plane of
a workshop organized by Lambda-X in Birmingham, to whom
the instrument, between lens systems L1 and L2, as shown
all of the UK contact lens manufacturers where invited, Susan
schematically in figure 2, identified as TL. When there is no
Bowers described very well the diffi culties optometrists
lens to measure, the camera is illuminated uniformly.
encounter fitting those lenses to patients (see GlobalCONTACT
When a test lens TL is placed at the object plane of the
3/2009). While fitting a spectacle lens is relatively easy using
instrument, it causes the light rays coming from the source
GlobalCONTACT 3-10
26.11.2010 12:22:08 Uhr
Lens Inspection TECHNOLOGY
1
© GlobalCONTACT 2010
2
© Gl
GlobalCONTACT
b lCONTACT 2010
3
5
4
Fig. 1: Schematic layout of NIMO based on the PSS technique, Fig. 2: Schematic representation of light ray travel in the Schlieren
method when a positive power contact lens is placed in the instrument, Fig. 3: Live image of a single vision IOL as produced by this
instrument, Fig. 4: A single vision spherical soft contact lens observed in a wet cell, Fig. 5: TR1504 for contact lenses
to deviate or deflect. The light intensity reaching the camera
sphere power is deduced from the defocus coefficient while
pixels is also modulated by the LCD pattern. So-called
cylinder and axis are deduced from the astigmatism terms.
schlieren fringes are generated by this; the higher the power
For IOLs, the Modulation Transfer Function is calculated
of the test lens, the greater the deviation of the light beams
from the wavefront. In addition, power maps with a very
and the higher the number of schlieren fringes.
high spatial resolution are calculated for each pixel within
the optic zone of the lens. The instrument soft ware also allows
In addition to the power data, the inclusion of a high resolution
wavefront analysis via Zernike polynomial decomposition
camera allows lens surface features such as orientation marks
to be made at any aperture diameter of the lens being
or engraving to be observed, as shown in fi gure 4
measured.
The measurement operation consists of applying the phase-
The instrument offers very good reproducibility and accuracy.
shift ing principle 2 to obtain the mapping of the light beam
As determined by a ring test according to ISO 5725, the
deviation at as many points as available on the camera. The
reproducibility standard deviation is better than 0.02D for
instrument resolution is therefore directly related to the
sphere power and 0.026D for cylinder power in the case of
number of pixels in the camera; this represents a significant
rigid contact lenses, and better than 0.05D for sphere power
increase in resolution by comparison to previous methods.
of spherical soft contact lenses 3. Similar performances have
been achieved on IOLs. Precise positioning of the lens in the
Both x and y components of the light beam deviation are
instrument is not required and does not impact the accuracy
measured which taken together fully characterize the lens
of the measurements, making the integration of this
power because they are the derivative of the wavefront on
instrument in automated production lines easy. A proprietary
each point on the lens. As with all power mapping devices,
soft ware plug-in allows for communication between the lens
the instrument then uses customized soft ware to calculate
map computer and a remote server via TCP/IP.
many different aspects of the power-related dimensions of
the lens. The paraxial power, sphere, cylinder and axis, is
We now give a few examples of measurements obtained with
calculated in any zone. To do this, the calculated wavefront
both lens mapping instruments for IOL (TR0815) and CL
is fitted to a Zernike polynomial combination; the paraxial
(TR1504).
GlobalCONTACT 3-10
42_44_LambdaX.indd 43
43
26.11.2010 12:22:08 Uhr
TECHNOLOGY Lens Inspection
8
10
6
7
9
Fig. 6: Automatic measurement of toric IOL with detection of optical (green) and mechanical axis (red), Fig. 7: Radial power map of a J&J
Acuvue bifocal (-5D, Add 2.5D) contact lens., Fig. 8: Radial power map of a J&J Acuvue bifocal (-5D, Add 2.5D) contact lens, Fig. 9: Radial
power map of a Ciba Vision progressive (+6D) contact lens, Fig. 10: Radial power map of of a Ciba Vision progressive (+6D) contact lens
Toric IOLs are a challenge to measure automatically. NIMO
To conclude, this is a useful instrument to assist IOL and CL
however will automatically detect the optical toric axis and
manufacturers as well as optometrists or lens dispenser. The
compare it with the axis of the engraving marks recognized
dedicated soft ware packages, including image processing and
with image processing-soft ware, an optional feature on the
TCP/IP protocols for interfacing, allow the measurement of
NIMO TR0815 (figure 6).
all type of lenses including toric ones to be automated. „
Figure 7 shows the power map of a J&J Acuvue bifocal (-5D,
Add 2.5D). It is a multifocal zonal contact lens. Figure 8 shows
the corresponding radial power profi le extracted from figure
7 and automatically calculated by the lens mapper. The high
resolution power map gives the detail of the power distribution
power map and the position of the near focus zones.
[1] Joannes L., et al, Phase-shifting schlieren : high-resolution quantitative schlieren that uses the
phase-shifting technique principle. Appl Opt 2003;42:5046-53
[2] Creath K., Phase-measurement techniques, in Progress in Optics, E. Wolf Eds, Amsterdam:
Elsevier, 1988:26,346-93
[3] Joannes L., et al, The reproducibility of a new power mapping instrument based on the phase
shifting schlieren method for the measurement of spherical and toric contact lenses, To appear
Figure 9 and 10 are repetitively radial power map and radial
in Contact Lens and Anterior Eye
power profi le for a Ciba Vision progressive contact lens (+6D).
Luc Joannes obtained his University PhD in
For both figure 7 and figure 9, the right hand side of the picture
systems for shape and deformation at University
(ring 0), and average powers in successive concentric annular
of Louvain (Belgium). In 1998, he joined the
University of Bruxelles (Belgium) and its spin-off
Lambda-X where he is now Technical Director.
progression is limited in the central 2mm diameter area. Th is
He is participating together with the technical
kind of information is primordial to help the optometrist to
team to the development of space and industrial
fit the contact lens.
42_44_LambdaX.indd 44
in 1994. Then he developed optical metrology
gives the average power in the central 1mm diameter aperture
zones with growing diameters by steps of 1mm.Power
44
integrated optics at the University of Lille (France)
optical solutions and metrology products for ophthalmic applications.
GlobalCONTACT 3-10
26.11.2010 12:22:14 Uhr