A new non-contact optical device for ocular biometry
J Santodomingo-Rubido, E A H Mallen, B Gilmartin, J S Wolffsohn
Br J Ophthalmol 2002;86:458–462
Background: A new commercially available device (IOLMaster, Zeiss Instruments) provides high reso-
lution non-contact measurements of axial length (using partial coherent interferometry), anterior cham-
ber depth, and corneal radius (using image analysis). The study evaluates the validity and repeatability
of these measurements and compares the findings with those obtained from instrumentation currently
used in clinical practice.
Method: Measurements were taken on 52 subjects (104 eyes) aged 18–40 years with a range of
mean spherical refractive error from +7.0 D to −9.50 D. IOLMaster measurements of anterior chamber
depth and axial length were compared with A-scan applanation ultrasonography (Storz Omega) and
those for corneal radius with a Javal-Schiötz keratometer (Topcon) and an EyeSys corneal videokerato-
Results: Axial length: the difference between IOLMaster and ultrasound measures was insignificant
(0.02 (SD 0.32) mm, p = 0.47) with no bias across the range sampled (22.40–27.99 mm). Anterior
chamber depth: significantly shorter depths than ultrasound were found with the IOLMaster (−0.06
(0.25) mm, p <0.02) with no bias across the range sampled (2.85–4.40 mm). Corneal radius:
IOLMaster measurements matched more closely those of the keratometer than those of the videokerato-
scope (mean difference −0.03 v −0.06 mm respectively), but were more variable (95% confidence
0.13 v 0.07 mm). The repeatability of all the above IOLMaster biometric measures was found to be of
a high order with no significant bias across the measurement ranges sampled.
Conclusions: The validity and repeatability of measurements provided by the IOLMaster will augment
future studies in ocular biometry.
distinct advantages over traditional ultrasound methods of
measurement. The IOLMaster does not require contact with
the eye and hence avoids risk of corneal abrasion, claims sig-
nificantly higher resolution measures of axial length com-
pared with ultrasound methods (plus or minus 0.01 mm ver-
sus plus or minus 0.15 mm), anterior chamber depth (plus or
minus 0.01 mm versus plus or minus 0.15 mm),and has addi-
tional facilities to measure corneal curvature (plus or minus
0.01 mm).1 2Whereas the principal application of the device
will be in cataract surgery (the IOLMaster has onboard
software menus for the computation of intraocular lens
power),the relatively high order of dioptric resolution for axial
length (∼plus or minus 0.03 D) is especially valuable in stud-
ies of myopia. With regard to the former, PCI has been shown
to be independent of cataract grade.3Concerning the latter, it
is well documented that an increase in posterior vitreous
chamber depth is the principal structural correlate of
myopia,4–7the IOLMaster is likely to figure significantly in
future myopia research. While previous laboratory studies
have shown good agreement between PCI and ultrasound
methodsofmeasurement,3 8 9themainpurposeofthispaperis
to compare the reliability and repeatability of data obtained
clinically with the IOLMaster with those using a standard
applanation ultrasound method of the type employed in many
previous studies on myopia. In addition, as the image analysis
techniques to measure anterior chamber depth and corneal
curvature used by the IOLMaster have not been previously
assessed,the accuracy and repeatability of these methods have
been examined in this study.
he advent of a new commercially available device which
uses partial coherent interferometry (PCI) to measure
axial length (IOLMaster, Carl Zeiss Jena GmbH) presents
MATERIALS AND METHODS
Fifty two subjects (32 male and 20 female), with a mean age
of 25.2 (SD 4.7) years (range 18–40 years, median 23.6 years)
participated in the study. Subjects with ocular pathologies,
abnormal binocular vision, and previous allergy to the topical
anaesthetic benoxinate hydrochloride were excluded from the
study. Measurements were performed on 104 eyes by two
qualified optometrists (JS-R and EAHM), after the purpose of
the study was explained and informed consent given. The
monitor of the instrument was used to align the right eye,fol-
lowed by the left, with the instrument’s internal fixation tar-
get. The eyes were in focus when the instrument head was
approximately 5.5 cm away from the subject’s eyes. Subjects
were asked to perform a complete blink just before measure-
ments were taken in order to spread an optically smooth tear
film over the cornea.
Corneal curvature was measured with the IOLMaster and
compared with measures from a videokeratoscope (EyeSys
Instruments, Houston, TX, USA) and a Javal-Schiötz kerato-
meter (Topcon, Capelle a/d IJssel, Netherlands). The IOLMas-
ter reflects six points of light, arranged in a 2.3 mm diameter
hexagonal pattern (measured by digital callipers), from the
air/tear film interface. The separation of opposite pairs of
lights is measured objectively by the instrument’s internal
software and the toroidal surface curvatures calculated from
three fixed meridians.10In comparison, the Javal-Schiötz
keratometer requires the user to align the keratometer mires
along the principal meridians and corneal curvature is
measured by subjective alignment of the mires, reflected from
the central 3.4 mm of the cornea. Videokeratography is an
image analysis technique for measuring corneal topography
with eight concentric rings of light, of known separation and
width,reflected from the air/tear film interface.The separation
of the 16 ring edges is measured objectively by the internal
image analysis software of the instrument, at 1 degree
intervals over 360 degrees, over a corneal diameter of 3 mm.11
In addition, an eccentricity value is calculated to indicate the
mean rate of corneal flattening using all rings, over a corneal
diameter of approximately 9.2 mm.12
See end of article for
Professor B Gilmartin,
School of Life and Health
Sciences, Optometry and
Vision Sciences, Aston
University, Aston Triangle,
Birmingham B4 7ET, UK
Accepted for publication
21 November 2001
Anterior chamber depth was measured with the IOLMaster
and compared with measures from an A-scan applanation
ultrasound device (Storz Omega Compu-Scan Biometric
Ruler,Storz International,St Louis,MO,USA).The IOLMaster
directs a 0.7 mm width slit beam of light through the anterior
segment of the eye at an angle of 38 degrees to the visual axis.
The instrument camera is aligned so that the light beam forms
an optical section and the internal software measures the dis-
tance between the anterior corneal pole and the anterior crys-
talline lens surface to calculate the anterior chamber depth.
The A-scan applanation device calculates anterior chamber
depth from the time taken for ultrasound waves to reflect back
to its receiver from an optical surface.13One drop of topical
anaesthetic, benoxinate hydrochloride 0.4% (Minims, Chau-
vin Pharmaceuticals Ltd),was instilled in each of the subject’s
eyes 2 minutes before ultrasound measurement. Special care
was taken in aligning the transducer beam probe along the
optical axis and to exert minimal corneal pressure. Ten meas-
urements were taken for each eye and the mean calculated.
Axial length was measured with the IOLMaster and
compared with measures from the A-scan applanation
ultrasound device. The IOLMaster measures optical axial
length by partial coherence interferometry, based on the
Michelson interferometer (Fig 1).9The laser diode (LD)
generates infrared light (λ = 780 µm) of short coherence
length (CL= 160 µm),which is reflected into the eye by mirrors
M1 and M2,after being split into two equal coaxial beams CB1
and CB2 by the beam splitter B1. The separation of the two
coaxial beams is twice the displacement d of the mirror M1.
Both coaxial beams enter the eye, where reflections take place
at the corneal (C) and retinal (R) interfaces. On leaving the
eye, the difference in frequency between the coaxial beams is
detected by a photodetector (PHD), after passing through a
second beam splitter (BS2). During measurement, the mirror
M1 is moved at constant speed, producing a Doppler modula-
tion in the frequency of the reflected coaxial light at the pho-
todetector. The displacement d of the mirror M1 can be
precisely determined and related to the reflected signals
detected at the photodetector, allowing accurate measure-
ments of the length AL between the cornea and the retina.
Measurements with this device are not affected by longitu-
dinal eye motion.3 9The calculation of axial length is depend-
ent on the refractive index of the medium in which the light
travels, and therefore the optical path length is divided by the
mean group refractive index (taken as n = 1.3549) in order to
obtain the geometrical axial length.9 14Laser light is reflected
from the retinal pigment epithelium, in contrast with
ultrasound waves which are reflected from the internal limit-
ing membrane.Hence,in order to make the IOLMaster results
comparable with previous ultrasound measures, a conversion
factor has been incorporated into the instrument software.8 9
time taken for ultrasound waves to reflect back to its receiver
from the internal limiting membrane.13
Three separate measurements were recorded for both axial
length and corneal curvature,whereas a single shot automati-
cally generated five measures of anterior chamber depth. In
around 5% of subjects, measurements were automatically
rejected by the instrument as, compared with the respective
mean, they were different by 0.05 mm for corneal curvature,
0.15 mm for anterior chamber depth or the signal to noise
ratio (an indicator of measurement quality) for axial length
was lower than 2.0. In these cases, an error message was dis-
played on the monitor and the measure repeated.
Objective refraction was performed with the Shin-Nippon
SRW-5000 autorefractor and the mean spherical equivalent
calculated from six readings.15The repeatability of the
IOLMaster was examined by measuring corneal curvature,
anterior chamber depth,and axial length on the same subjects
after a period of 1–10 days from the initial measurement.
The bias between measures (the mean difference, standard
deviation, and 95% confidence limits) was calculated and pre-
sented graphically.16Comparison between measures was
performed using paired two tailed t tests. Corneal curvatures
Operating principal of
A new non-contact optical device for ocular biometry 459
were converted from dioptres into a vector representation in
millimetres for analysis17: a spherical lens of power MSE (the
mean spherical equivalent = sphere + [cylinder/2]); Jackson
cross cylinder at axis 0° with power J0(= −[cylinder/2] cos[2 ×
axis]); Jackson cross cylinder at axis 45° with power J45(=
−[cylinder/2] sin[2 × axis]). The level of agreement between
biometry measures and ocular refraction was tested using the
Pearson’s product moment correlation coefficient.
Corneal curvature measurement differences between the IOL-
Master, Javal-Schiötz keratometer, and videokeratoscope are
shown in Table 1. The mean difference in corneal curvature
measured by the IOLMaster was in better agreement with the
Javal-Schiötz keratometer than the videokeratoscope, but
more variable (Fig 2). The broken lines on the graphs (Figs
2–4) indicate the extent to which the IOLMaster might over-
read or under-read compared to the alternative methods
examined (that is,the IOLMaster could be expected to read as
much as 0.01 mm above or 0.13 mm below the corneal
videokeratoscope for mean corneal curvature).
Anterior chamber depth, as measured with the IOLMaster,
was significantly shorter (by −0.06 (0.25) mm, p <0.02) than
that measured by applanation ultrasound (Fig 3). There was
no significant mean difference (bias) in the accuracy of the
instrument for the whole range of anterior chamber depths
evident in this study (that is, 2.85–4.40 mm). The IOLMaster
could be expected to read as much as 0.43 mm above or 0.54
mm below ultrasound for anterior chamber depth.
Axial length, as measured with the IOLMaster, was similar
to that measured by applanation ultrasound (difference 0.02
(0.32) mm, p = 0.47; Fig 4). Again, there was no significant
bias in the accuracy of the instrument for the whole range of
axial lengths evident in this study (that is, 22.40–27.99 mm).
The IOLMaster could be expected to read as much as 0.65 mm
above or 0.61 mm below ultrasound for axial length.
The repeatability of corneal curvature measurements made
with the IOLMaster are given in Table 2. The mean difference
in anterior chamber depth (−0.01 (0.08) mm, p = 0.24; Fig 3)
and axial length (0.00 (0.04) mm, p = 0.75; Fig 4) measured
with the IOLMaster was insignificant and there was no bias
over the whole range. A significant correlation with ocular
refraction was found for anterior chamber depth (r = 0.51, p
<0.01) and axial length (r = −0.77, p <0.01).
The study shows that the validity, repeatability, and clinical
utility of optical and image analysis methods of assessing
ocular biometric measures, match at least those offered by
measures and Javal-Schiotz keratometer or EyeSys corneal
topographer; repeatability of IOLMaster (n=104 eyes).
Corneal curvature: difference between IOLMaster
Mean corneal curvature (mm)
IOLMaster v videokeratoscope
IOLMaster v keratometer
95% Confidence limits
Difference in corneal curvature (mm)
measurements between the IOLMaster and
Javal-Schiötz keratometer and videokeratoscope
Vector comparison of corneal curvature
Javal Mean k
EyeSys Mean k
and A-Scan ultrasonography; repeatability of IOLMaster (n=104
Anterior chamber depth: difference between IOLMaster
Mean anterior chamber depth (mm)
limits of validity
Difference in anterior chamber depth (mm)
ultrasonography; repeatability of IOLMaster (n=104 eyes).
Axial length: difference between IOLMaster and A-Scan
Mean axial length (mm)
Difference in axial length (mm)
2524 22 23
limits of validity
curvature measurements repeated after an interval of
between 1 and 10 days
Vector comparison of IOLMaster corneal
FunctionMean difference (mm)95% confidence limits
460 Santodomingo-Rubido, Mallen, Gilmartin, et al
instrumentation currently used in clinical practice and it is
envisaged that the IOLMaster will be well received in both the
clinical and research environment.
The close match found between axial length measurements
using the IOLMaster and applanation ultrasound (0.02 (0.32)
mm) is consistent with those reported previously by
Hitzenberger,8who found a difference between Doppler inter-
ferometry and water immersion ultrasonography of 0.01
(0.13) mm. This study has found a closer agreement between
PCI and applanation ultrasonography than most of the previ-
ous reports.3The IOLMaster image analysis procedure for
measuring anterior chamber depth gave values that closely
matched those for applanation ultrasound. Although there
was a statistically significant difference between these
measures (on average −0.06 mm), this is smaller than the
resolution of applanation ultrasound and clinically insignifi-
cant (<2% of the anterior chamber depth). IOLMaster meas-
ures of corneal curvature were similar to those of keratometry
The coefficient of repeatability for IOLMaster axial length
measurements found in this study (0.00 (0.04) mm) is clearly
impressive when compared with those generally found for
ultrasound, in which a difference between the first and the
mean of three or five readings has been found to be of the
order of 0.12to 0.15 mm.1Previous studies using optical meth-
ods have used the mean standard deviation of single session
repeated measures to assess repeatability and reported values
of 0.025 mm8and 0.019 mm.3The repeatability of anterior
chamber depth image analysis measures with the IOLMaster
was also good, but slightly more variable than that found for
axial length. IOLMaster measures of corneal curvature had
repeatability levels similar to those previously reported for
keratometry and videokeratoscopy.1 11 18 19
The facility for taking biometry measurements without the
need for corneal contact is a particular advantage in both
clinical and research environments and especially in child
studies as, once fixation has been achieved, it is claimed that
only 0.5 seconds is required to perform a measurement. The
significant correlation between axial length and ocular refrac-
tion reported in this study is well documented.5 20In contrast,
a weaker, but significant, correlation was found between
anterior chamber depth and ocular refraction. Previous
studies have failed to support this latter finding.5 21The
IOLMaster currently cannot provide measures of lens thick-
ness but, despite this, its higher level of precision and repeat-
ability for anterior chamber depth may provide further insight
into the role of anterior segment dimensions in emmetropisa-
tion. Drexler and colleagues22
commercial instrument which uses PCI to measure lens
thickness with a precision of 8–10 µm and a resolution of 9 µm
and is thus able to monitor temporal change in the accommo-
dation response,but the wavelength of laser light used cannot
penetrate the eye as far as the posterior pole for concomitant
measures of axial length. Accurate axial length determination
is also of particular relevance in the quantitative analysis of
fundus structures23 24such as the optic disc,25 26the neuroreti-
nal rim area,27 28and the retinal nerve fibre layer thickness.29
Determination of retinal contour is relevant to our
understanding of the mechanisms of eye growth.30–34Previous
studies have determined retinal contour using relatively com-
plex computer software analysis which requires input data
obtained from A-scan ultrasonography,keratometry,and peri-
pheral refraction,30 31 35magnetic resonance imaging,36or the
indirect estimation of retinal contour from peripheral
have developed a non-
refraction.32 33Determination of retinal contour with PCI
methods has been previously reported for a single individual8
and represents a valuable facility for future studies on poste-
rior segment dimensions.The axial length:corneal radius ratio
has been proposed as a biometric predictor for the onset and
development of myopia, particularly in children.20 21 37Exam-
ination of the utility of this ratio will be enhanced by the abil-
ity of the IOLMaster to provide non-contact, successive,
repeatable, high resolution measurements of axial length and
J Santodomingo-Rubido, E A H Mallen, B Gilmartin, J S Wolffsohn,
Neurosciences Research Institute, School of Life and Health Sciences,
Aston University, Birmingham, UK
The authors have no proprietary interest in the IOLMaster.
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462 Santodomingo-Rubido, Mallen, Gilmartin, et al