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The effect of refractive correction on ocular optical quality
measurement using double pass system
Wan Xiu-hua M.D PhD, Cai Xiao-gu M.D, Qiao Li-ya M.D, Zhang Ye M.D, Tan Jia-xuan M.D,
Vishal Jhanji M.D and Wang Ning-li M.D PhD
1 Beijing TongRen Eye Center, Beijing TongRen Hospital, Capital Medical University;
Beijing Ophthalmology & Visual Science Key Lab, Beijing, China.(Wan XH,Cai XG,Qiao
LY,Zhang Y,Tan JX,Wang NL);
2 Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong,
Hong Kong, PR China(Vishal J);
3 Beijing Institute of Ophthalmology, Beijing TongRen Hospital, Capital University of
Medical Science, Beijing, China.(Wan XH,Wang NL).
Correspondence to: WANG Ning-li, MD, PhD
Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University
2 Chongwenmennei Street, Dongcheng District, Beijing 100730, China
E-mail: [email protected]. +86 10 58269968, Fax +86 10 58269930
Conflict of interest: None of the authors have any conflict of interest of any sort.
Financial Support: No
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摘要
关键字 : 屈光不正,眼镜,点扩散函数,调制传递函数,人眼光学成像质量
目的：通过比较不同矫正方式下的人眼光学成像质量函数,研究眼镜这种屈光矫正方式对人眼光
学成像质量的影响。方法：前瞻性、自身对照研究。对70例（70眼）健康志愿者进行双通道客
观成像质量分析系统(Optical Quality Analysis SystemⅡTM 西班牙)在人工瞳孔4mm条件下分别
使用传统框架眼镜矫正与内置矫正方法进行光学成像质量函数测量。分析两者方法测量以下成
像参数的均值：调制传递函数截止空间频率（MTF cutoff）、斯特列尔比值（Strehl Ratio SR）、
点扩散函数(PSF)在10%最大高度的宽度(PSF10)和点扩散函数(PSF)在10%最大高度的宽度
(PSF10) ，并分析造成两者测量差异的相关因素。结果：眼镜矫正与仪器内置矫正法所测得的
光学成像质量参数均不具有统计学差异；Bland-Altman 一致性分析显示两种矫正方式一致性较
好，且两种矫正方式的重复性系数显示OQAS仪器成像质量参数测量一致性均较佳。多元线性逐
步回归模型结果显示发现MTFcutoff 、SR和PSF50 测量差值只与两种矫正方法选择的最佳聚焦
点差值相关. PSF10差值与最佳聚焦点差值、内置矫正法是否使用外置散光镜片矫正、年龄相关。
我们并未发现屈光状态与两种矫正方法测量差值存在线性相关关系，然而在高度近视眼组中，
使用眼镜矫正法与仪器内置矫正法相比，最初聚焦点（0.50±0.44 D）具有远视偏倚及较好的
成像质量参数结果。结论：双通道客观成像分析系统是一种临床上可靠的视觉质量评估技术，
眼镜矫正和仪器内置矫正技术均可获得可靠的人眼光学质量，然而在高度近视眼患者中，使用
仪器内置矫正技术时需谨慎分析结果。
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Abstract
Key words: refractive error; spectacles; point spread function; modulation transfer
function; ocular optical quality
Background: Optical Quality Analysis SystemⅡ(OQAS, Visiometrics, Terrassa, Spain),
that uses double-pass (DP) technique is the only commercially available device that
allows objective measurement of ocular retinal image quality. This study is to evaluate
the impact of spectacle lenses on the ocular optical quality parameters and the validity
of the optometer within OQAS.
Methods: 70 eyes of healthy volunteers were enrolled. Optical quality measurements were
performed using OQAS with an artificial pupil diameter of 4.0mm.Three consecutive
measurements were obtained from spectacle correction corresponding with subjective
refraction, and from the OQAS built-in optometer separately. Modulation Transfer
Function cutoff frequency (MTFcutoff), Strehl ratio, width of the Point Spread Function
(PSF) at 10 per cent of its maximal height (PSF10) and width of the PSF at 50 per cent
of its maximal height (PSF50) were analyzed.
Results: There were no significant difference in any of the parameters between the
spectacles and the optometer correction (all P >0.05, paired t-test). We found a good
agreement between both the methods and a good intra-observer repeatability in both the
correction methods. Difference in best focus between two methods was the only parameter
associated significantly with optical quality parameters differences. Best focus
difference, built-in optometer correction with or without external cylinder lens and
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age were associated significantly with PSF10 difference. No linear correlation between
refractive status and optical quality measurement difference was observed. A hyperopic
bias(best focus difference of :0.50±0.44 D),and a relatively better optical quality
using spectacles correction in high myopia group was found.
Conclusion: OQAS based on double-pass system is a clinically reliable instrument. In
patients with high myopia, measurements using built-in optometer correction should be
considered and interpreted with caution.
BACKGROUND
Diffraction, aberration and scattering leads to the deterioration of optical quality
in human eyes1. A most precise evaluation of ocular optical quality should
comprehensively consider different sources of image degradation, i.e. residual
lower-order aberrations, higher-order aberrations and scattering caused by refractive
correction methods and visual aids.
The point spread function (PSF) represents the actual light distribution spread over
the entire retina. Different domains of this distribution are indicated, dominating
different aspects of visual function2. Typically, PSF of the human eye encompasses hugely
different domains: a small-angle, high-intensity domain, called the 'PSF core', and
a large-angle, low-intensity domain, referred to as straylight. The small angle PSF
that dominates visual acuity, contrast sensitivity and aberrations can be assessed by
using a double-pass or other optical techniques. For the second domain, psychophysical
techniques could be used, in particular, the Compensation Comparison or CC technique3.
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Every area of PSF is important for the quality of vision. Spectacles are one of the
most useful visual aids used for the correction of refractive errors. It has been shown
that, on the large-angle domain (>4 degree) of the point-spread function (PSF), clean
spectacle lens obtains at least an order of magnitude effect lower than of the eye whereas,
unclean spectacle lens may approach the PSF of the eye4. However, the effect of spectacles
lens on the small-angle domain (<1 degree) is unknown. It is also important to note
that subjective refraction wearing spectacle lenses may alter the angle of incident
rays onto the cornea and therefore have the potential to make a large impact on the
aberrations although the spectacle lens by itself has little aberration. As an optical
aid that does not move with obliquity of gaze, the optical quality measurement with
spectacles may also be affected by subject’s obliquity of gaze during the fixation
test5.. However, the effect of an external spectacles lenses
on the optical quality
measurement is unknown.
OQAS, based on the double-pass (DP) technique6 is the only commercially available
device that allows measurement of the effect of optimal aberrations and the loss of
ocular transparency on the optical quality of the human eye7. For more smaller visual
angles point spread function (smaller than 1◦), it has been proven that OQAS can capture
the complete optical information in this area8,9. Unlike the other known techniques, an
important inventive aspect of this system comprises a double-pass ophthalmoscopic
system for correcting low-order aberrations with a built-in optometer. Subject's lower
order aberrations are corrected prior to recording so that image at the plane of the
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retina is free from the influence of lower order aberrations. This property of OQAS
offers a feasible way to compare the in-vivo optical quality of eyes in different states
of refractive correction.
The aims of the present study are: (1)compare the optical quality obtained by a trial
spectacle correction and a built-in optometer within OQAS, (2) assess the validity of
the built-in optometer refractive correction by OQAS.
METHODS
Subjects
70 healthy volunteers with simple ametropia were enrolled from the outpatient
department of the Beijing TongRen Hospital. Only one eye was chosen for data collection
based on a random number sequence. Eligibility criteria of subjects included:
best-corrected visual acuity (BCVA) of 0.20 or better (logMAR); pupil diameter equal
to or greater than 4 mm.Eyes with active ocular pathologies including refractive media
opacity and retinal disease, or previous ocular surgery were excluded from the study.
Subjects who used contact lenses were asked to remove their lenses at least one day
prior to the testing. Informed consent was obtained from each subject after the nature
and possible consequences of the study were explained. The study was conducted in
accordance with the tenets of the Declaration of Helsinki, and the study protocol was
approved by the Beijing TongRen Hospital Ethics Committee.
Protocol
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All optical quality parameters were measured with OQAS, based on the asymmetric scheme
of a double-pass system layout incorporating new and improved features adapted for
routine measurements in clinical practice10. The system records images after reflection
in the retina and a double pass (DP) through ocular media. Near-infrared light consisting
of laser diode (wavelength, 780 nm) is used because it is more comfortable for the subject
and provides retinal image quality estimates that are comparable to those obtained with
visible light6,11. From the point spread function images recorded by DP system, the
modulation transfer function (MTF) that yields the relationship between the contrast
of an object and its associated image as a function of spatial frequency was obtained1.
The MTF represents the loss of contrast as a function of the spatial frequency. A
two-dimensional radically averaged profile of monochromatic MTF is used to describe
the optical quality in the instrument. To simplify the data and facilitate the clinical
comparison of retinal image quality between subjects, OQAS provides several parameters
that are related to the MTF: the MTF cutoff and the Strehl ratio7. The MTF cutoff in
the double-pass system is the frequency at which the MTF reaches a value of 0.0112. To
avoid the artifacts due to high frequency noise, the device uses an MTF threshold value
of 0.01, which corresponds to 1% contrast. Thus, the MTF cutoff frequency in this study
refers to the frequency up to which the eye can focus an object on the retina with 1%
contrast1. The double-pass system computes the Strehl ratio in two dimensions as the
ratio between the areas under the modulation transfer function curve of the measured
eye and that of the aberration-free eye13. The width of the point spread function at
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10 per cent of its maximal height (PSF10) and width of the PSF at 50 per cent of its
maximal height (PSF50) are also provided by OQAS software. To obtain measurements with
a determined value of pupil diameter, the instrument incorporates a circular diameter
diaphragm. The diaphragm is conjugated with the eye pupil plane and therefore acts as
an effective entrance pupil when the natural pupil of the eye is larger than this value14.
The OQAS also incorporates a modified Thorner optometer, formed by 2 achromatic doublets,
used to compensate for the patient’s spherical refraction. By moving 1 doublet, it
is possible to correct the ametropia. The spherical refractive error was automatically
measured and corrected by the double-pass system by means of a motorized optometer within
a range of -8.00 to +6.00 D. External cylindrical lenses are required for astigmatism
﹥0.50 D. The system automatically obtains the condition that corresponds to the best
focus condition; therefore, the measurements are not affected by the patient’s
ametropia15.
Subjects underwent the following examination by an experienced optometrist or
ophthalmologist: visual acuity testing with Early Treatment Diabetic Retinopathy Study
(EDTRS) chart on a LogMAR scale, manifest refraction, subjective refraction, slit lamp
examination to exclude ocular media opacities without cycloplegia. Optical quality
measurements were performed by the same-trained observer at 4-mm artificial pupil, which
was controlled by means of a diaphragm wheel located inside the DP system.After
subjective refraction, subjects were instructed to wear a standard trial frame and to
remain stationary. Head and chin rest was provided and the subjects were asked to fixate
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on the target while maintaining normal blinking. Subjects were required to blink prior
to each measurement to maintain a good tear film during the examination. Three repeated
measurements were obtained with the spectacles but not using the built-in optometer.
The pupil center was realigned between each measurement. After that, subjects were asked
to take off the trial frame and close their eyes for 5 minutes. Subsequently, another
three repeated measurements were obtained after correction of the refractive error using
modified Thorner optometer. For subjects with more than 0.5D cylindrical refractive
errors that could not be automatically corrected by OQAS, the spherical refractive error
was automatically corrected by the double-pass system while the astigmatism was
corrected with an external cylindrical lens to achieve the best possible retinal image.
The external lens was not placed exactly in the perpendicular direction to the optical
axis of the OQAS but it was slightly tilted to avoid possible reflections on the lens
to avoid influencing the measurements9. The difference in best focus between two
correction methods was recorded. The room illumination was kept at 42.0 lux (measured
by digital lux meter, LX-1010B) during all measurements.
Statistical analysis
All statistical analyses were performed using the SPSS software (version 17.0, SPSS
Inc., Chicago, IL, USA) for Windows. Normality of all data distributions was confirmed
by means of the Kolmogorov–Smirnov test and then parametric statistics were applied.
Values are presented as mean (± SD), and the corresponding range (minimum and maximum).
All tests were two tailed and p-values less than 0.05 were considered statistically
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significant. Bland-Altman analysis16 were used to compare the agreement of optical
quality parameters between two conditions. The Pearson correlation coefficient (r)
between each method was also evaluated. The student’s paired t-test was used to compare
the difference in the average parameters between the two methods. The association
between various independent variables were analyzed by linear regression stepwise
models.
RESULTS
70 subjects were enrolled in the study that included 39females and 31 males. The mean
age of the subjects was 35.5 ± 13.1 years (range 16 to 68 years). 36 right eyes and
34 left eyes were selected. The mean UCVA (logMAR) was 0.54 ± 0.42 (range 0.04 to 2.0)
and the mean BCVA (logMAR) was 0.00 ± 0.06 (range -0.10 to 0.20). The spherical
equivalent (SE) range from -7.63 to +2.75 D(Mean: -2.09 ± 2.27 diopters). The spherical
manifest refractive error ranged from -7.25 to +3.00 D (Mean: -2.00 ± 2.37 D), and
cylinder from -3.50 to 0.25 D (Mean: -0.50 ± 0.71 D). The mean best-focus difference
between spectacles correction and built-in optometer correction was 0.025 ± 0.39
diopters (range -1.25 to +1.25 D).
Table 1 summarizes the mean values of optical quality parameters and the results of
the student’s paired t-test using the two refractive correction methods. Among the
selected eyes, 26 eyes were using the Thorner optometer with the external cylindrical
lens correction, and the MTFcutoff, Strehl ratio,PSF50 and PSF10 difference in this
group were 0.37 ±8.49, 0.01 ±0.04, -0.30 ±1.35, -1.08 ±4.21 respectively. 44 eyes
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used the total Thorner optometer without external cylindrical lens correction, and the
MTFcutoff, Strehl ratio,PSF50 and PSF10 difference in this group were -1.22 ±6.59,
-0.01±0.05, 0.09 ±0.72, 1.00 ±2.58 respectively. There was no significant difference
in all the optical quality measurements between the two methods (all the P value >0.05,
paired t-test). Table 2 and Figure 3 demonstrate the results of Bland-Altman analysis
and indicated a good agreement between both methods.
Table 3 summarizes the optical quality differences in different refractive group.
Moderate correlation coefficients were observed for all parameters (r = 0.75 to 0.78)
except the PSF 50 (r=0.65). In this study, intra-observer repeatability was analyzed
by using the coefficient of repeatability (COR; 1.96 times the within-subject standard
deviation, Sw), mean COR for each session was then obtained by adding the square of
the individual CORs for each individual eye, and calculating the square root of the
mean value. The percentage of mean value for COR of 50% is often selected as the highest
acceptable value for metrological purposes in biology
1,12
.The mean COR (%)of MTFcutoff
using spectacles and Thorner corrections:3.55(8.60%) and 3.64(8.27%), Strehl
ratio:0.03(10.94%) and 0.03(11.39%),PSF50:0.12(14.90%) and
0.08(14.83%),PSF10:0.12(8.62%) and 0.12(8.28%).The COR in all above parameters were
below 15% indicating good intra-observer repeatability in both measurement methods.
Multivariate linear stepwise regression models were created for difference of above
optical quality parameters between two refractive methods: only best focus difference
were associated significantly with MTFcutoff difference(Beta:0.52,P<0.01),Strehl
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ratio difference (Beta:0.45,P<0.01) and PSF50 difference(Beta:-0.38,P<0.01).Best
focus difference (Beta:-0.32,P<0.01), Thorner correction with or without external
cylinder lens(Beta:-0.34,P<0.01) and age (Beta:0.34,P<0.01) were associated
significantly with PSF10 difference.
DISSCUSION
In this study, we compared the optical quality obtained by a trial spectacle correction
and a built-in optometer correction in OQAS. The results demonstrate the effect of
spectacle lenses on small-angle point spread function and modulation transfer function
in a double pass system. The mean values of optical quality indicators provided by the
OQAS suggests that the optical quality was good in all these eyes. These results are
similar to the previous studies in which target population was also healthy young
individuals
7,12
. Compared to the optical quality parameters in eyes' refractive state
totally corrected by spectacles, no statistically significant difference was observed
in all optical quality parameters in eyes with using modified Thorner optometer
incorporated or using Thorner optometer plus external cylindrical lens correction.
Although to a different aspect of point spread function, our findings seems to be in
a agreement with those of De Wit et al, who measured the effect of spectacles lens in
large-angle domain (>4 degree) of the point-spread function and concluded that, in
general, auxiliary optics such as a pair of glasses generate relatively little stray
light. This statement is only valid under two conditions: (1) the lens is carefully
cleaned and (2) there is not an obviously visible large amount of damage to the lens.
12
All the spectacle trial lenses we used in the present study were carefully cleaned and
free of any visible damage. This suggested that cleaned spectacle lenses per se may
not affect the on axial optical quality.
The agreement of optical quality measurement between two correction methods was also
measured. When the Bland-Altman plots (Figure 3) were analyzed, a far smaller and no
statistically significant mean difference between the measurements was demonstrated.
However, considering the earlier studies that the reported values of optical quality
parameters obtained by DP system in a healthy population were 44.54 cpd (MTF cutoff),
0.27 (Strehl ratio), 3.64 minutes of arc (PSF50), 0.19 minutes of arc (PSF10)7,17, a spread
of data with 95% LOA: MTFcutoff (-12.93 to 13.59), Strehl ratio(-0.08 to 0.09),
PSF50(-1.77 to 1.82), PSF10(-6.41 to 5.62) was obtained. We also found statistically
significant correlations between all optical quality parameters difference with best
focus difference in two correction methods. This suggests that a part of inconsistency
between the two correction methods observed in our data was as a result of the difference
in best focus point chosen by subjective refraction and modified Thorner optometer.
It is easy to understand that determination of the best-focus point in DP technology
is based on the use of infrared light (780nm), whereas in subjective refraction, it
is based on the balance between green and red light. Thus the defocus caused by
longitudinal chromatic aberration between two refractive corrections may lead to the
optical quality bias. Besides, it is important to realize that the bias and agreement
between two refractive correction methods must be considered in the context of the
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inherent variability of the subjective refraction. This has been previously performed
in a well-designed study which found a bias between two clinicians of -0.12 D and 95%
LoA of -0.90 D to 0.65 D for mean spherical error18. This bias is considerably more than
was observed in the present study (Mean best-focus difference: 0.025 ± 0.39 D). The
most important drawback of this technology is the use of infrared light
13,19.
The artifact
due to this issue dominates the recording grossly outside the central peak area of PSF,
particularly limiting estimation of the scattering effect20..However, determination of
the best-focus point reflects the central peak area of the recording. No linear
correlation was observed between refractive status or spherical equivalent with optical
quality measurement difference between two methods but a hyperopic bias(best focus
difference :0.50±0.44D) was observed and a relatively better optical quality using
spectacles correction in high myopia group(See Table 3). The cause of discrepancy in
this group could possibly be due to the longitudinal chromatic aberration between two
refractive corrections that may increase in long axial eyes and unstable performance
of the motorized optometer when subject's refraction is close to its -8D correction
limits. However, the number of high myopic eyes in this study is relatively small,
whether such high myopic spectacles affect ocular optical quality measurement requires
further investigation. Using the built-in optometer in DP system to correct subject's
refractive error is a valid method, but in high myopia patients, measurements using
built-in optometer correction should be considered and interpreted with caution.
Recently, several studies suggested that OQAS has good intra- and intersession
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repeatability1,12. However, Tomas et al suggested that the consistency of measurements
using OQAS seems to be limited, especially in eyes with poor optical quality. In the
present study, almost identical and relatively small CORs for all parameters were
observed in the two refractive correction methods. This indicates that the double-pass
system has a high degree of intra-observer repeatability even in measurements using
spectacles. These results are slightly better than that reported in the previous
studies12,17. Hence, wearing spectacles may not bring a significant variation in
repeatability of optical quality measurements in a DP system. It is reliable to use
spectacle correction in optical quality measurements when subject's refractive state
beyond the application range of refractive correction incorporated in the OQAS.
One of the limitations of this study was that we did not have cycloplegia before the
measurement, hence some accommodation might come into play in both measurements. In
order to reduce influence of this confounding factor, subjects were strictly required
to have a 5-minute rest between two refractive correction measurements. The DP technique
which recorded using near infrared light is an advantage for the patient’s comfort
during image acquisition. However, the magnitude of scatter in infrared light can be
different than in visible light. The impact of retinal scatter could also be larger
in infrared since light penetrates deeper in the retina. This is a limitation for the
absolute characterization of ocular optical quality. This technique, although useful,
should be combined with complementary techniques routinely6
In summary, our results demonstrate that there is no difference in ocular optical
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quality parameters obtained using spectacles correction or built-in optometer and in
addition, there is a good agreement through a DP system. When a large difference was
found between subjective refraction and built-in optometer base on DP system, the
subjective refraction and optical quality results should be considered and interpreted
with caution. In conclusion, OQAS based on DP system is a reliable instrument. Spectacles
lens per se would not affect the small-angle domain point spread function. To obtain
a valid ocular optical quality, either external spectacles correction or the built-in
optometer correction could be used，but in high myopia patients, optical quality
measurement using built-in optometer correction should be considered and interpreted
with caution.
ACKNOWLEDGMENTS
The authors thank Jaume Pujol, professor of Centre for Sensors, Instruments and Systems
Development (CD6), Universit at Polite`cnica de Catalunya, Terrassa, Barcelona, Spain,
for his technical support
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REFERENCES
1.
Saad A, Saab M, Gatinel D. Repeatability of measurements with a double-pass system. J Cataract Refract Surg,
2010, 36: 28-33.
2.
van den Berg T, Franssen L, Coppens J. Ocular media clarity and straylight. Encyclopedia of the Eye, 2010, 3:
173-183.
3.
van den Berg TJ, Franssen L, Coppens JE. Straylight in the human eye: testing objectivity and optical character of
the psychophysical measurement. Ophthalmic Physiol Opt, 2009, 29: 345-350.
4.
De Wit GC, Coppens JE. Stray light of spectacle lenses compared with stray light in the eye. Optom Vis Sci, 2003,
80: 395-400.
5.
Code SM, Remole A. Retinal image quality during oblique gaze through spectacle lenses: plastic vs. glass. Am J
Optom Physiol Opt, 1985, 62: 240-245.
6.
Artal P, Benito A, Perez GM, Alcon E, De Casas A, Pujol J, et al. An objective scatter index based on double-pass
retinal images of a point source to classify cataracts. PLoS One, 2011, 6: e16823.
7.
Martinez-Roda JA, Vilaseca M, Ondategui JC, Giner A, Burgos FJ, Cardona G, et al. Optical quality and
intraocular scattering in a healthy young population. Clin Exp Optom, 2011, 94: 223-229.
8.
van den Berg TJ, Franssen L, Kruijt B, Coppens JE. History of ocular straylight measurement: A review. Z Med
Phys, 2013, 23: 6-20.
9.
Vilaseca M, Romero MJ, Arjona M, Luque SO, Ondategui JC, Salvador A, et al. Grading nuclear, cortical and
posterior subcapsular cataracts using an objective scatter index measured with a double-pass system. Br J
Ophthalmol, 2012, 96: 1204-1210.
10.
Kamiya K, Umeda K, Kobashi H, Shimizu K, Kawamorita T, Uozato H. Effect of Aging on Optical Quality and
Intraocular Scattering Using the Double-Pass Instrument. Curr Eye Res, 2012.
11.
Lopez-Gil NaA, P., Artal P. Comparison of double-pass estimates of the retinal-image quality obtained with green
and near-infrared light. J Opt Soc Am A Opt Image Sci Vis, 1997, 14: 961-971.
12.
Vilaseca M, Peris E, Pujol J, Borras R, Arjona M. Intra- and intersession repeatability of a double-pass instrument.
Optom Vis Sci, 2010, 87: 675-681.
13.
Guirao A, Gonzalez C, Redondo M, Geraghty E, Norrby S, Artal P. Average optical performance of the human eye
as a function of age in a normal population. Invest Ophthalmol Vis Sci, 1999, 40: 203-213.
17
14.
Artal P, Iglesias I, Lopez-Gil N, Green DG. Double-pass measurements of the retinal-image quality with unequal
entrance and exit pupil sizes and the reversibility of the eye's optical system. J Opt Soc Am A Opt Image Sci Vis,
1995, 12: 2358-2366.
15.
Benito A, Perez GM, Mirabet S, Vilaseca M, Pujol J, Marin JM, et al. Objective optical assessment of tear-film
quality dynamics in normal and mildly symptomatic dry eyes. J Cataract Refract Surg, 2011, 37: 1481-1487.
16.
Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement.
Lancet, 1986, 1: 307-310.
17.
Tomas J, Pinero DP, Alio JL. Intra-observer repeatability of optical quality measures provided by a double-pass
system. Clin Exp Optom, 2012, 95: 60-65.
18.
Bullimore MA, Fusaro RE, Adams CW. The repeatability of automated and clinician refraction. Optom Vis Sci,
1998, 75: 617-622.
19.
Rodriguez P, Navarro R. Double-pass versus aberrometric modulation transfer function in green light. J Biomed
Opt, 2007, 12: 044018.
20.
Pinero DP, Ortiz D, Alio JL. Ocular scattering. Optom Vis Sci, 2010, 87: E682-696.
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Table 1: Mean values of the optical quality parameters of different corrections
Spectacles Total Thorner Paired t
(n=44)
Parameters Mean ± std
Spectacles
Thorner Plus
Paired t
(n=44)
Test
(n=26)
Lens(n=26)
Test
Mean ± std
P-Value
Mean ± std
Mean ± std
P-Value
MTFcutoff 40.65 ±9.59 41.96 ±9.62
0.20
35.29 ±8.31 35.57 ±10.65
0.865
SR
0.22±0.06
0.23±0.06
0.23
0.19±0.05
0.19±0.04
0.729
PSF50
3.16±1.12
3.05±0.94
0.28
3.49±0.94
3.74±1.33
0.393
PSF10
10.51±4.22
9.49±3.34
0.21
12.74±4.44
13.29±4.81
0.519
Comments: Spectacles =totally spectacles refractive correction. Total Thorner= OQAS
built-in Thorner optometer correction without external cylindrical lens. Thorner Plus Lens
= built-in Thorner optometer with external cylindrical lens correction.
19
Table 2: Results of Bland-Altman analysis and correlation coefficients.
Difference between 2 methods
LOA
B & A analysis
Mean
SD
r value
Lower 95%
Upper 95%
Width of 95%
MTFcutoff
0.33
6.76
0.75
-12.93
13.59
26.52
Strehl ratio
0.004
0.04
0.75
-0.08
0.09
0.17
PSF10
-0.39
3.06
0.78
-6.41
5.62
12.03
PSF50
0.02
0.91
0.65
-1.77
1.82
3.59
Comments: B&A analysis= Bland-Altman analysis; LOA=limits of agreement; Lower 95%
LOA=mean-1.96SD.Upper 95% LOA=mean+1.96SD; r=correlation coefficient;
20
Table 3: optical quality measurement differences in different refractive group
Best-focus
MTFcutoff
SR
PSF50
PSF10
Difference
difference
difference
difference
difference
N=12
0.04±0.32
0.44±5.77
0.01±0.02
0.02±0.81
0.52±2.84
N=25
-0.03±0.36
-1.50±7.13
-0.01±0.05
0.16±0.87
1.39±2.97
N=23
-0.04±0.43
-2.12±6.36
-0.02±0.04 -0.06±0.96
0.24±2.58
N=10
0.50±0.44
3.39±10.20
0.05±0.04
-2.62±4.88
Refractive state
Eyes
Hyperopia
(+0.5,+3.0D)
Mild myopia
(-0.5D,-3.0D)
Moderate Myopia
(-3.0D,-6.0D)
High Myopia
(<-6.0D)
-0.58±1.41
Comments: Difference (Spectalces correction-Thorner Corrections)
21
Figure Legends
Figure.1: Scattergram showing the relationship between the best focus difference in
spectacles correction and built-in optometer correction (Focusdif) and modulation
transfer function cut-off point difference (Graph A:MTFdif),Strehl ratio(Graph
B:SRdif), width of the Point Spread Function at 50 per cent of its maximal height (Graph
C:PSF50dif) and width of the PSF at 10 per cent of its maximal height (Graph D:PSF10dif).
Figure 2: Bland-Altman plots showing the mean of differences and mean±1.96 SD for
Spectacles correction and using the OQAS built-in modified Thorner optometer correction;
Graph A=Modulation transfer function cutoff frequency (MTFcutoff);Graph B= Strehl
ratio(SR);Graph C= width of the point spread function at 50 per cent of its maximal
height (PSF50);Graph D=width of the PSF at 10 per cent of its maximal height(PSF10).
22
Figure 1. Scattergram of the best focus difference in spectacles correction and OQAS
built-in optometer correction (Focusdif) and optical quality parameters difference
23
Figure 2: Bland-Altman plots of optical quality parameters using spectacles correction
and OQAS built-in modified Thorner optometer correction
Graph A
30
Graph B
.20
.15
20
.10
10
Difference
Difference
Mean + 1.96SD
Mean
0
Mean + 1.96SD
.05
Mean
0.00
-.05
-10
Mean - 1.96SD
-.10
Mean - 1.96SD
-20
-.15
0
10
20
30
40
50
60
.10
.15
.20
Average of MTFcutoff
.30
.35
.40
Average of Strehl ratio
Graph C
4
.25
Graph D
15
3
10
2
Mean + 1.96SD
Mean + 1.96SD
Difference
Difference
5
1
Mean
0
0
Mean
-5
-1
Mean - 1.96SD
-10
Mean - 1.96SD
-2
-15
-3
1
2
3
4
5
6
4
7
6
8
10
12
14
16
Average of PSF10
Average of PSF50
24
18
20
22
24