Fast Photoelectric Estimation of Oxygen Transmissibility of Silicone
Hydrogel Contact Lens
Hsin-Yi Tsai
1
, Chih-Ning Hsu
1
, Yu-Hsuan Lin
1
, Kuo-Cheng Huang
1
and Patrick Joi-Tsang Shum
2
1
Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan
2
Department of Ophthalmology, Cathay General Hospital, Taipei, Taiwan
Keywords: Silicone-Hydrogel Contact Lens, Water Content, Oxygen Transmissibility, Photoelectric Method.
Abstract: Nominal oxygen transmissibility (Dk/t) values on commercial product packages are usually of the lens
material instead of the actual values of contact lenses (CLs) derived based on their power. To evaluate the
Dk/t values of CLs of different powers, we developed a rapid photoelectric method. In the experiment, a
photodiode was employed to detect variations in the light intensity passing through silicone hydrogel (Si-Hy)
CLs over a period. Light intensity variations can indicate the water content (WC) and power of Si-Hy CLs
and help calculate the Dk/t of Si-Hy CLs of different powers. Experimental results indicated that the WC and
specific power of Si-Hy CLs were determined by the initial attenuation voltage, which ranged from 0.73 to
1.15 V for the lenses tested herein, whereas the WC varied from 38% to 56%. The Dk/t of Si-Hy CLs at -3.00
D was determined from the voltage variation over 3 min after reaching the peak voltage and the Dk/t
corresponding to a specific power could be evaluated from the ratio of initial attenuation voltage of a lens
having a specific power to that having -3.00 D. The results of this study can serve as reference information
for quality control in factories.
1 INTRODUCTION
Contact lenses (CL) are classified as hard and soft
based on the hardness of the lens. Traditionally,
polymethyl methacrylate (PMMA) and hydrogels
have been used as the main materials to manufacture
hard and soft CLs, respectively. However, in recent
years, rigid materials with gas permeability have been
developed as a substitute for PMMA for
manufacturing hard CLs to improve oxygen
permeability and fit on the eyes (Bergenske et al.,
1987; Harmano et al., 1994). Soft CLs have typically
been manufactured using water-containing and gel-
like plastics, and they are more pliable than hard CLs,
which makes them more comfortable to wear.
However, over several hours of use of soft CLs, the
cornea gradually becomes hypoxic and the eye feels
dry because soft CLs do not allow air to permeate
through. Therefore, the oxygen permeability (Dk) of
a CL is an important parameter when selecting CL
products. According to the ISO 11539 standard for
the classification of CLs, the material type is
described using a six-part code depending on the
material composition, namely water content (WC),
percentages of ionic and nonionic monomer, contents
of silicone and fluorine, and oxygen permeability
(ISO11539, 1999). The main materials used to
manufacture soft CLs include 2-hydroxyethyl
methacrylate, poly-2-hydroxyethyl methacrylate,
methacrylic acid, and vinyl pyrrolidone (Tranoudis et
al., 2004); in all of these materials, the higher the WC
is, the higher the oxygen permeability is. In general,
the typical oxygen permeability of hydrogel lenses
ranges from 25 to 50 ((cm
3
[O
2
] × cm)/(cm
2
× s ×
mmHg)), but oxygen permeability is limited to 80
((cm
3
[O
2
]×cm)/(cm
2
×sec×mmHg)) even if the WC of
a lens is 100% (Sweeney et al., 2006), as shown in
Fig. 1.
Figure 1: Relationship between oxygen permeability and
WC of hydrogel and Si-Hy CLs (Jones, 2002).
Tsai, H-Y., Hsu, C-N., Lin, Y-H., Huang, K-C. and Shum, P.
Fast Photoelectric Estimation of Oxygen Transmissibility of Silicone Hydrogel Contact Lens.
DOI: 10.5220/0006619702050212
In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 205-212
ISBN: 978-989-758-286-8
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
205
The oxygen transmissibility Dk/t of a lens is
defined as the ratio of the oxygen permeability Dk of
a CL material and the thickness t of a local region of
the CL. Moreover, the oxygen permeability Dk is
determined by the diffusion coefficient D and
solubility coefficient k, where D and k represent the
speed of gas movement through the CL material and
the degree of dissolved oxygen contained in the CL
material, respectively. In addition, the unit of Dk is
10
-
11
(cm
3
[O
2
] × cm)/(cm
2
× s × mmHg), which equals
1 barrier (Cerle, 1972).
Harvitt et al., (1998) suggested an oxygen
transmissibility value Dk/t of 125 × 10
-
9
(cm × mL
[O
2
])/(s × mL × mm Hg) to prevent stromal anoxia,
and hydrogel-based CLs cannot satisfy this
requirement. Hence, silicone hydrogel (Si-Hy) was
developed as a novel and revolutionary material with
a Dk/t value of more than 100; this material facilitates
the passage of a greater amount of oxygen through the
CL to the cornea than hydrogel. With this feature,
problems such as blurred vision, red eyes, and corneal
swelling can be prevented over extended periods of
use of soft CLs.
Coulometric (Alvord et al., 1998) and
polarographic (Efron et al., 2007) methods have been
employed in previous studies to measure the Dk value
of a CL. ISO 9913-1 (1996) describes the
polarographic method for determining Dk and Dk/t,
especially for Dk values of 0–75 barrier. In the
measurement process, gas flow is generated and
passed through a CL, and a gold/platinum cathode
and silver anode are placed centrally under the tested
CL. In addition, ISO 9913-2 (2000) describes the
coulometric method for determining Dk and Dk/t of
rigid and nonhydrogel flexible CLs, especially for Dk
values higher than 75 barrier. However, this method
cannot be applied to hydrogel CLs. The polarographic
method underestimates or overestimates the value of
Dk at the edge of a CL owing to the difference in
oxygen partial pressure; this phenomenon is called
the boundary layer effect or edge effect. It can be
corrected by considering the thickness of the side of
the CL. To overcome the limitations of the
polarographic and coulometric methods, ISO 18369-
4 (2006) specifies tests of the physicochemical
properties of CL materials, including extraction, rigid
lens flexure and breakage, oxygen permeability,
refractive index, and WC. In particular, the standard
aims at simultaneous measurement of the
physicochemical properties of hydrogel and
nonhydrogel CLs.
Table 1: Comparison of methods for measuring oxygen
permeability of CLs.
Method
Technique
Characteristic
Polarographic
Generate gas
flow/
Electrode detect
difference of
voltage
1. For Dk value
range 0-75 barrier.
2. For Hydrogel CL.
Coulometric
Generate gas
flow/
Use coulometric
sensor to detect
pass oxygen
through CL
1. For Dk value
higher than 75
barrier.
2. For Rigid and
non-hydrogel CL.
Photoelectric
Irradiate light on
CL/Detect
variation of
transmission light
1. For Hidrogel and
silicone hydrogel
CL.
2. No limitation of
Dk value.
3. Rapidly
measuring process
and without gas
chamber.
Lee et al., (2015) used the polarographic method
to measure the Dk value of CLs in phosphate-
buffered saline (PBS) and borate-buffered saline
(BBS) solution, and the boundary effect was
corrected by stacking four layers. The results showed
that the Dk values of all CLs measured in BBS
solution were more stable than those measured in
PBS solution. Lewandowska et al., (2015) analyzed
the porosity of CLs to determine the relationship
between oxygen permeability and porosity. The
results revealed that the highest Dk value can be
obtained by using a CL mould material with the
fewest pores of the largest size.
According to previous studies, methods for
measuring Dk and Dk/t are based on the
electrochemical or the diffusion method, and these
methods require oxygen gas, a gas chamber, and
several precise components, as summarized in Table
1. Moreover, the measured value is usually the ideal
value instead of the actual value of the wearable.
An instant measurement model for measuring the
WC and oxygen permeability of hydrogel CLs was
developed by analyzing light attenuation over a
specific spectrum (Hung, 2017). A spectrometer was
used to detect light intensity, and the light intensity at
wavelengths of 500–600 nm was analyzed over a
period of 8 min. However, light intensity was
measured optically, and the oxygen transmissibility
of a CL considering its thickness and power was not
evaluated.
Therefore, in the present study, we developed a
photoelectric method for observing variations in the
light transmitted through Si-Hy CLs over a specific
period and for evaluating the WC and actual oxygen
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
206
transmissibility value directly. In this method, optical
components such as LEDs and photodiodes (PDs) are
employed to replace halogen lamps and
spectrometers. Moreover, we designed and fabricated
a signal amplifier circuit to read the voltage
transferred by the light intensity; the resulting device
is portable and can serve as a substitute to
conventional equipment, which are expensive.
2 FUNDEMENTAL THEORY
AND EXPERIMENTAL
SYSTEM
2.1 Characteristics of Silicone
Hydrogel Contact Lens
Different materials used to prepare Si-Hy CLs exhibit
different light reflectivity, transmission, and
absorption levels when illuminated by light of
different wavelengths. Hale et al. (1973) found that
water exhibits relativity stable and lower light
absorption at wavelengths of 400–600 nm, as
illustrated in Fig. 2.
Figure 2: Absorption spectrum of water. (Hale et al., 1973).
In addition, it exhibits lower absorption and O
2
evolution rates for light of wavelengths 500–650 nm,
as shown in Fig. 3. Therefore, we employed an LED
light source with a wavelength of 520 nm to irradiate
Si-Hy CLs for minimizing the measurement errors
caused by water and oxygen.
Figure 3: Action and absorption spectrum of oxygen in
visible spectrum. (Taiz et al., 2015).
2.2 Experimental System and
Measurement Process
2.2.1 Preparation of Silicone Hydrogel
Contact Lenses
Si-Hy is a novel material used for fabricating soft
CLs, and its excellent properties include higher
oxygen permeability than other CL materials. In the
experiment in this study, four types of Si-Hy CLs,
with different WC levels, oxygen permeability levels,
and eight different powers for each type of Si-Hy CL,
were used to investigate the voltage variation caused
by Si-Hy CLs; their nominal specifications are listed
in Table 2.
Table 2: Specifications of Si-Hy CLs.
Brand
Specificatio
n
ACUVU
E
OASYS
ACUVU
E
TruEye
Hydro
n
Eye
Secert
Cooper
Vision
Clariti
Material
Senofilco
n
A
Narafilco
n A
Filcon
I
Somofilco
n A
Water
Content
(%)
38
46
47
56
Dk × 10
-11
(cm
3
[O
2
] ×
cm)/(cm
2
×
sec ×
mmHg)
122
100
120
60
Central
Thickness
@-3.00D
(mm)
0.085
0.085
0.08
0.07
Dk/t × 10
-9
(cm
2
[O
2
] ×
cm)/(cm
2
×
sec ×
mmHg)
144
118
150
86
In general, the oxygen permeability (Dk) of Si-Hy
CLs decreases with increasing WC; however, Dk is
affected by CL thickness as well. Therefore, the
oxygen transmissibility (Dk/t) of CLs provides more
accurate information about the oxygen transmission
ability of CLs. Furthermore, the nominal central
thickness of a CL is almost for the power of -3.00 D,
but the central and edge thicknesses and their ratio
change with CL power. Lira et al. (2014) used an
electronic thickness gauge to measure the central and
peripheral thicknesses of several CLs of different
powers and re-estimated the oxygen transmissibility
of each CL. Their results indicated that the CL
thickness increased with increasing CL power,
especially in the range of -3.00 to -6.00 D. In this
study, we used a portable optical microscope to
Fast Photoelectric Estimation of Oxygen Transmissibility of Silicone Hydrogel Contact Lens
207
acquire cross-sectional images of Si-Hy CLs to
determine their actual thickness.
2.2.2 Experimental Setup
A schematic of the experimental setup is presented in
Fig. 4. A green LED emitting light of wavelength 520
nm and triggered by a current of 1 mA was used as
the light source, and the voltage measured using a
Tekronix DPO3014 oscilloscope represented the
intensity of light received by the PD; the measured
value tended to decrease when the Si-Hy CL was
dehydrated. In addition, a signal processing circuit
was connected between the PD and the oscilloscope
to transform the light signal into an electrical signal
and amplify tiny variations in the signal.
Figure 4: Schematic of experimental setup.
2.2.3 Design of Signal Amplifier Circuit
Regarding the signal amplifier circuit used herein,
which was designed based on reverse amplification
technology, as the amount of light detected by the PD
increased, the output voltage became lower.
Accordingly, the light intensity would affect only the
voltage signal, and the frequency would remain
unaffected. Therefore, the voltage value displayed on
the oscilloscope increased when the Si-Hy CL was
placed on the measuring setup, which reduced the
intensity of the transmitted light, and the voltage
variation was 2 V, as shown in Fig. 5. Here, the
frequency was unaffected by the light intensity.
Figure 5: Voltage measured by oscilloscope with and
without CLs.
2.2.4 Photoelectric Method and
Measurement Process
Given that the light transmission intensity is influen-
ced by the WC of the CL, CL material, and CL
thickness, the photoelectric module used herein was
set up to detect intensity variations caused by
different types of Si-Hy CLs and to determine their
WC and power levels. Moreover, the actual oxygen
transmissibility of Si-Hy CLs with different powers
could be evaluated. Six main steps were involved in
evaluating the actual oxygen transmissibility Dk/t of
Si-Hy CLs of unknown or known power. A flowchart
of this process is shown in Fig. 6, and the details of
each step are described as follows.
Step (I): We used a green LED light to provide
stable irradiation on Si-Hy CLs, and we applied a PD
and oscilloscope to detect the intensity of light
transmission and display the measured signal,
respectively. In addition, we designed and deployed a
signal amplifier circuit between the PD and the
oscilloscope to amplify the voltage to amplify the
minute variations in light intensity.
Step (II): We measured the voltage from the
oscilloscope without wearing any Si-Hy CL and used
this value as the reference in the following calculation
process.
Step (III): After placing a Si-Hy CL on the stage
of the measurement setup, we started to count time.
Step (IV): We recorded the voltage measured by
the oscilloscope for Si-Hy CLs of different power and
WC levels every 1 min.
Step (V): We analyzed variations in the voltage
when using Si-Hy CLs relative to the reference value
and then determined the WC and Dk/t values of Si-
Hy CLs at -3.00 D.
Step (VI): The actual Dk/t of CL of specific power
could be calculated from the ratio of initial
attenuation voltage between the specific power and -
3.00 D.
Figure 6: Flowchart for evaluating WC and actual Dk/t of
Si-Hy CLs.
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
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3 EXPERIMENTAL RESUTLS
AND DISCUSSION
3.1 Dehydration and Evaluation of
Water Content of Contact Lenses
The material used to prepare Si-Hy CLs contains
many micropores, and water fills these holes, thus
blocking oxygen diffusion and reducing light
transmission. Therefore, the oxygen permeability of
Si-Hy CLs decreases with increasing WC, and light
transmission increases when Si-Hy CLs are
dehydrated. From the measured voltage values of four
different types of Si-Hy CLs with a power of -3.00 D,
the Si-Hy CLs with higher WC initially blocked a
large amount of light to be transmitted, but they
dehydrated rapidly, and the voltage signal gradually
stopped changing after 3 min, as shown in Fig. 7. In
addition, the dehydration ratio of Si-Hy CLs with low
WC was less than that of Si-Hy CLs with high WC.
On the basis of these features, users of Si-Hy CLs
with lower WC would not feel strong sensations of
dryness in the eye over extended periods of use;
moreover, this type of Si-Hy CL has high oxygen
permeability.
Figure 7: Attenuation voltage over 9 min in cases of Si-Hy
CLs with four different WC levels.
The voltage variation between the cases of CL use
and no CL use were employed to determine WC; WC
was found to vary linearly with the voltage variation,
as shown in Fig. 8. The initial attenuation voltage
(V
iκ
) was determined according to the voltage
variation between the cases of initial CL use (V
0min
)
and no CL use (V
reference
), as expressed by Eq. (1).
referencemin0i
VVV
(1)
Accordingly, the relationship between the initial
attenuation voltage (V
iκ
) and WC of Si-Hy CLs at a
power of -3.00 D can be given by Eq. (2). From this
equation, an increase in the WC of Si-Hy CLs would
result in the blockage of greater amounts of light and
an increase in the attenuation voltage simultaneously.
The constant in this expression was obtained using
the linear equation in Fig. 8.
30956.0WC0256.0V
i
(2)
Figure 8: Initial attenuation voltage affected by WC of Si-
Hy CLs.
3.2 Relationship between Oxygen
Transmissibility and Light
Transmission through Contact
Lenses
When the Si-Hy CL was placed on the stage of the
measurement setup, the voltage measured by the
oscilloscope initially increased and peaked at a
certain value. In this period, the transmission of light
was affected by the WC of the Si-Hy CL. Then, the
Si-Hy CL was dehydrated, the measured voltage
decreased gradually, and the oxygen transmissibility
of the Si-Hy CL started to affect the intensity of light
transmission. Therefore, the voltage variation at 3
min after the appearance of the peak voltage could be
employed to evaluate the oxygen transmissibility of
Si-Hy CLs. The results showed that the oxygen
transmissibility of the four different types of Si-Hy
CLs (the same as those listed in Table 2) with a power
of -3.00 D had a polynomial relationship with the
voltage variation, as shown in Fig. 9. When the
relationship between the voltage variation and
oxygen transmissibility of 80–150 was established,
the unknown oxygen transmissibility (Dk/t) of a Si-
Hy CL with a power of -3.00D could be evaluated
from the voltage variation (V) measured using the
developed method and the following equation.
32
V39.10996V76.5626
V39.110144.175t/Dk
(3)
Fast Photoelectric Estimation of Oxygen Transmissibility of Silicone Hydrogel Contact Lens
209
The constant in the Eq. (3) was obtained from the
fitting curve in Fig. 9. In addition, the WC decreased
as the oxygen transmissibility of the Si-Hy CLs
increased and the rate of CL dehydration decreased
simultaneously. Owing to this phenomenon, the
voltage variation of the CLs with high oxygen
transmissibility was lower than that of the CLs with
low oxygen transmissibility.
Figure 9: Relationship between oxygen transmissibility and
voltage variation at 3 min after the voltage peaked.
3.3 Thickness and Shape of Contact
Lenses
In general, the nominal thickness of a CL is defined
by the central thickness and set at a power of -3.00 D.
However, the thicknesses in the central and the edge
regions are different, and the values change with the
power of the CL. In the experimental results, we
found that the central and edge thicknesses differed
only slightly, and the thickness ratio of CLs with
powers of -1.00 to -8.00 ranged from 2.2 to 2.6, as
shown in Fig. 10. In addition, the thickness ratio
ranges were similar for different types of Si-Hy CLs.
Figure 10: Central and edge thicknesses, and thickness ratio
of Si-Hy CLs.
Although the central and edge thicknesses were
similar for lenses of different powers, the sagittal
length varied with the power of the Si-Hy CLs, as
shown in Figs. 11 and 12. As illustrated in the figures,
the diameter of the Si-Hy CL was fixed, and the
sagittal length was determined at the position where
the lens thickness was less than 0.1 mm; the length
ratio (L
r
) can be expressed by Eq. (4). The results
showed that the sagittal length decreased and length
ratio increased as the Si-Hy CL power increased. This
is because the central and edge thicknesses are
controlled to maintain wearing comfort; thus, the
shapes of Si-Hy CLs are slightly adjusted to facilitate
fabrication and to meet the requirements of different
powers. In addition, variations in lens shape would
affect the transmission intensity of light passing
through Si-Hy CLs.
Sagittal
Diameter
L
r
(4)
Figure 11: Schematic of diameter and sagittal length of Si-
Hy CLs with powers of -1.00 and -8.00 D.
Figure 12: Diameter and sagittal length, and length ratio of
Si-Hy CLs.
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
210
3.4 Actual Oxygen Transmissibility of
Contact Lenses of Different Powers
From the preceding results, the shape of Si-Hy CLs
would be affected by the lens power, and the initial
attenuation voltage would be affected simultaneously.
The initial attenuation voltage decreased with
increasing Si-Hy CL power because the length ratio
between the lens diameter and sagittal length
increased, thus causing a greater amount of
transmission light to be focused on the PD. Therefore,
the power of Si-Hy CLs could be determined using
the initial attenuation voltage. In addition, the trends
could be applied to different types of Si-Hy CLs, as
shown in Fig. 13.
Figure 13: Effect of Si-Hy CL power on initial attenuation
voltage.
Although the WC and oxygen permeability (Dk)
of Si-Hy CLs of the same material are fixed, the
actual oxygen transmissibility may differ slightly
depending on the length ratio. An increase in the
length ratio would lead to a wider region in the lens
being thicker than 0.1 mm, and the actual oxygen
transmissibility would decrease. Accordingly, the
actual oxygen transmissibility of Si-Hy CLs at a
specific power can be evaluated using Eqs. (5) and
(6),
)D00.3(@V
)xD(@V
'R
i
i
(5)
'RD00.3(@t/Dk)xD(@t/Dk
(6)
where x is the specific power of a CL, and R’ is the
ratio of initial attenuation voltage between the lens of
a specific power and that of -3.00 D. For example,
the nominal oxygen transmissibility Dk/t of Cooper
Vision Clariti at -3.00 D is 86, and the initial
attenuation voltages (V
iκ
) at -6.00 and -3.00 D are
0.83 and 1.15, respectively. The ratio of initial
attenuation voltages (R’) of the same lens of two
powers is 0.72, and the actual oxygen transmissibility
of Cooper Vision Clariti at -6.00 D is 62.07.
4 CONCLUSIONS
In this study, we developed a rapid photoelectric
measurement and evaluation method for measuring
the WC of Si-Hy CLs from the initial attenuation
voltage; the WC values of the lenses considered
herein ranged from 38% to 56%. In addition, the
power of the Si-Hy CLs influenced their initial
attenuation voltage, sagittal length, and oxygen
transmissibility. Thus, the initial attenuation voltage
could be employed to evaluate the specific power of
the Si-Hy CLs; additionally, the actual oxygen
transmissibility for a given specific power of Si-Hy
CLs could be evaluated from the ratio of the initial
attenuation voltage of a CL having a specific power
to that of a CL having a power of -3.00 D. Through
this method, the WC, power, and actual oxygen
transmissibility of a Si-Hy CL of unknown material
and power can be evaluated rapidly instead of
obtaining the nominal oxygen transmissibility at the
power of -3.00 D. The advantages of the proposed
method include high speed and low cost. In the future,
a portable measurement instrument will be developed
by designing a printed circuit board of a read circuit
to replace the oscilloscope used herein in order to
facilitate rapid measurement of CLs for quality
examination in a factory setting.
ACKNOWLEDGEMENTS
This work was supported in part by the Ministry of
Science and Technology, TAIWAN, under Grants
MOST 106–2221–E–492–017– and MOST 106–
2221–E–492–011–.
REFERENCES
Bergenske, P. D., Polse, K. A., 1987. The effect of rigid gas
permeable lenses on corneal sensitivity, Journal of the
American Optometric Association, 58, 212-215.
Harmano, H., Waranabe, K., Hamano, T., Mitsunaga, S.,
Kotani, S., Okada, A., 1994. A study of the
complications incduced by conventional and disposable
contact lenses, Journal of the Contact Lens Association
of Ophthalmologists, 20, 103-108.
Fast Photoelectric Estimation of Oxygen Transmissibility of Silicone Hydrogel Contact Lens
211
ISO 11539, 1999. Ophthalmic Optics- Contact Lenses-
Classification of Contact Lenses and Contact Lens
Material.
Tranoudis, I., Efron, N., 2004. Water properties of soft
contact lens materials, Contact Lens Anterior Eye, 27,
193-208.
Sweeney, D., 2006, Silicone Hydrogels: The Evolution of a
Revolution, Contact Lens Spectrum.
Jones, L., 2002, Modern Contact Lens Materials: A Clinical
Performance Update, Contact Lens Spectrum.
Cerle J., 1972, Hydrophilic lenses for continuous wearing,
Clinical and Experimental Optometry, 55, 343-346.
Harvitt, D. M., Bonanno, J. A., 1998 Minimum contact lens
transmissibility values for daily and extended wear
contact lenses, Optometry and Vision Science Journal,
75, 189.
Alvord, L., Court, J., Davis, T., Morgan, C. F., Schindhelm,
K., Vogt, J., Winterton, L., 1998, Oxygen Permeability
of a New Type of High Dk Soft Contact Lens Material,
Optometry and Vision Science, 75, 30-36.
Efron, N., Morgan, P. B., Cameron, I. D., Brennan, N. A.,
Goodwin, M., 2007, Oxygen Permeability and Water
Content of Silicone Hygrogel Contact Lens Materials,
Optometry and Vision Science, 84, 328-337.
ISO International Standard 9913-1, 1996, Optics and
optical instruments, Determination of Oxygen
Permeability and Transmissibility by the Fatt Method,
Geneva, Switzerland: International Organization for
Standardization.
ISO International Standard 9913-2, 2000, Optics and
optical instruments, Determination of Oxygen
Permeability and Transmissibility by the Coulometric
Method, Geneva, Switzerland: International
Organization for Standardization.
ISO International Standard 18369-4, 2006, Ophthalmic
optics-Contact lenses. Part 4: Physicochemical
Properties of Contact Lens Materials, Geneva:
International Organization for Standardization.
Lee, S. E., Kim, S. R., Park, M., 2015, Oxygen permeability
of soft contact lenses in different pH, osmolality and
buffering solution, International Journal of
Ophthalmolgy, 8, 1037-1042.
Lewandowska, M., Jasińska-Kwaśnik, W., Jóźwik, A.,
Siedlecki, D., 2015, Measurement of oxygen
permeability of contact lenses based on analysis of
porosity, Interdisciplinary Journal of Engineering
Science, III, 1-5.
Hung, C. W., Huang, K. C., Tsai, H. Y., Lin, Y. H., Shum,
P. J-T, 2017, Development of Instant Measuring Model
for Oxygen Permeability and Water Content of
Hydrogel Contact Lens, Intertional Conference on
Photonics, Optics and Laser Technology, 1, 175-82.
Hale, G. M., Querry, M. R., 1973, Optical constants of
water in the 200nm to 200µm wavelength region,
Applied Optics, 12, 555-563.
Taiz, L., Zieger, E., 2015, Plant Physiology and
Development, Sinauer Associates, Inc.6
th
Edition.
Lira, M., Pereira, C., Real Oliveira, M. E., Castanheira, E.
M., 2014, Importance of contact lens power and
thickness in oxygen transmissibility, Contact Lens &
Anterior Eye, 38, 120-126.
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
212