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A Better Look at Polycarbonate Lens

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Abstract and Figures

Polycarbonate lenses are commonly used in many optical applications. Their high impact resistance, low weight, and cheap cost of high-volume production makes them more practical than traditional glass in various applications [1]. Some of these applications require safety (e.g. safety eyewear), complexity (e.g. Fresnel lens) or durability (e.g. traffic light lens) criteria that are difficult to meet without the use of plastics. Its ability to cheaply meet many requirements while maintaining sufficient optical qualities makes plastic lenses stand out in its field. Polycarbonate lenses also have limitations. The main concern for consumers is the ease at which they can be scratched. To compensate for this, extra processes can be carried out to apply an anti-scratch coating. Nanovea takes a look into some important properties of plastic lens by utilizing our three metrology instruments: Profilometer, Tribometer, and Mechanical Tester.
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INTRODUCTION
Polycarbonate lenses are commonly used in many optical applications. Their high impact resistance, low
weight, and cheap cost of high-volume production makes them more practical than traditional glass in
various applications [1]. Some of these applications require safety (e.g. safety eyewear), complexity (e.g.
Fresnel lens) or durability (e.g. trac light lens) criteria that are dicult to meet without the use of plastics.
Its ability to cheaply meet many requirements while maintaining sucient optical qualities makes plastic
lenses stand out in its eld. Polycarbonate lenses also have limitations. The main concern for consumers is
the ease at which they can be scratched. To compensate for this, extra processes can be carried out to
apply an anti-scratch coating.
Nanovea takes a look into some important properties of polycarbonate lenses by utilizing our three
metrology instruments: Prolometer, Tribometer, and Mechanical Tester.
A BETTER LOOK AT
POLYCARBONATE
LENSES
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IMPORTANCE OF TESTING POLYCARBONATE LENSES
Surface data of a lens is ideal for obtaining the surface roughness and radius of curvature. These properties
inuence the optical quality of the lens. Radius of curvature aects the lens’ optical power while surface
roughness will inuence the scattering of light. In addition, the thickness of the lens will be measured.
Lens thickness of the lens will aect its eective focal length.
The quality of the lens will decrease as more defects are on the surface of the lens. Material with high
scratch resistance tends to wear less over time and are less prone to defects caused by external sources.
Scratch hardness will dene the resistance of the sample to scratch defects. This can be used to determine
the scratch hardness of the bulk material or eectiveness of a scratch resistant coatings. Additionally,
adhesive scratch testing can be conducted to determine the quality of adhesion between the coating and
the lens.
Coecient of friction (COF) can be obtained from tribology testing against various materials. Since poly-
carbonate lenses are used in many dierent applications, it would be practical to understand how other
materials will behave when interacting with the lens. Thus, friction can be minimized (or maximized) when
selecting complementary materials. Wear testing demonstrates the durability of the sample in dierent
conditions.
The testing and results found during this study is representative of how the sample will perform in real life
applications. The results can be used to determine which type of material, process, or design is ideal for the
user’s application. Quality control testing can also be repeatedly conducted with our highly accurate
instruments.
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MEASUREMENT OBJECTIVE
In this case study, a general investigation on several important properties of a polycarbonate lens is
conducted. The following properties will be obtained from our prolometer, tribometer, and mechanical
tester: surface roughness, radius of curvature, thickness, scratch hardness, COF against various materials,
and wear rate.
Example of polycarbonate lens about to be tested on Nanovea Prolometer
Example of polycarbonate lens about to be tested on Nanovea Tribometer
Example of polycarbonate lens about to be tested on Nanovea Mechanical Tester
EQUIPMENT FEATURED
PROFILOMETRY
NANOVEA HS2000
Advanced Automation
Customizable Options
High Speed
Precision Flatness Measurement
Rigid and Stable Structure
User Friendly Technology
Learn More at
https://nanovea.com/instruments/?p=prolometers
NANOVEA PS50
50mm x 50mm XY
Compact Benchtop
Ideal Upgrade From Stylus and Laser Technologies
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The results from the prolometry measurements can be seen in Figure 1 and 2. The Figures below
are 2D and 3D images of the lens’s true form.
Figure 1: False-color view of front side (left) and back side (right) of polycarbonate lens
Figure 2: 3-D view of front side (left) and back side (right) of polycarbonate lens; amplied 10%
µm
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
µm
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
0 2 4 6 8 10 mm
mm
0
1
2
3
4
5
6
7
8
9
10
µm
0
50
100
150
0 2 4 6 8 10 mm
mm
0
1
2
3
4
5
6
7
8
9
10
µm
0
50
100
150
PROFILOMETRY
RADIUS OF CURVATURE AND ROUGHNESS
Test Parameter
Value
Instrument
Nanovea HS2000
Optical Sensor
L1 Lens (200µm Z-range)
Scan size (mm)
10mm x 10mm
Step size (µm)
5µm x 5µm
Scan time (h:m:s)
00:01:02
Table 1: Test parameters for roughness and radius measurements on the lens
MEASUREMENT PARAMETERS
RADIUS
PROFILOMETRY
Conducting an area scan on the lens ensures the radius of curvature is captured at the apex of the curve. To
observe the symmetry of the lens, radius of curvature was calculated from both the X- and Y-axis. Values of
142.1 and 135.5mm were obtained for the front side and 137.0mm and 139.2mm were obtained for the
back side.
Figure 5: Prole extraction in the X-axis (left) and Y-axis (right) of front side of polycarbonate lens
Figure 6: Prole extraction in the X-axis (left) and Y-axis (right) of back side of polycarbonate lens
0 1 2 3 4 5 6 7 8 9 10 mm
µm
-100
-50
0
50
Paramet ers 0-1 Unit
Radius 142.1 mm
01
0 1 2 3 4 5 6 7 8 9 10 mm
µm
-100
-50
0
50
Paramet ers 0-1 Unit
Radius 135.5 mm
01
0 1 2 3 4 5 6 7 8 9 10 mm
µm
-50
0
50
100
Paramet ers 0-1 Unit
Radius 137.0 mm
0 1
0 1 2 3 4 5 6 7 8 9 10 mm
µm
-50
0
50
100
Paramet ers 0-1 Unit
Radius 139.2 mm
0 1
ROUGHNESS
of 0.25mm was applied to obtain roughness height parameters. An Sa value of 26.76nm was obtained for
the front side of the polycarbonate lens, and 18.16nm for the back side of the plastic lens. Their respective
Sq values were 37.77nm and 36.02. These values are very low and are ideal to minimize scattering of light
when light interacts with the lens’s surface.
PROFILOMETRY
nm
0
100
200
300
400
500
600
700
nm
0
50
100
150
200
250
300
350
400
450
500
550
600
650
Utilizing our point sensor system, the thickness of the polycarbonate lens was obtained. This measurement
works by having a focal point at each surface. Having multiple focal points are possible due to our axial
chromatism technique. The dierence in refraction between air and the sample is corrected using the
sample’s index of refraction. The two surfaces, top and bottom, can be seen in gure 7. Since the coposition
of our sample is unknown, this was set to a value of common plastic: polycarbonate – 1.58. By subtracting
the two surfaces scanned, the thickness can be obtained. The mean thickness of the sample, scanned near
the apex of the curvature, was found to be approximately 2.611mm.
Figure 7: False-color view of top surface (left) and bottom surface (right)
Figure 8: False-color view (left) and height parameters (right) for thickness of polycarbonate
0 1 2 3 4 5 mm
mm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
µm
16200
16210
16220
16230
16240
16250
0 1 2 3 4 5 mm
mm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
µm
13590
13600
13610
13620
13630
13640
0 1 2 3 4 5 mm
mm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
µm
2590
2600
2610
2620
2630
Test Parameter
Value
Instrument
Nanovea PS50
Optical Sensor
PS5 (10000µm Z-range)
Scan size (mm)
5mm x 5mm
Step size (µm)
10µm x 10µm
Scan time (h:m:s)
00:36:32
MEASUREMENT PARAMETERS
Table 2: Test parameters for roughness and radius measurements on the lens
THICKNESS
PROFILOMETRY
EQUIPMENT FEATURED
MECHANICAL TESTING
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Spacious Platform with Adjustable Height Clearance
Upgradeable, robust and low cost of ownership
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SCRATCH HARDNESS
MEASUREMENT PARAMETERS
Table 3: Parameters used for scratch hardness testing on polycarbonate lens
Test Parameter
Value
Load type
Constant
Final Load (N)
15
Scratch Length (mm)
5
Scratching speed (mm/min) 18
Indenter geometry
120° cone
Indenter material (tip)
Diamond
Indenter tip radius (μm)
200
SCRATCH HARDNESS
MECHANICAL TESTING
The scratch test was conducted in accordance to ASTM-G171. Scratches were made at the apex of the lens
to minimize error caused by the curvature of the lens.
A scratch hardness of 420.59 ± 8.69MPa was obtained. As expected, the scratch hardness value is quite low
due to the nature of plastics. For reference, the scratch hardness testing we previously conducted on
aluminum, copper, and steel in the past were 0.84, 0.52, and 3.20GPa respectively [2]. Even though testing
conditions were dierent, the scratch resistance of the polycarbonate lens appears to be in the same
magnitude as a soft, scratch-prone metal like copper.
Figure 10: Scratch hardness measurement conducted under an optical microscope.
The blue dotted lines are positioned at the edge of the scratch to obtain scratch width.
Figure 9: Friction graph obtained from the scratch test
Measurement 1 (MPa)
Measurement 2 (MPa)
Measurement 3 (MPa)
Scratch 1
432.19
418.89
412.52
Scratch 2
431.51
416.25
413.4
Scratch 3
431.71
421.55
409.8
Table 4: Results from scratch hardness test
SCRATCH IMAGING WITH OPTICAL PROFILOMETRY
MECHANICAL TESTING
The polycarbonate lens was proled with our prolometry instrument to closely inspect the outcome of
the scratch test. A great deal of material was found to be surrounding the area where the scratch took place
(Figure 13). The volume of material around the scratch (Peak) and the volume lost (Hole) are about the
same. From this study, it is observed that the soft plastic seems to have been easily displaced during the
scratch. This allows us to make the conclusion that the material has a low scratch resistance.
The mean depth of the scratch ended up being 7.864 ± 0.2652µm into the surface. This was obtained by
extracting a series of prole across the scratched area and averaging the maximum valley depth (Pv) of
each prole (Figure 14).
Figure 14: Extracted series of proles (left) and their primary prole parameters (right). Red line indicates the mean prole.
0 1 2 3 4 5 6 mm
µm
0
500
1000
µm
0
10
20
µm
0
5
10
15
20
01002003004005006007008009001000 µm
µm
-10
-5
0
5
10
15
Parameters Unit Hole Peak
Surface µm² 8423751147975
Volume µm³ 43607284307441
Max. depth/height µm 11.79 10.65
Mean depth/height µm 5.177 3.752
Figure 12: 3-D view of scratch made on the lens
Figure 13: Volume of a hole/peak analysis on the
scratch created
Figure 11: False-color view of a scratch made on the lens
EQUIPMENT FEATURED
TRIBOLOGY
NANOVEA T50
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COEFFICIENT OF FRICTION
MEASUREMENT PARAMETERS
Table 5: Parameters used for coecient of friction testing on polycarbonate lens
0.5
5
10
0.0-5.0
0.78
Ball
Pin Material
Rubber, PTFE, ZrO
2
,
Al
3
O
2
, SS440C
6
Combination
Disk Material
Pin Material
1
Polycarbonate
Rubber
2
Polycarbonate
PTFE
3
Polycarbonate
ZrO2
4
Polycarbonate
Al3O2
5
Polycarbonate
SS440C
Table 6: Pin-On-Disk material combinations
Figure 15: COF graphs of 1) Rubber, 2) PTFE, 3) ZrO2, 4) Al2O3, 5) SS440C
1) 2)
3) 4)
5)
Table 6: Results of COF testing on Plastic Lens
Pin Material
Max COF
Min COF
Average COF
Rubber
0.947
0.277
0.734
PTFE
0.210
0.027
0.089
ZrO
2
0.193
0.043
0.106
Al
3
O
2
0.215
0.051
0.120
SS440C
0.175
0.022
0.072
COEFFICIENT OF FRICTION
A Pin-On-Disk Spiral Test was performed to ensure that the pins would pass over an unworn region throughout
all tests. The rst ve revolutions were cropped from the graphs. This was done to remove data when the radius
was near 0 (minimal tangential movement). The curvature of the lens must be kept in consideration when analyz-
ing the COF data.
The test results rank the following material from highest COF to lowest COF: Rubber, Al2O3, ZrO2, PTFE, SS440C.
The tests conducted were performed with a small normal force to minimize the eects of wear on the sample.
TRIBOLOGY
LINEAR WEAR
TRIBOLOGY
MEASUREMENT PARAMETERS
Figure 16: Friction graph from linear wear testing
Table 8: Parameters used for linear wear testing on polycarbonate lens
Test Parameter
Value
Load (N)
20
Test Duration (min)
20
Speed (rpm)
100
Amplitude (mm)
10
Total Distance (m)
40
Ball Material
ZrO
2
Ball Diameter (mm)
6
The linear test was conducted near the apex of the lens to minimize eects from curvature. From the COF
graph, two stages of wear can be observed. At 0-200 revolutions, the two surfaces are adapting to surface of
the sample. After 200 revolutions, signicant wear begins to occur. Loose particles created from the wear test
are now rampant along the surface of the worn area, creating three-body abrasion wear. To accurately calculate
wear rate, the volume loss was calculated by proling the wear track, analytically removing the curvature from
the lens, and conducting a volume of a hole study (Figure 18). A total volume of 577,479,379µm3 was lost. The
zirconium oxide wore an average of 61.69 ± 6.830µm into the plastic lens.
LINEAR WEAR
TRIBOLOGY
Figure 17: 3-D View of wear track created by linear wear test
Figure 18: Volume of a hole analysis
conducted on worn area
Sample of the lens with a wear track
Sample of polycarbonate lens with wear track
Figure 19: Extracted series of proles (left) and their primary prole parameters (right). Red line indicated the mean prole.
Table 9: Linear wear testing results
Max COF
Min COF
Average COF
Volume Loss
(µm
3
)
Wear Rate x 10-5
(mm
3
/Nm)
0.459
0.037
0.336
577479379
72.185
radius, and thickness measurements.
Scratch resistance or scratch hardness testing
was conducted on our mechanical tester
The COF and wear rate of the plastic lens
was obtained by using the tribometer.
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CONCLUSION
Important properties of polycarbonate lens were investigated using Nanoveas
metrology instruments. The ability to accurately measure and quantify properties
of materials is important for material selection and quality control processes.
easily applied with our temperature, humidity, lubrication, and corrosion modules.
RECCOMENDED READING
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Progressive Tribology Mapping of Flooring
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[1] Kogler, Kent. "Selection of plastics for optical applications. Advanced materials and
processes technology (1999).
[2] Li, Duanjie. "SCRATCH HARDNESS MEASUREMENT USING MECHANICAL TESTER."
(2014).
REFFERENCES
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