Post on 16-Mar-2018
Tech Paper
Anti-Sparkle Film Distinctness of Image Characterization
Anti-Sparkle Film Distinctness of Image Characterization
Brian Hayden, Paul Weindorf Visteon Corporation, Michigan, USA
Abstract: The amount of sparkle associated with automotive
anti-glare display surfaces is generally worsened with increasing
display resolutions. One sparkle countermeasure recently
introduced by 3M is the use of an anti-sparkle optically clear
adhesive (OCA). The distinctness of image (DOI) properties of the
3M anti-sparkle OCA are further investigated.
Keywords: Anti-glare; anti-sparkle; sparkle; distinctness;
image; DOI; OCA, MTF
1. Introduction Anti-glare (AG) surface treatments are often utilized on
automotive displays in order to minimize specular reflection
components by scattering the rays from the light sources as shown
in Figure 1-1.
Figure 1-1. Diffuse Reflection from Anti-Glare Surface
[1]
The use of AG surface treatments may cause a sparkle
phenomenon often described as a grainy or scintillating effect
when used in conjunction with a display. Sparkle is caused by the
“randomized light refraction coupled with the pixel orientation
that leads to non-uniform lighting” [2] as shown in Figure 1-2.
Figure 1-2. Depiction of Sparkle Production [2]
Most current attempts to reduce sparkle have concentrated on
reducing the size and pitch of the anti-glare surface features as
shown in Figure 1-3.
Figure 1-3. Wyko 3D Surface Analyses - High
Sparkle (Left), Low Sparkle (Right) [3]
3M has developed a unique anti-sparkle (AS) optically clear
adhesive (OCA) film that uses a diffraction grating structure to be
situated between the display and the anti-glare cover lens as
depicted in Figure 1-4.
Figure 1-4. Anti-sparkle Film Diagram
[Courtesy of 3M]
The 3M AS OCA is based on a 2-dimentional diffraction grating
approach that essentially replicates the light from each sub-pixel
into nine equally illuminated dots as shown in Figure 1-5.
Figure 1-5. Individual Light Dot Multiplied by 2-Dimensional Anti-Sparkle Grating [Courtesy of 3M]
Although the 3M AS OCA does a good job of significantly
reducing sparkle, there are other optical consequences that need to
be understood. 3M wrote a helpful paper [2] that addressed most
of the optical effects associated with the use of the 3M AS OCA:
Sparkle Reduction – Showed significant reduction, but did
not address effect as a function for distance from the display
Gloss – the AS film causes a reduction in the gloss level
Image Sharpness – the AS film reduces image clarity, but
generally in the area of acceptability.
In addition, Visteon wrote a paper [4] regarding the reflection
properties of the anti-sparkle film and concluded that the 3M AS
film contributes only a very small amount of reflectance if the
optical stack is optically bonded. One final area that merits further
investigation is the effect of the AS film on the image quality
(image sharpness). The objective of this paper is to provide some
guidance as to the expected image quality degradation with the
use of the AS film.
2. Background/Objective The objective of this paper is to measure the clarity of image
performance of the 3M anti-sparkle OCA and to make an
assessment of what image degradation would be seen by the user.
The sample configurations measured are depicted in Figures 2-1.
Figure 2-1. Optical Sample Configurations
The various materials in the optical samples were:
AG film HM01 – Mitsubishi super extra minute AG
AG film LM302 – Clear version of Bayer LM296 tinted film
AG film HM02 – Mitsubishi extra minute AG
3M Anti-Sparkle OCA film – Easy 160829-5x7
3M OCA – 8146-5
Glass – 3mm soda lime type
3. Measurement Results The Display Messtechnik SMS-1000 was utilized to capture
images of the optical performance. Images collected were:
MTF utilizing sinusoidal arrays from Applied Image Inc.
Knife edge measurements using black to white density
targets on the sinusoidal arrays from Applied Image Inc.
The sinusoidal array from Applied Image Inc. is shown in Figure
3-1.
Figure 3-1. Applied Image Sinusoidal Array SINE
M-14
The sinusoidal array description is outlined in Figure 3-2 where
the cycles/mm is notated for each of the target sinusoidal patterns.
Figure 3-2. SINE M-14 Pattern Description
Figure 3-3 shows the SMS-1000 raw image of the sinusoidal array
which has been annotated. The SMS-1000 had a resolution of
71.99 camera pixels/mm with the following set up parameters:
Objective lens: 50 mm
Aperture: f/5.6
Imaging distance: 300 mm
Figure 3-3. SMS-1000 Image of the M-14 Sinusoidal
Array
Images utilizing the various anti-glare samples with and without
the AS OCA are depicted in Figure 3-4. Figure 3-5 shows the
modulation measurement results using the SMS-1000.
Figure 3-4. Images for the Various AG Samples
Sinusoidal Target Array (no AG sample)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 82% 81% 82% 79% 75% 73% 69%
HM01 (Standard OCA)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 81% 79% 78% 77% 70% 71% 65%
HM01 (Anti-Sparkle OCA)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 37% 21% 32% 22% 8% 9% 24%
HM02 (Standard OCA)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 76% 68% 65% 60% 50% 51% 45%
HM02 (Anti-Sparkle OCA)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 35% 26% 28% 19% -- 11% 15%
LM302 (Standard OCA)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 60% 32% 20% 7% 3% 2%
LM302 (Anti-Sparkle OCA)
Cycles / mm 1 2 3 4 5 6 8
Contrast CM 31% 18%
Figure 3-5. Modulation Depiction for the Various AG Films
Table 3-1 summarizes the contrast modulation response data for
the various optical configurations which are plotted in Figure 3-6.
Figure 3-6 shows a considerable reduction in the modulation
contrast response for the frequencies tested when the AS film is
utilized.
Table 3-1. Contrast Modulation Summary
Figure 3-6. Contrast Modulation Summary
4. Analysis In order to better understand the frequency response
characteristics of the AS film, a knife edge test using the black to
white density pattern on the M-14 sinusoidal array was utilized.
Knife edge test methods to extract the modulation transfer
function (MTF) are discussed per Information Display
Measurements Standard (IDMS) section 7.7 “EFFECTIVE
RESOLUTION” [5]. The knife edge results were analyzed by
taking the derivative of the edge transition and performing a Fast
Fourier Transform (FFT) on the derivative function (line-spread
function) to obtain the MTF function [6] as shown in Figure 4-1.
Furthermore in reference [6], for reasonable clarity at a 24 inch
viewing distance, an MTF > 0.6 at 5.8 cycles/mm was asserted.
However this was based on AG films that are “Gaussian” in
nature and generally have a monotonically decreasing MTF as a
function of frequency.
The first column of the knife edge results in Figure 4-1 shows the
captured image for the black to white density patterns. The second
column in Figure 4-1 shows the actual optical edge transition
(blue curve) and the associated derivative function (purple curve).
Finally the third column in Figure 4-1 shows FFT of the
derivative function which is the MTF of the system.
It is interesting to note that per Figure 3-5, the HM01 with the AS
film shows an increase in the MTF at 8 cycles/mm with the anti-
sparkle film which is consistent with results per Figure 4-1, thus
confirming the validity of knife edge test. Also for the HM01 with
the AS film, at spatial frequencies of 5 and 6 cycles/mm, the MTF
is almost zero for both Figures 3-5 and 4-1. One thing to keep in
mind is that for the knife edge test, the frequency response of the
optical system has a monotonically decreasing MTF and is not
perfect as can be observed in the edge transition reference with no
AG MTF plot in Figure 4-1 (upper right corner).
For the LM302 AG film the, the DOI is reduced below the
recommended MTF of 0.6 at 5.8 cycles/mm at a 24 inch viewing
distance [6]. However it should be noted that the LM302 film is a
clear version of the tinted LM296 film that has been used
successfully in front of a TFT display for “dead front” instrument
applications although DOI is reduced. Therefore if the LM302 is
used as a reference limit of acceptability, it suggests that the 3M
AS film may provide suitable performance with the use of less
aggressive AG films such as HM01 and HM02 films.
HM01 (Standard OCA)
HM01 (Anti-Sparkle OCA)
HM02 (Standard OCA)
HM02 (Anti-Sparkle OCA)
LM302 (Standard OCA)
LM302 (Anti-Sparkle OCA)
Step Reference (no AG)
Figure 4-1. Knife Edge Image, Edge Transition and FFT MTF Results
Another metric useful to understand the performance
characteristics of the AS film is the contrast sensitivity. The
contrast sensitivity function of the human eye may be
approximated by Equation 4-1 [7] and is “an analytical
approximation for the “average” threshold curve.”
(4-1)
It should also be noted that the term “Contrast Sensitivity” is
related to contrast threshold by the relationship per Equation 4-2.
(4-2)
The two curves are plotted on Figure 4-2A where the Contrast
Sensitivity axis is on the left and the Contrast Threshold axis is on
the right. The meaning of the curves is that everything below the
Contrast Sensitivity curve can be seen. Likewise everything above
the Contrast Threshold curve can be seen.
Figure 4-2A: Contrast Sensitivity and Contrast
Threshold
Figure 4-2A is plotted in terms of cycles/degree for the ordinate
axis. It is helpful to convert cycles/degree to cycles/mm at a
viewing distance of 24 inches as all of the data is presented in
terms of cycles/mm. Equation 4-3 shows how the conversion is
accomplished where D is the viewing distance.
(4-3)
When the contrast sensitivity function is plotted in terms of spatial
frequency at a 24 inch viewing distance, Figure 4-2B shows that
at 2 to 4 cycles/mm, the human eye contrast sensitivity has
dropped significantly from the peak value further supporting the
position that the AS film performance may be acceptable
Figure 4-2B. Contrast Sensitivity and Contrast
Threshold at 24” in cycles/mm
As a practical example, images of the various AG films with a
standard test pattern were taken with a Radiant Imaging
colorimeter. The test equipment setup is shown in Figure 4-3 and
an image of the test pattern with no film is shown in Figure 4-4.
The characters in the test pattern (“1x,2x,3x”) were measured to
be approximately 4.7 mm high with the measurement distance
24 inch. The 1x line pair grille pattern measured approximately
1.9 cycles/mm. Figures 4-5, 4-6 and 4-7 show the various AG
films both with and without the AS OCA.
Figure 4-3. Radiant Imaging Colorimeter Test Setup
Figure 4-4. Test Pattern with no Films
Figure 4-5. HM01 (left), HM01 with AS (right)
Figure 4-6. HM02 (left), HM02 with AS (right)
Figure 4-7. LM302 (left), LM302 with AS (right)
5. Conclusion/Summary The 3M AS film does degrade image clarity and therefore its
application may be somewhat dependent on the type of AG film
used and the viewing distance. For a viewing distance of 24
inches and a 3 mm substrate thickness, some image clarity
degradation will be visible due to the AS film, but may be
acceptable based on its outstanding performance in reducing
sparkle to an acceptable level. As this study was performed with
only a 3 mm substrate thickness, one area that warrants further
investigation is how the DOI varies as a function of the distance
between the AS film and the TFT image plane.
Another area which requires further investigation is to reduce the
substrate thickness from 3 mm used in this study, to determine if
the MTF would be improved. In general for AG films, a reduced
distance between the AG film and the TFT image plane will
improve the image sharpness [6], however it is not known
whether this principle applies also to the AS film.
6. References [1] Diffuse Reflection,
<https://upload.wikimedia.org/wikipedia/commons/b/bd/Lam
bert2.gif> (June 2017).
[2] Sitter, B., Tebow, C., Zhang, Z., “Anti-Glare Solutions for
Automotive Displays,” Society for Information Display 2016
Vehicle Displays and Interfaces Symposium, Digest of
Technical Papers.
[3] Hayden, B., et al, “Anti-Glare Sparkle Optical Modeling &
Prediction Method,” Society for Information Display 2015
Vehicle Displays and Interfaces Symposium, Digest of
Technical Papers.
[4] Weindorf, P., Hayden, B., Lor, K., “Characterization of Anti-
Sparkle Film for Automotive Applications,” Society for
Information Display 2017, International Symposium
Technical Paper, 40.2.
[5] Society for Information Display, International Committee for
Display Metrology, Information Display Measurements
Standard, Version 1.03, <http://www.icdm-sid.org/>.
[6] Weindorf, P., Hayden, B., “Anti-glare Film Sharpness
Measurement Investigations,” Society for Information
Display 2012, Vehicle Displays and Interfaces Symposium,
Digest of Technical Papers.
[7] Kopeika, Norman S., “A System Engineering Approach to
Imaging,” Bellingham, Washington: SPIE Publications,
1998.
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About Visteon Visteon is a global company that designs, engineers and manufactures innovative cockpit electronics products and connected car solutions for most of the world’s major vehicle manufacturers. Visteon is a leading provider of instrument clusters, head-up displays, information displays, infotainment, audio systems, telematics and SmartCore™ cockpit domain controllers. Visteon also supplies embedded multimedia and smartphone connectivity software solutions to the global automotive industry. Headquartered in Van Buren Township, Michigan, Visteon has approximately 10,000 employees at more than 40 facilities in 18 countries. Visteon had sales of $3.16 billion in 2016. Learn more at www.visteon.com.
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