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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