Dai Nippon Printing Co., Ltd. (DNP) has clarified an optical principle for the measurement of sparkle contrast, which is used in the field of electronic displays equipped with an anti-glare film. It can be generally applied to a variety of imaging cameras for the fair comparison of the measured values.

So far, it has been difficult to simply compare sparkle contrast, when using other measurement equipment. In this latest study, DNP has clarified the principle of optical measurement which is necessary for obtaining comparable and reliable data, even when using a variety of imaging devices under various measurement conditions.

As a result, the sparkle contrast can now be quantitatively compared and evaluated, helping to accelerate the development efficiency of anti-glare film.

Sparkle tends to be a problem with increasing a pixel density of the display, and it has become more important to evaluate this phenomenon more accurately.

Looking ahead, DNP will use this newly acquired knowledge to answer customer requests promptly, and provide new anti-glare film for the latest large displays, mobile terminals, and in-vehicle displays.

It is also expected that this knowledge will support discussions on the standardization at the International Electrotechnical Commission (IEC), a standard body that establishes international standards in the field of electrical and electronic technology.

[DNP's Efforts]

In order to reduce annoying reflection of ambient light, anti-glare film has been developed and applied to electronic displays, such as PCs and smartphones.

However, as a side-effect of applying anti-glare film, sparkle pattern becomes obvious as fine and irregular luminance distribution caused by the combination of uneven surface-structure of anti-glare film and display matrix structure.

In the past, the degree of sparkle was evaluated visually by skilled personnel based on professional experience, but individual differences affected the reproducibility of the results. Furthermore, there was another problem regarding how to quantify the measurement results.

This led to a series of developments in the market of quantitative sparkle measurement methods, i.e. measurement of display sparkle contrast.

However, even with the same sample, it has been difficult to get reliable and quantitatively comparable data from different parties. This is largely because the measurement parameters from each party were individually different, even while sparkle contrast strongly depends on measurement conditions. This was caused by the fact that the basic optical principles of sparkle contrast measurement had not been perfectly established.

As a result, it has been difficult to compare measured data from components manufacturers with those from their customers.

In addition, the regional standard of sparkle contrast measurement in Japan, the Determination of Magnitude of Display Sparkle (JIS C1006), established in December 2019, did not support the comparison of the measured data as an absolute value, but focused on the correlation with the visual sensory evaluation test and the quantitative measured value obtained under fixed measurement conditions.

In this situation, DNP, as a manufacturer of anti-glare film, aims to achieve smooth communication with the final product manufacturer, who is also our client, and to increase the speed of development through reliable and comparable measured data.

In 2015, research on the optical measurement principle of sparkle measurement was commenced in order to achieve quantitative comparisons of the measurement results, in addition to the correlation with visual sensory tests.

In 2018, we successfully revealed the basic optical principle of sparkle contrast using theoretically measurements. And in 2019, we established guidelines on how to set the measurement conditions to obtain the comparable data as absolute values.

[Optical Principle for Measurement of Display Sparkle Contrast]

Measurement of sparkle pattern is performed by capturing an image of the display surface covered with an anti-glare film. Sparkle contrast is defined as a standard deviation of luminance distribution of sparkle pattern divided its average value.

DNP confirmed that the size of the minimum resolution area on the display surface is determined by the angle at which the measurement plane is viewed from the both edge of the aperture of the camera lens (called the aperture angle), and that the size of the minimum resolution area on the display surface is inversely proportional to sparkle contrast.

As a result, the same sparkle contrast was obtained on the image plane by appropriately selecting a focal length of the imaging lens and measurement distance so as to keep the same minimum resolution area on the display surface.

In addition, when considering the minimum resolution area on the image sensor of an imaging lens, which generally depends only on the F-number of the imaging lens (the ratio of the focal length of the imaging lens to the aperture diameter), the sparkle contrast could be compared among imaging lenses with different focal lengths.

By using this knowledge, it is possible to analyze the causes of differences in measured values and compare data with the same criteria, even with different measurement equipment and conditions.

[DNP's Anti-Glare Film]

Since 1999, DNP has been developing a measurement and evaluation method to quantify the visual sensory evaluation test of display sparkle, and also has reported at academic conferences, as well as obtaining patents.

DNP's anti-glare film suppresses sparkle on display surfaces and reduces glare caused by ambient light. It has been adopted in many displays from various manufacturers since the early 2000s, supporting high-definition and high-contrast images. Based on this contribution, DNP has gained the world's No. 1 share in display surface treatment films, according to "the Current Status and Future Prospects of Functional Polymer Films", 2019 edition, from Fuji Chimera Research Institute, Inc., 2018 forecast.

[Looking Ahead]

It is becoming increasingly important to be able to quantitatively compare and evaluate sparkle contrast, because the degree of sparkle is getting stronger in large and high- resolution displays such as 4K and 8K. DNP will continue to develop better anti-glare films based on the research of the optical measurement principle of sparkle contrast, and will present the latest knowledge related to sparkle phenomenon at academic conferences. We believe that this will facilitate theoretically-driven discussions with our client companies, leading to more appropriate anti-glare solutions.

IEC has started standardizing the measurement of display sparkle contrast. In this latest development, we believe it is of great significance that we will contribute to establish a pathway for “interoperability” of measured data, which the IEC focuses upon. We hope that this knowledge will support future discussions on IEC standardization.

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