Repeatable Sparkle Measurement for Quality Control of Anti-Glare Displays
Displays are increasingly integrated into environments where their positions are fixed relative to the viewer and variable lighting conditions are common, such as vehicle consoles and dashboards. Because of their role in operation and safety, these in-vehicle displays must be viewable in all types of ambient lighting conditions. Typically, anti-glare (AG) films are applied to displays to minimize reflections from light sources such as sunlight and streetlamps and ensure that important information is viewable to occupants and operators of the vehicle.
Ambient light can cause mirror-like reflections on automotive display screens, obscuring images and making them difficult to read.
AG films and coatings have microstructure surfaces that allow ambient light to be diffused evenly off display screens to reduce glare. However, this surface structure also interferes with the light emitted by a display’s pixels. The interference of the AG surface with the display’s pixel geometry causes a visual effect called “sparkle.” Sparkle reduces display clarity and can appear grainy or hazy to a human observer.
Measurement images captured by an image-based display test system show how the application of the AG layer (right) introduces visual properties including sparkle, haze, and diffusion.
The tiny, faceted microstructures that make up the AG layer act as small lenses. Sparkle occurs when light emitted from each display pixel is refracted by these lenses (surface variations) that cross the pixel area, causing interference and scatter. This refracted light appears “sparkly,” like a kind of visual noise.
An example of anti-glare microstructure, from Nuijs and Horikx (1994).1
A key challenge in the manufacture of AG displays is to ensure sufficient variation in the AG surface to reduce unwanted reflections, without sacrificing the clarity and resolution of the displayed image through the AG layer.
Measuring Sparkle in AG Displays
To mitigate the sparkle effect while preserving the anti-glare benefits for display users, automakers and display manufacturers need to measure perceived sparkle to ensure a satisfactory and safe user experience. A method for sparkle measurement should provide a repeatable sparkle value that consistently reflects the perceived severity of sparkle as ranked by human observation. This value can be used to set tolerances in automated quality control that are consistent with human perception. Such a measurement can be used not only in display production, but also in supplier qualification (IQC or OQC) by OEMs who source third-party displays for integration.
Until recently, the lack of a repeatable measurement method for sparkle has been a barrier to defining standard quality control processes. Based on extensive lab testing at its headquarters facility in Redmond, Washington, U.S.A.—as well as studies at automotive OEM customer facilities—Radiant has defined a standard sparkle measurement method.
Our approach produces quantifiable results that consistently correlate measured sparkle to human visual perception of quality in AG displays, enabling automakers to meet objective quality standards using automated display test processes. Using high-resolution ProMetric® Imaging Colorimeters and Photometers, the Radiant Sparkle Measurement Method is the first to allow OEMs to set a numeric tolerance (for instance, less than 2% measured sparkle) for displays supplied by their vendors, ensuring a consistent level of quality across all displays regardless of supplier, product, or time and location of testing.
Sparkle test set up with the ProMetric® I Imaging Colorimeter.
Key Factors in Effective Sparkle Measurement
To define a standard sparkle measurement method, Radiant tested multiple variables to determine which measurement system parameters and test conditions yielded the most repeatable sparkle values. The variables we tested were system resolution, moiré patterns, system focus (modulation depth), gradient, size of analysis region, detector size, and working distance.
Our study found, for example, that the sensor resolution of the imaging system used to perform sparkle measurement is not a critical factor for obtaining accurate results. Display gradient, on the other hand, can have a significant impact on sparkle measurements. Gradient may be either an inherent quality of a non-uniform display or introduced in measurement images by the measurement system optics. Because sparkle is measured as a standard deviation of luminance across a display, gradient can be incorrectly measured as sparkle, and thus a gradient filter is needed to ensure uniformity of the display measurement image before sparkle is calculated.
Standard Sparkle Measurement Method for AG Display Testing
Standardized through Radiant’s study, the Radiant Sparkle Measurement Method has been proven at automotive OEM facilities for repeatability with direct correlation to human perception. Using this method, automotive and display manufacturers can set precise tolerances for sparkle and define standards for quality control.
A key component of the solution is the TT-AutomotiveDisplay™ software module, part of the TrueTest™ Software family. It provides all of the benefits of TrueTest to efficiently perform light, color, and mura measurements with specific tests to evaluate the quality of displays integrated within the vehicle.
TT-AutomotiveDisplay relies on a ProMetric Imaging Colorimeter or Photometer to test all display characteristics simultaneously, capturing and processing data much faster and more consistently than a human inspector or a spot measurement device. The combined solution enables users to perform rapid, automated visual inspection of displays integrated in dashboards and consoles.
TT-AutomotiveDisplay™ software outputs the visible effect of sparkle (standard deviation of pixel-level luminance variation) for each region of a display, then calculates an average across the display. This value can be passed or failed based on a human visual tolerance for display quality.
To learn more about sparkle measurement and Radiant’s study identifying standard test system specifications and measurement parameters, watch our recent webinar: “Defining a Sparkle Measurement Standard for Quality Control of Anti-Glare Displays.” In the webinar, Matt Scholz, Automotive Business Leader at Radiant Vision Systems, presents a method for repeatable sparkle measurement across users, devices, and systems, with quantifiable results that correlate to human visual perception of display quality. Topics covered include:
- The effect of sparkle on visual display quality
- Standard system specifications and method for repeatable sparkle measurement
- Setting a sparkle tolerance based on human visual perception
- Customer case study at automotive OEM.
- Nuijs, A., and Horikx, J., “Diffraction and scattering at anti-glare structures for display devices,” Applied Optics 33,18 (1994) p.4058