DISPLAY METROLOGY

Automotive displays must fulfill high requirements including outstanding optical performance. We report on the reduction of power consumption by image enhancement improving bright readability and by local dimming of LCDs (FALD). We also address halo and non-uniformity effects for FALD, which reduce image quality, by measurements and thresholds for perception.

In this contribution we research calibration related impacts on
MTF measurements using the Slanted Line approach in
oversampling conditions. Based on this we propose a
simplified workflow of the Slanted Line MTF method and
conclude benefits and limitations of these setup conditions

In this contribution, we qualitatively and quantitively compare the recently proposed slanted line MTF resolution measurement with the established pixel crosstalk resolution measurement. This is done by comparing evaluation results and reproducibility from measurements of nine different samples in three different setups for each method.

This paper provides an overview of image stitching and its general advantages and challenges. Further, we introduce a novel stitching concept based on our advanced pixel registration (APR) procedure. It allows easy and comparable flexible stitching setups for DeMURA and uniformity measurements in laboratory and production environments.

Modern single-pixel emitter displays such as OLED, MicroLEDs and LEDs suffer from production-related non-uniformity. The luminance and chromaticity can vary locally from pixel to pixel, resulting in a high-frequency non-uniformity and globally leading to a low-frequency non-uniformity. In order to correct these effects, luminance data of individual subpixels need to be measured. However, this is a very challenging and time-consuming task, especially for modern high-resolution displays.
To ensure a correct pixel registration (assigning the luminance to the correct pixel) in state of the art methods, display pixels are partially switched off [Patent US9135851B2]. However, this reduces cycle time and changes the average pixel level, which can affect the results.
We present a method to overcome these issues, called Advanced Pixel Registration (APR). It is based on a specific registration pattern applied during a teach-in process. An example pattern is provided in Figure 1 (left). After this initial registration, DeMURA measurements can be performed with only one image capture per input signal. The same is true for following displays during EOL testing, as small misalignments, which occur in production control environments as slight shifts, inclinations or rotations of the DUT (see Figure 2) can be corrected automatically.
This contribution validates the APR method using a flat and free-form curved display with methods similar to [] Feng, X. (2019), 78-2: Measurement and Evaluation of Subpixel Brightness for Demura. SID Symposium Digest of Technical Papers, 50: 1122-1125.]. The results show that the APR method can significantly improve the efficiency of DeMURA processes required for high-quality LED, OLED and MicroLED displays, regardless of their shape.

This contribution examines the influence of ILMD noise on the reproducibility of different sparkle evaluation setups. Sparkle measurements at different sampling rates and aperture numbers are simulated for different ILMD sensors. Especially at low sparkle levels, the SNR can become very critical for some evaluation techniques such that the number of measurements needs to be increased significantly to ensure reproducibility among the different ILMD sensors.

This contribution proposes a sparkle evaluation based on a spatial frequency filter, taking into account various setup influences. Furthermore, the effect of flexible setup conditions on the reproducibility of measurement results is investigated. The procedure and concepts are derived for sampling resolutions between 15 and 30 cpx/mm with display pixel pitches between 183 and 224 ppi and validated by a round-robin experiment with different test devices, including glass and foil-based Anti-Glare Layers. The findings serve as a basis for the measurement conditions of an automotive display sparkle measurement specification.

The alignment quality and reproducibility in ILMD (Imaging Luminance Measurement Device) based display metrology has a great influence on the reproducibility of the obtained measurement data. In this context, this contribution outlines and introduces several advanced measurement and alignment concepts that can be performed with “photometric robotics”. The term describes machine vision performed with an ILMD supported by robotic movements.

Evaluation of single emitter-based display technologies like OLED and μLED at modern display resolutions requires high-resolution measurements. The typically used oversampling often negatively affect cycle times and ILMD complexity. In this contribution, we present, explain, and validate an alternative to performing high-resolution measurements despite the Moiré phenomenon.

As head-up displays play an increasingly important role in modern vehicle cockpits, there is a growing demand for measurement procedures to characterize them. There are two general approaches to measuring virtual image distance, a parallax-based triangulation method and a focus-based technique. They can be performed using Imaging Luminance Measurement Devices with type II calibration, making them suitable for photometrical and geometrical measurements. This paper examines the advantages and drawbacks of both methods using mathematical models and measurement data.

In this contribution, we present and validate a DeMURA procedure using a one-shot approach that does not require massive oversampling. It bases on on-site calibration of the imaging condition in the setup by a specific teach pattern. It has only to be performed for the first sample. Slight misalignments of following displays are then automatically corrected. After that, the luminance of each display can be measured with a single measurement. The proposed method has the potential to facilitate high-precision calibration of pixel-level luminance under much more relaxed sampling conditions and higher speed compared to current methods.

This contribution uses the recently proposed Slanted Line approach to evaluate the MTF of a display as a measure of display resolution. It investigates the sensitivity for macroscopic imaging conditions and compares the reproducibility between four different macroscopic lenses using three different ILMDs. The samples consist of two displays combined with different structured top layers, which negatively affect the perceived resolution of the displays.

The increasing display sizes and changing form factors of displays, including automotive displays, lead to impractical measurement distances for lateral uniformity measurements. This contribution suggests and exemplarily applies two alternative and combinable methods to allow lateral uniformity measurements at low distances and describes an adjusted BlackMURA compliant validation procedure. The proposed methods are validated with a high-quality display device and are compared to results using the standard long-distance measurement procedure.

The increasing display sizes and changing form factors of displays, including automotive displays, lead to impractical measurement distances for lateral uniformity measurements. This contribution suggests and exemplarily applies two alternative and combinable methods to allow lateral uniformity measurements at low distances and describes an adjusted BlackMURA compliant validation procedure. The proposed methods are validated with a high-quality display device and are compared to results using the standard long-distance measurement procedure.

Standardization of measurements is crucial for comparing measurement results. Here, three authors from TechnoTeam describe the steps needed for the standardized evaluation of image-resolved luminance measurement systems and their relation to basic display measurement tasks.

The reproducible quantification of anti-glare layer based display sparkle with Imaging Luminance Measurement Devices (ILMD) is essential for testing and conformity assessment of many displays aiming at outdoor applications. This study systematically researches relevant setup and system influences of a Fourier space-based sparkle evaluation. These include 13 different antiglare layers, two displays with different PPI, and several different system setups featuring 12 different camera/lens combinations and more than 10000 individual luminance images. Based on the resulting sensitivities, the measurement procedure is optimized with respect to the achieved reproducibility. The resulting procedure serves as basis to define a new automotive specification for reproducible sparkle measurements and may do so for other applications, that need to quantify sparkle in a reproducible way.

Imaging Luminance Measuring Devices (ILMDs) have become indispensable in the field of display characterization. The ability to capture the entire display with one measurement and to examine it for specific characteristics is only possible with such a spatially resolving system. With the completion of work on CIE TC2-59, ILMDs can now be characterized and compared. The following paper describes the basic terms surrounding the use of ILMDs. Furthermore, the characterization will be explained with some examples and the appropriate selection of the measuring instruments will be shown with examples of some tasks with respect the display ratings (i.e. average luminance and uniformity).

This contribution evaluates a frequency-filter based sparkle evaluation and investigates the compromise between reproducible measurement results and flexible setup conditions. It bases on measurements with two displays and 9 AGLs. The findings serve as basis for the measurement conditions of an upcoming automotive display sparkle measurement specification

The variety of different measurement tasks that can be performed with Imaging Luminance and Color Measurement Devices (ILMD/ICMD), also called luminance cameras, is constantly increasing with the number of different specialized lenses. One examples is the one-shot quantification of luminance as a function of angle with conoscopic lenses. Other examples are special AR/VR lenses used to quantify near eye displays [1]. For spatial measurements, high magnification lenses can be used to quantify luminance characteristics at the subpixel level. This includes microscopic and macroscopic lenses.
In this contribution, we will show exemplary results on various spatial measurements obtained with microscopic and macroscopic lenses. The analyzed displays are an automotive LC display with a PPI of 224 and an OLED smartphone display with a PPI of 458. Figure 1 shows examples of a captured luminance image of both displays and both lenses in a deep grey state with the full field of view of the respective lenses.
We then use a method similar to [2] to analyze the crosstalk between different sub-pixels as a measure for image sharpness [3]. Here, we modify the crosstalk by applying different anti-glare layers (AGL), which are important components in both applications since both automotive displays and smartphones are used in direct sunlight. Figure 2 shows a zoomed-in microscopy-based luminance image of the OLED display with and without AGL on a logarithmic scale. Finally, we analyze the capabilities of both the macroscopic and microscopic lens to quantify the resolution and image quality parameters for both displays.

With increasing performance parameters and decreasing costs OLED displays are getting more relevant for premium automotive application. Therefore, the German Flat Panel Forum (DFF) extends its current LCD specification and measurement methods to also cover OLED displays. Challenges and solutions for lifetime, Burn-In (Image Sticking) and viewing angle procedures are presented more in detail.

Measurements with conoscopic lenses together with ILMDs (Imaging Luminance/Color Measuring Devices) can support the characterisation of displays and materials during the R&D and production process. Using a conoscopic lens the angular dependence of luminance/color can be measured with only a single measurement. Using different geometrical relations between the device under test, the conoscopic lens and different illumination conditions a lot of display and material properties can be measured with one or a few measurements only. The article shows the working principle of conoscopic lenses in detail. Furthermore the measurement of the angular dependent luminance/color, the angular dependent contrast and the measurement of selected angular dependent material properties (e.g. transmission and reflection characteristics of glasses) are presented. For all applications the advantages and limitations of the conoscopic lens measurements are explained to suggest the right fields of application for this measurement method.

Modern automotive displays may be sensitive to static content, which remains as an undesired effect of either temporary vanishing or being a static ghost image within the refreshed content. Therefore, there is a development of measurement procedures to quantize the degree of image sticking [1- 6]. Different aspects of these methods as the grey level dependencies or the importance of a temp-oral alignment were considered in [7]. But the mathematically necessary separation of unavoidable initial non-uniformities and the actual image sticking was excluded in that research.
This contribution concentrates on the performance of two selected image sticking evaluation methods [5, 6] from the automotive community and [1] for reference. After briefly introducing the three methods, this contribution focusses on their capability of separating initial non-uniformities from the actual sticking image effect of the target display. Therefore, a mathematical analysis, which is based on a simple but physically motivated sticking image model, is performed. Based on that, an additional non-uniformity correction is proposed. This additional correction has a positive influence on the precision but a negative influence on the measurement time of the fastest measurement method [6]. Thus, we propose a workflow, that decides based on the properties of the DUT whether the correction is necessary or not. All conclusions are supported by simulations and validated using measurement results of a randomly chosen non-uniform automotive LC display.
The aim of this paper is on the one hand to quantize the mathematical influence of the methods and on the other hands to suggest a workflow, which utilizes an existing method and optimizes its application with respect to precision and overall measurement time.

There are several optical display attributes, which require optical and photometric measurements during R&D activities or quality control and conformity assessment for automotive applications. The aim of this research paper is to assess typical properties of luminance based display measurements with respect to different optical display attributes. The measurement properties include on the one hand characteristics of the measurement equipment and on the other, the characteristics of the measurement method.
Based on the general working principle of ILMDs (imaging luminance measurement device), we briefly explain typical ILMD characteristics as well as the concepts of repeatability, reproducibility, precision and measurement uncertainty [1]. Afterwards, optical display properties as well as measurement methods [2, 3, 4] are introduced.
Finally, we qualitatively and quantitatively assess chosen influences of ILMD specifications and different measurement procedures on the obtained results.
Thus, this work shall convey a feeling for photometric display metrology requirements under the consideration of the measurement task and the desired precision

More and more, the suppliers of near-to-eye displays (NED) are seeking cost-effective imaging luminance and color measurement solutions that are adaptable to their instrument structures. Here, the authors offer a set of formulas that determine the basic parameters of imaging luminance and radiance measurement devices and lenses for different NED-concepts.

Imaging Luminance and Color Measuring Devices (ILMD, ICMD) also called Array Detectors in combination with adapted measuring lenses provide effective one-shot solutions to evaluate modern Near-Eye Displays (NED). Basic parameters of those lenses depend on the NED-arrangements under test.

Corresponding equations are deducted. The application of two new measuring lenses shows a way to generate a series of measuring lenses.

The “Uniformity measurement standard for Displays”, which is used for automotive applications, describes precise setup and alignment procedures to ensure reproducible measurement results. However, the influences of the tested device and the ILMD are not considered in detail. This contribution shows experiments and simulations to estimate these influences as well.

Image Sticking is an undesired display property, which requires time-consuming and expensive testing. This study summarizes some proposed Image Sticking evaluation methods and identifies relevant differences. It derives aspects, which should be considered prior to a time-consuming and potentially irreversible Sticking Image measurement. Further, parameter analysis associated with the Imaging Luminance Measurement Device is briefly outlined.

This contribution aims to describe the challenges associated with the characterization of NED (Near-Eye Displays) using Imaging Luminance and Color Measuring Devices. Based on the special attributes of NED systems, the requirements for the measurement set-up are deduced. Then two special lens designs, the hypercentric lens and the front stop lens, which both overcome these challenges, are introduced. This includes their concepts, applications and presentation of exemplarily obtained results.

To decide whether a display works according to the specification it is not only necessary to take the measurement values into account. The measurement uncertainty also plays a significant role in this decision. Besides some basic knowledge about the measurement uncertainty evaluation, the paper shows the main contributions and their influences for the key values of the BlackMURA standard. Using the rules of conformity assessment, the use of tolerance intervals and acceptance intervals is explained in detail to give all parties (OEMs, manufacturer of display and measurement system manufacturer) the chance to discuss their quality metrics in a reasonable manner.

The requirements of automotive displays differ to a large extent from consumer and industrial displays. In order to reduce the effort in specifying and evaluating high quality displays, German automotive OEMs, Tier 1/2 and the German Flat Panel Forum have launched a cooperation.

A method is proposed that characterizes the loss of image clarity caused by anti-glare coatings. The method uses high resolution imaging, giving a distribution of scattered light that can be related easily to the clarity impression of the human eye. Results of different AG treated cover glasses are shown and a new parameter is proposed to characterize the loss of image clarity. This can be applied to the specification and characterization of the cover glasses as well as the characterisation of the final product for quality assurance.

We introduce the scattering-induced spreading of a narrow linear light source in transmission as an alternative method for characterization of image blurring induced by anti-glare layers. This novel method is compared to the recently presented concept of pixel crosstalk and to the transmissive scatter distribution function of anti-glare layers measured in two different ways. The reduction of transmittance modulation is presented as an additional method for characterization of image blur. The ranking of the results is compared to a generalized haze level of the AG samples as obtained from the transmittance distribution functions measured with two different methods.