Display Metrology and Image Quality Are Relevant, Vital for Development

Display Metrology and Image Quality Are Relevant, Vital for Development

Display Week 2018 featured a variety of exhibits, seminars, and symposia on topics related to display metrology and image quality. These topics are (or should be) of interest to anyone who’s serious about developing displays or integrating displays into a system where visual performance is of paramount concern.

by Tom Fiske

THE adage “If you can’t measure it, you can’t improve it” certainly applies to the optical performance of front-of-screen displays. Valid and robust display characterization is crucial to the timely development of display technology and of devices that use that technology. In fact, it’s a critical component of the “fail fast” methodology of product and technology development. Engineers need trusted data to succeed. Data that confirms a null result is just as valuable as data that supports an idea, if not more so. Display metrology continues to be an important component of SID’s Display Week, as evidenced by the strong metrology showing in 2018 at the symposium, in seminars, in standards meetings, and on the exhibit floor. But there’s a lot more to display metrology than pointing a luminance meter at the screen and pushing a button. Good data comes from understanding the idiosyncrasies of different display technologies, proper measurement techniques, and a competent analysis and understanding of the measurement result and how it relates to the purpose of the display in the system.

Metrology on Display

The exhibition floor at Display Week sported many examples of display-measurement devices and systems to aid enterprising engineers in their search for measured truth.

Gamma Scientific impressed with tightly packaged GS-1160 and GS-1160B handheld and portable spectroradiometers. The GS-1160, in particular, is a marvel. It detects the spectral power distribution of a 10-mm spot via contact or proximity measurement. It also measures temporal quantities, such as flicker as specified by the Japan Electronics and Information Technology Industries Association (JEITA) and Video Electronics Standards Association (VESA) It can be operated in stand-alone, battery-operated mode or in a connected mode via USB. It comes with software to operate the device in connected mode and reports spectra, luminance, chromaticity, and temporal results. A display on the device facilitates a quick readout of the results.

In a similar vein, Konica Minolta showed its newly released CA-410 Color Analyzer. The CA-410 is the follow-up to the CA-310, a staple in many of the labs I’ve worked in over the years. The CA 410 is a filter colorimeter that’s very fast and has a high-luminance dynamic range (0.001 to 5,000 cd/m2, depending on the configuration). It also measures JEITA flicker and is designed to be used in a production environment. It can be purchased with a CA-DP40 data processor that can operate 10 CA 410 optical heads at once for multipoint measurements. It can also be operated from a PC or Mac using the CA-S40 software package.

TechnoTeam Bildverarbeitung GmbH (located in the German Pavilion) was showing its “LMK 5 color” imaging colorimeter with accompanying software and system components. The LMK 5 color is similar in principle to the devices offered by Radiant Vision Systems in that it’s a monochrome 2D sensor combined with a color filter wheel that matches the CIE color-matching functions. The resulting data sets are color and luminance maps of whatever object is captured by the imaging system: displays (LCDs, OLEDs, augmented reality/virtual reality [AR/VR]), light sources, illuminated symbology, etc. TechnoTeam offers a variety of sensor resolutions and lenses for spatial measurement as well as a conoscopic lens for angular characterization. The accompanying analysis software can report parameters for uniformity, image retention, and mura artifacts, to name a few. I was personally interested in the mura analysis feature, BlackMURA, which conforms to the Deutsches Flachdisplay-Forum (DFF, or German Flatpanel Display Forum) standard, “Uniformity Measurement Standard for Displays, v1.2.” There are many mura analysis routines, including BlackMURA, one based on the International Display Measurements Standard (IDMS) Section 8.2.3, the proprietary TrueMura software sold by Radiant Vision Systems, and one from Konica Minolta. I’d like to see a “mura shoot-out” sometime to determine which one is most effective at finding and correctly classifying mura artifacts. Anyone game?

One of the most interesting things I saw on the floor this year was at the ELDIM booth. Start-up company United Visual Researchers uses spectral and polarization measurements from any surface at all angles and then uses the information in a powerful graphics engine to render the appearance of the surface in a wide variety of ambient lighting environments. In the context of displays, this means that you can measure the spectral emissive properties of the display in a dark room, then measure the reflective characteristics of the front surface of a display and the reflectance characteristics of the bezel and backside (at all angles). After feeding the device optical characteristics to the model (and letting the model do a few calculations), the user sees what the display will look like in a variety of scenarios: on a city street on a cloudy day, in the park on a sunny day, in the office, at home – from any angle. The model allows professionals in R&D, engineering, marketing, and management to see how design choices will affect the appearance of a proposed display device. The rendering engine will operate in real time – that is, the user can move the device around in the virtual environment and see the immediate result of a change in viewing angle, a device brightness adjustment, or some other design change. This seems to me to be a very powerful engineering and business tool for display development.

Seminars Measure Up

Seminars and Short Courses at Display Week give attendees the opportunity to broaden their knowledge of recent and prominent topics in display technology. SID brings together experts in various fields of technology and makes them available for those wishing to develop their expertise in these areas. These are marvelous opportunities for career development and not to be missed.

At the Monday Seminars this year, Michael Becker from Instrument Systems gave a wonderful overview of display-measurement foundations in his seminar, “Display Metrology: Basics, Framework, & Applications.” He reviewed the basic definitions of the field; i.e., display metrology is the measurement of the electro-optical properties of electronic visual displays. There are four main organizations dealing with different aspects of light and display measurement: International Commission on Illumination (CIE), International Electrotechnical Commission (IEC), International Organization for Standardization (ISO), and International Committee for Display Metrology (ICDM). One must be mindful of the intended application scenario, the type of display technology in use, the parameters to be measured, the type of measurement instrument most suited to the task, the most effective way to present the data, the ambient conditions of the test, and other factors. Becker did a great job in defining the basic types of display technology, different types of measurement instruments, perceptual limit considerations, resolution, and how to evaluate reflections. All in all, it was a great introduction to the field of display measurement and well worth attending. The notes make a good reference as well.

James Larimer of Image Metrics gave a nice seminar called “Display Color Standards and Image Quality.” He provided a brief history of color imaging and talked about why we need a standard for color: standards determine the signal coding and primaries for image capture; they set rules and formats for transmitting imagery data; the image data must be decoded and the imagery reconstructed, rendered, on the display screen; and though not generally part of a standard, color management systems enable display of images in different modes to address the desires and requirements of the viewer and to differentiate display products. The seminar focused on color gamut and color space definitions and characteristics and the electro-optic transfer function – or how gray levels are rendered and black-and-white levels defined.

“High-Dynamic-Range: A Consumer Ecosystem” is the title of the seminar given by Timo Kunkel and Rob Wanat from Dolby Laboratories. Dolby does have a definite point of view regarding HDR, and it has proprietary IP for handling HDR content as part of its licensing business model. But Dolby’s seminars and papers are always well presented and backed up by good research done by knowledgeable engineers. Kunkel and Wanat enriched their talk with a description of how the human visual system sees the world and responds to high-dynamic-range scenes in the real world. This informs their approach of how to best acquire, deliver, and display HDR content. They build a good case for the Dolby Perceptual Quantizer as embodied in SMPTE 2084 as a good way to encode the electro-optic transfer function (EOTF) for HDR displays. It will be interesting to see how HDR continues to evolve as displays improve and HDR standards and pipelines are developed.

Metrology Papers of Note

Papers in the Symposium part of Display Week focus on recent advances in display technology, processing, and measurement methods. They are generally presented by the scientists and engineers from industry, academia, and other institutions who created the work. This gives everyone in the industry the chance to share knowledge of exciting new advances, and this year was no exception. We summarize here two papers – one about reflective-display measurement and one about measurement of AR/VR displays. Both topics are of intense interest in display technology today.

Dirk Hertel of E Ink gave a very informative paper, “Application of the Optical Measurement Methodologies of IEC and ISO Standards to Reflective E-Paper Displays (EPDs).” He presented an extension of the method for measuring reflective displays from the International Electrotechnical Commission (IEC) and IDMS and showed that “the optical characterization methodology specified in IEC 62679 has been extended with methods from ISO 09241 to account for unwanted, disturbing reflections. This was done by adding a glare term to the ambient display luminance model.” The IEC model includes emissive characteristics of the display – in this case, the light from an integrated front light unit on the EPD, light from a diffuse source, and light from a directed (or specular) source. The addition leverages the work of the International Organization for Standardization (ISO), National Institute of Standards and Technology (NIST), Becker, and others to include wavelength and ambient-lighting geometry to measure meaningful reflective characteristics of EPDs in relevant scenarios.

Another paper worth a look, “Standardizing Fundamental Criteria for Near-to-Eye Display Optical Measurements: Determining Eye Point Position,” by Draper, Penczek, Varshneya, and Boynton from the Army Night Vision Lab, NIST, and University of Colorado at Boulder, gave a good take on how to establish eye position for a near-to-eye display (NED). Most developers ask users to wear the NED systems, then optimize some optical feature as a function of eye position so that the eye is placed at the design eye position within the eye box of the display. This procedure is not very repeatable, will differ between users, and may result in a less than satisfactory experience for the user (Fig. 1). The authors review four different methods for objectively determining the eye-box extent and design eye position for a NED: crosshair, center luminance, center resolution, and full field-of-view method. These methods use cameras or luminance probes of various types to determine the luminance, line width, resolution, and so on of a NED as a function of eye position and orientation. It was found that all methods give reasonable results, though some are more complicated and time-consuming than others. The authors suggest that these methods can adapted for production line use.

Fig. 1:  This slide from a symposium presentation shows the wrong way to find the design eye point.

Supporting Standards Committees

Part of SID’s mission is to encourage and support the development of display standards. As part of this mission, SID makes available meeting space and food and beverage service to standards development organizations including the IEC and the ISO. There was some robust discussion at the IEC meetings around color topics this year (judging from the request for a large meeting room and conversations with participants). Administrative matters dominated this year’s meeting of the ICDM, as the committee held its biannual election of officers. SID has also offered more financial and logistical support in the next few years to help with ICDM organizational development and the publication of a follow-on version of the Information Display Measurements Standard.

The Ongoing Importance of Display Measurement

The interesting thing about display measurement and the assessment of image quality is that they touch on all display technologies – direct-view OLEDs or LCDs, reflective displays, and AR/VR systems. These devices exist to deliver photons of a certain type and spatio-temporal pattern to the human visual system. The photon patterns convey visual information in the form of human-perceivable images with the purpose to inform, warn, or entertain. This purpose can only be optimally achieved if all the subsystems in the device work together to deliver the visual image in the way the system designers intended. This means that the quality of the image has to meet certain standards. And this is where display measurement connects all things display. If display developers and manufacturers cannot guarantee that their system meets image quality standards, then why would a user buy it?

Display measurement enables display sellers and buyers across the supply chain to know that the display meets objective image-quality standards and will be able to fulfill the developer’s intent and the user’s expectation. Display measurement is where we connect human perception to objective physical quantities that can be evaluated against a threshold informed by our understanding of how people see the world. The way I see it, display measurement enables the entire display industry. So the next time you see a display metrologist, shake her or his hand and say thank you. •

Tom Fiske is currently a Senior Electrical Engineer at Microsoft Surface working on display technology, image quality, and optical metrology. He has been on the technical staff at Qualcomm, Rockwell Collins, Philips Electronics, dpiX LLC, and Xerox PARC. He can be reached at tom.fiske@microsoft.com.