Metrology and Image Quality Play Starring Roles at Display Week 2017
Metrology technology sessions and exhibits were much in evidence this year, highlighted by the first-ever metrology training course at Display Week. Image quality also took center stage, with high-dynamic-range (HDR) technology featured on and off the show floor.
by Tom Fiske
Display metrology at Display Week (and in the display community at large) has experienced a rise in prominence in recent years. This is due in part to the popularity of the Information Display Measurements Standard (IDMS) document published in 2012. The authors of the document, the members of the International Committee for Display Metrology (ICDM), now a part of SID, are hard at work preparing the next edition of the metrology methods standards volume, which is expected in the next year or two.
Display manufacturers and system integrators recognize the value of agreed-upon methods and definitions for display characterization. To be assured that display products adhere to display performance standards, the right measurements have to be made in the right way. It is therefore important to have up-to-date methods that are created and approved by experts in the display metrology field. The ICDM, in fact, has hosted robust discussions about various aspects of display metrology development at several of its recent meetings. Part of the Society for Information Display’s charter is to encourage advances in display measurement and as a part of that effort, SID provides support for the ICDM, publishes the IDMS, and sponsors educational seminars such as the training course at this year’s Display Week. In addition, SID supports the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO), which are also working on display metrology topics.
Among the many activities related to display metrology at Display Week 2017 were the display measurement sessions in the symposium program, a display metrology track in the exhibitors’ forum, several measurement companies on the exhibit floor, and a metrology seminar on Monday. And, for the first time, SID and ICDM offered an Introductory Display Metrology Training Course (Fig. 1) in parallel with the other Sunday Short Courses. The ICDM also had a booth on the exhibit floor, and the participation of many companies in the ICDM was highlighted at the exhibits.
Fig. 1: Attendees listen to a speaker at the Introductory Display Metrology Training Course at Display Week 2017. All images courtesy of Tom Fiske.
Display Metrology Training Course
The Introductory Display Metrology Training Course, organized by ICDM Chair Joe Miseli, featured a high-quality slate of speakers and a handful of hands-on demos of some representative types of light measurement devices (LMDs). The course was conceived as a service to the display community and to highlight the activity of the ICDM. It may be offered again in future years, depending on demand.
The introductory-level aspect was one of the keys for the first couple of presentations. Edward Kelley of Keltek Research covered basic radiometry, photometry, and colorimetry in his usual engaging style. This extremely useful information is the foundation for making any light-based measurement on devices that use light for making images for human consumption. Kelley’s notes from this course are a useful compilation of definitions and formulae that serve as an essential reference.
He began with the definition of light as electromagnetic radiation and moved on to the concepts of spectra, descriptions of essential geometrical considerations, and definitions of key concepts and quantities for radiometry and photometry. Along the way, Kelley did a very good job of letting his audience know when they should pay attention to the material with notations such as “!Main Idea!” as well as when listeners could safely check their Facebook feed (e.g., “Boring Alert.”) Thankfully there was a lot of the former and not much of the latter. His definitions were emphatic and clear; e.g., don’t use the term “brightness” (a perceptual construct) when you mean “luminance” (which can be directly measured). He effectively used the concepts in the beginning of the session to describe the basics of how optical detectors work, how to characterize an LED, and how to understand the subtleties of characterizing color gamut and color volume.
Michael Becker from Instrument Systems, one of today’s leading experts on display metrology, also gave a very informative talk on the basic framework for display metrology, reviewing the definitions of and motivations for display metrology; i.e., the science of metrology as applied to electronic visual displays for the purpose of establishing the usability of a display for a particular situation. He described the main standards organizations that are most relevant to display metrology: the Commission Internationale de l’Eclairage (CIE) for fundamentals of photometry and colorimetry; the IEC for applicability of display components and systems; the ISO for usability and ergonomics; and the ICDM for practical methods and tutorials.
Becker also discussed the importance of knowing the intended application scenario for a display system. This drives which characteristics need to be measured and what thresholds should be met to satisfy the usage requirements. He reviewed the various components of a comprehensive display measurement system. One can measure electro-optical, spatial, angular, and temporal quantities as well as how these parameters vary as a function of ambient conditions (e.g., temperature, age, lighting conditions). The following questions were also addressed: How should those measurements be reported? Tables, graphs, pictures, or a combination? What test patterns should be used? Becker continued with an overview of different types of measurement devices: spot meters, spatial colorimeters, and detectors for temporal and angular measurements. The last section covered best practices (e.g., if in doubt, repeat the measurement) and cautions (e.g., make sure the measurement conditions are well described).
Becker also gave a seminar on display metrology as part of the Monday Seminar series. Although there was some overlap of material, Monday’s session included more details about pixel resolution, various types of angular and temporal measurements, and reflectance measurement considerations.
Hands-On LMDs
The hands-on demos of different types of LMDs at the training course were well received. Represented types of LMDs included a colorimeter/photometer from Konica Minolta, a spectroradiometer from Gooch and Housego, and an imaging colorimeter from Instrument Systems (Fig. 2). This segment of the course gave attendees insight into the practical considerations of how to mount displays and LMDs, as well as the trade-offs among types of instrumentation. There is no substitute for being able to try something out yourself.
Fig. 2: The LumiTop 2700 from Instrument Systems, included as part of the hands-on demonstrations in the display metrology course, combines spectral accuracy with the convenience of 2D color imaging.
Subjects that were prominent the whole week of the show also showed up in the course: augmented reality and virtual reality (AR/VR) display systems. The AR/VR display characterization session given by Tom Lianza from Photo Research was very well attended. He recounted lessons learned as he told the story of the processes he followed to eventually capture good measurements for AR and VR display systems.
Lianza began with the four distinct categories for measurement: VR (near-eye display and no ambient illumination); AR (near eye and some ambient); digital eyeglasses (near eye and full ambient); and automotive/avionics (head-up display and full ambient). All of these display types produce a virtual image – there is no physical structure producing light at the apparent image location. This is important because virtual images require more care to measure accurately due to the limited volume (aka eyebox) over which the images can dependably be viewed. Some of the most important challenges involve matching the optical characteristics of the exit pupil of the display with those of the entrance pupil of the LMD. Lianza suggested a fixed-measurement-aperture solution for some situations.
He also discussed how the LMD had to be designed, adjusted, and used to give accurate and valid results – presenting various real-world examples that were especially useful. He emphasized a common theme in measurement practice – repeat your measurements and make sure your results are sensible for the given situation. (I can heartily endorse this practice – as learned from hard experience.)
Other topics of the day included measurement considerations for high-dynamic-range (HDR) displays, and displays for automotive applications. A special bonus session was included (with wine and cheese!) on reflectance measurements. For this presentation, Edward Kelley returned to give us a good grounding in reflectance basics and their relevance to display characterization. Kelley began with precise definitions of canonical reflection terminology, including “reflectance factor,” “luminance factor,” “diffuse,” “specular,” etc. It is very important to rigorously define your light source, sample, and LMD geometrical arrangements, since these will critically impact the accuracy, repeatability, and applicability of your results. Kelley also reviewed the four types of reflectance found in a display context: specular, diffuse, haze, and matrix scatter.
The bidirectional reflectance distribution function (BRDF) is the complete geometrical and wavelength-dependent description of how an object reflects light. Unfortunately, high-resolution BRDF is very difficult to fully characterize. Fortunately, the reflective characteristics of a display can be adequately measured in many situations with much simpler methods. Kelley’s practical guidelines for the acceptability of such methods are that they must be robust, reproducible, and unambiguous. He finished his presentation by describing many different reflectance-measurement methods and ranked them against these criteria. It was another great talk and the material provided will prove to be very useful reference information.
HDR Displays
HDR was a popular topic at Display Week. Judging from the technical presentations at the show, no one technology or company has a lock on HDR displays, the HDR ecosystem, or even on how to describe HDR performance. Standard-dynamic-range (SDR) displays (the ones that we’re used to seeing every day on our TVs, PCs, tablets, and phones) have a maximum luminance of around 300 to 500 candelas per square meter (cd/m2) and a black-to-white luminance range of about 1,000:1 for LCDs – OLEDs can be much higher. HDR promises more realistic and exciting images with more black-to-white luminance range and wider color gamut. The main technology contenders are OLED displays, dual-modulation LCDs, and dual-modulated projection light-valve systems. The ecosystem includes Dolby Vision and ACES encoding and delivery schemes, among others. The measurement protocols and standards for HDR are still in development.
OLED displays are still fairly new (and therefore expensive), but deliver a superb HDR experience. The black levels are amazing and almost unmeasurably low. The peak luminance can be up to several hundred cd/m2, but those values are limited to lower average picture level (APL). The dual-modulation techniques for LCDs generally use a local dimming backlight (i.e., a backlight that has areas with individual luminance control) as the second modulation source. These require sufficient backlight spatial “resolution” (i.e.>, controllable areas) for good HDR performance. Depending on implementation, halo artifacts can be a problem. A second LCD panel (with a normal backlight) can also be used as the second modulation source. Projection systems need an additional light valve in the optical path as the second modulation source to render HDR images. Direct-backlit LCDs have the advantage of high peak luminance at any APL since the light level is nominally independent of the image-forming function. Wide color gamut (WCG) is realized in OLEDs due to the inherent narrow spectral bandwidths of the RGB color primaries. LCDs achieve wide color gamut by using LED backlights with quantum-dot technology to get color primaries with narrow spectral bandwidths.
All this is to say that HDR and WCG will not be inexpensive to realize. The display devices are new, the workflow is not quite settled, and the pipelines – from acquisition, processing and content delivery – are bigger and more complicated than those for SDR content.
A few items of note around HDR at Display Week: LG Display showed off a beautiful (and thin – 6 mm!) “Wallpaper” 77-in. ultra-high-definition (UHD) OLED display at its booth on the exhibit floor (Fig. 3). In fact, this display won one of SID’s 2017 Display of the Year awards. It provides a truly stunning HDR image, with black levels at an amazingly low luminance and highlights up to 800 cd/m2 at 10 percent APL. This panel will also retail at around $20,000, so it won’t be for everyone. LGD executives showed me around the rest of the booth, where they had an impressive array of display technologies and applications, including flexible and curved displays, 8K resolution, HDR, displays with integrated speakers, and transparent displays.
Fig. 3: LG Display’s 77-in. “Wallpaper” OLED was difficult to miss on the Display Week 2017 exhibit floor.
In the technical program, authors from the University of Florida and AU Optronics delivered a paper, “High-Dynamic-Range LCD With Pixel-Level Local Dimming,” which describes a system with a fringe-field-switching (FFS) LCD cascaded with a twisted-nematic (TN) LCD with no color filter to give 1,000,000:1 contrast ratio (CR) and good angular performance. The display boasts a bit depth of more than 14 bits. A polarization preserving diffuser is introduced between the panels to eliminate moiré effects and maintain CR. There are still some challenges to address, such as cost, misalignment issues, decreased efficiency, and system thickness, but this approach could be a promising method for implementing HDR.
Dolby Laboratories authors delivered the invited paper, “Prediction of Overall HDR Quality by Using Perceptually Transformed Display Measurements.” Using a local dimming LCD system, measured parameters such as maximum luminance, minimum luminance, backlight resolution, color gamut, and bit depth were used in two different models to predict HDR display performance as compared to subjective ranking across variations in the above parameters. The physical model used the directly measured parameters. The perceptual model was based on a perceptually transformed set of analogous parameters. The authors provided a compelling case for an HDR quality metric that uses the perceptually transformed parameters. I would like to see this work extended beyond local dimming LCDs to include OLEDs as well.
Dolby authors also delivered a good Monday seminar on HDR. Dolby is obviously invested in a particular HDR ecosystem (Dolby Vision) and technology (dual-modulated LCDs). However, the company’s proposals and positions are always well presented and are backed by top-notch human factors research and display metrology. This is much appreciated.
A couple of other noteworthy HDR papers from a session called “Visual Quality of HDR Displays” included one from Samsung on the “Visual Quality of a Global Dimming Backlight with High-Contrast Liquid-Crystal Panel for High Dynamic Range Displays” and one from University of Central Florida on “Reproducing High-Dynamic-Range Contents Adaptively Based on Display Specifications.” The Samsung paper showed that a system with a global dimming backlight with a high CR LCD was preferable to one with a local dimming backlight with a moderate CR LCD. The UCF paper showed that an SDR display can reasonably render HDR content with an appropriate gamma and color-mapping algorithm and a dynamic range of 0.1 to 640 cd/m2.
HDR remains an active field of research and development. This is true on a few fronts: display technology, the delivery pipeline, and the most effective description of visual performance. So, no matter what your favorite flavor of HDR is or how you want to describe it, it looks like there’s still opportunity to make a contribution.
It’s still too early to tell which HDR ecosystem and workflow model will be widely adopted. For the consumer’s sake, one hopes that the industry will settle on a solution that has enough commonality to avoid a Blu-ray/HD-DVD debacle. Apart from which technical implementations, ecosystem, and workflow models prevail, the metrology around HDR needs some development. Pursuant to that need, there is a large group within the ICDM that is hard at work on defining and standardizing HDR measurement methods.
Among other areas the ICDM will address in the coming months is metrology in AR/VR displays. This will definitely be a hot area, as evidenced by the interest in AR/VR during the metrology course on Sunday and the prominence of the AR/VR papers during the symposium sessions. There is an obvious need for manufacturers of HDR and AR/VR technology to support the advancement of display metrology that will enable the delivery of great customer experiences in these areas. So get involved and contribute to the work of the ICDM and submit those groundbreaking technology papers to Display Week and the Journal of the SID. •
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.