Image Quality and Metrology
Display technology delivers the photons to the front of the screen; the human vision system detects the photons and perceives an image. Measurement devices capture and analyze image characteristics and deliver objective quantities that engineers use to inform optical designs and monitor manufacturing processes.
by Tom Fiske
DISPLAY WEEK is all about the presentation and demonstration of visually stunning displays – and this year’s event was no disappointment. Set in the middle of Silicon Valley in the first week of June, the show had an energy that was demonstrably high, as evidenced by the 10–15% increase in attendance across all events. Display Week is full of opportunities to network, learn, make deals, and feast your eyes on all the shiny new displays. Eye candy is a big part of the draw of Display Week – with various claims and demonstrations of the biggest, the brightest, the thinnest, and the best. Complementary to all the hoopla on the exhibit floor, and the biggest draw for engineers and researchers, are the opportunities to report on and learn about the latest technology required to create all that eye candy.
One important field of endeavor that touches all the various visual display technologies is the application of human vision concepts to the systematic evaluation of display image quality. Critical to this application are the devices and techniques that we use to measure display optical performance. Display technology delivers the photons and images to the front of the screen (or to the exit pupil); the human vision system (HVS) is there to perceive and appreciate them. Measurement device companies create systems to capture and analyze those photons and images and then deliver objective quantities that engineers use to inform optical designs and monitor manufacturing processes.
High Dynamic Range
One of the more compelling topics around display image quality this year was high dynamic range (HDR) and extended color gamut. For HDR, there was a Monday seminar,1 an invited paper in the Imaging Technologies and Applications track,2 and a presentation at the International Committee for Display Metrology (ICDM) meeting on Tuesday evening. Dolby Laboratories, Inc., is a strong proponent of HDR, given the company’s long incubation of HDR display and the Dolby Vision architecture for the capture, distribution, and display of HDR content. Scott Daly and Timo Kunkel delivered a Monday seminar that covered the basics of HDR display technology and human vision considerations. Daly and Kunkel described how the technology delivers more than 6 orders of magnitude of luminance dynamic range – yielding bright highlights and good shadow detail simultaneously. One way to do this is with a dual modulation display. In Dolby’s case, at least for the consumer market, it uses an array of LEDs in the LCD backlight that is independently controlled and in synchrony with the image on the LCD. The result is an HDR image created by a low-resolution luminance-only image on the backlight that is combined by the high-resolution image on the LCD. This type of backlight is also known as a local-dimming backlight.
Dolby’s studies show that 90% of subjects prefer images rendered with 6+ orders of magnitude of luminance dynamic range (from less than 0.01 cd/m2 to more than 10,000 cd/m2 (see Fig. 1).3 Typical LCDs can only deliver about 3.5 orders of magnitude of dynamic range and a peak luminance of several hundred cd/m2. Dolby Vision also accommodates expanded color gamut, high bit-depth gray scale (10–12 bits per color channel), and high frame rate (up to 120 Hz). Luminance dynamic range consistently ranks at the top of the list of those image-quality parameters that most people prefer, followed by color gamut, frame rate, and resolution. In other words, if you want to spend your gold on making your image look better, spend it on improving luminance dynamic range.
Fig. 1: Studies from Dolby show that 90% of subjects prefer images rendered with 6+ orders of magnitude of luminance dynamic range.3 Image courtesy Scott Daly, Dolby Laboratories, Inc.
HDR and wide color gamut enable greater creative choices by making a larger color volume available. However, care must be taken throughout the image capture, transformation, color grading, and mastering processes to preserve luminance and color information so that the intended image can be presented in either cinema or video contexts. James L. Helman of MovieLabs delivered an invited paper2 describing the background and reasoning behind some of the standards and architectures in the capture, mastering, and rendering tasks that take advantage of HDR. One process this paper focused on is the adoption and standardization of a perceptually based gray level to absolute luminance transfer curve to replace the traditional gamma curves used since the early days of video production. A 12-bit gray ramp as embodied in SMPTE ST 2084:2014 results in no gray-level banding artifacts and handles the wider primaries proposed for use in BT.2020.
Helman reported that the Academy of Motion Picture Arts and Sciences has developed an advanced color system and digital framework called the Academy Color Encoding System (ACES). ACES promises to simplify and improve the handling of multiple cameras, films, and mastering display devices through the definition of formats and standard color transforms. This will help manage the burden of adding and preserving HDR to content as it makes its way to various display devices. While it is still early, the infrastructure is being put in place to deliver HDR-enabled content in wide distribution. The goal is to realize a video system that delivers images below perceptual thresholds with a full-gamut-color system that matches the capacity of the HVS.
ICDM Tackles Contrast and Dynamic Range
At the ICDM meeting on Tuesday evening of Display Week, Daly and Darin Perrigo reviewed various ways that luminance contrast and dynamic range have traditionally been characterized and reported. Perrigo’s presentation focused on problematic issues when characterizing contrast in front-projector systems. Daly suggested extensions of current methods from the Information Display Measurements Standard (IDMS)4 that will give more relevant and useful information. Sequential contrast ratio (luminance of full-screen white divided by luminance of full-screen black, IDMS Section 5.10) is not adequate to fully describe the dynamic-range behavior of modern displays. This is especially true in regards to emissive displays (e.g., OLEDs), for which the black state is too dim to measure accurately as well as displays that use global or local backlight dimming. ANSI (aka black and white checkerboard) contrast (IDMS 5.26), another popular dynamic-range metric, has an average luminance that is not representative of most imagery (too high), overestimates internal display flare, and underestimates perceived contrast capability.
Daly reviewed several other extant methods for characterizing contrast, including full-white-signal contrast (IDMS 5.9.1), peak contrast (IDMS 5.11), starfield contrast (IDMS 5.12), and corner-box contrast (IDMS 5.13). He concluded his remarks with a description of an extension of the corner-box contrast method by adding measurements of images in which the position and gray level of the bright boxes are varied. These have the advantage of including in the characterization some of the beneficial effects from local-dimming displays in a relevant and realistic way. He also mentioned some potential ways to account for the spatio-temporal characteristics of the HVS. High-spatial-frequency-contrast detection is limited by glare and the MTF of the human vision system.5
Extended Color Gamuts
Another aspect of adding to the color-volume capability of displays is extending the color gamut by making the red, green, and blue color primaries more saturated. The SID exhibit floor offered examples of one of the main methods for realizing this technology. The two most common methods for achieving extended-color-gamut displays are laser (or laser/hybrid) projection or LCDs illuminated by backlights using blue LEDs and quantum-dot technology. Two of the most prominent implementations of quantum-dot backlights are from Nanosys and QD Vision. Both methods use blue LEDs as the light source to illuminate quantum dots that down-convert some of the blue light to green and red light. The result is narrow spectral bands of blue light (from the LEDs) and green and red light (from the quantum dots). The narrow spectral bandwidth of the resulting light – putting spectral power only in the red, green, and blue portions of the backlight spectra – is what enables the wider primaries and extended color gamut. The Nanosys approach uses a blue-LED-backlit light guide coupled with a quantum-dot-impregnated film (supplied by 3M) to deliver the uniformly distributed blue, green, and red light to the back of the LCD panel. The QD Vision approach is exclusively an edge-lit design. The light from a linear array of blue LEDs is coupled with a strip that contains quantum dots, and the resulting blue, green, and red light is uniformly distributed to the back of the LCD panel via a light guide. The QD Vision method has a cost advantage, but may be somewhat less efficient than the Nanosys/3M approach. Nanosys claims better efficiency due to effective light recycling, and its method is compatible with HDR displays because it can more easily accommodate a local-dimming backlight.
QD Vision has announced that its Color IQ technology is in sets from Philips, Hisense, TCL, and Konka. Nanosys quantum-dot-enhanced sets are also available from Samsung and AUO. Both quantum-dot companies were well represented on the Display Week exhibit floor, with stunning demonstration sets. The images in each booth effectively highlighted the visual power of wide-color-gamut displays. Nanosys won a Best-in-Show award (for the second year in a row). It was also the only booth at the exhibit that featured performances by acrobats (!) – see Fig. 2.
Fig. 2: The Nanosys booth at Display Week featured three displays (from left to right: a conventional LCD, with quantum dots, with Cd-free quantum dots) – and acrobats. Image courtesy Nanosys, Inc.
The exhibition also highlighted several advances on the metrology hardware front. One example is the new Tru-Image series of 2D imaging colorimeters (Fig. 3) from Photo Research. These instruments feature a thermoelectrically cooled 8- or 16-Mpixel CCD with a high-speed CIE color wheel. They come with Windows-based VideoWin 3 Pro software to control the instrument and analyze the data. Measuring capabilities include 2D-based luminance, chromaticity, correlated color temperature, and CIELAB analysis.
Fig. 3: Photo Research displayed its Tru-Image 2D Imaging Colorimeter. Image courtesy Tom Fiske.
Radiant Vision Systems showed off its line of automated-visual-inspection solutions. The company has been working on fielding configurations that reduce takt time, for example, with its ProMetric I series imaging colorimeter coupled with multiple spectrometers for testing smartphone displays. Their lineup also includes imaging spheres and imaging goniometers for angular measurements.
At the Gamma Scientific booth, we saw the company’s Robotic Display Measurement System that combines a 6-axis robot and high-performance spectroradiometers for fast, accurate display measurements (Fig. 4).
Fig. 4: This Robotic Display Measurement System from Gamma Scientific featured a six-axis robot (at right). Image courtesy Tom Fiske.
Display Week is an important venue for the presentation of new display technologies and applications. A significant goal of display technology is the continuous improvement of front-of-screen image quality. The chief method used to monitor progress and verify image-quality goals is by using proper display ptical-measurement methodologies and tools. Display Week 2015 highlighted the advancements of HDR and extended color gamut and how the standards community is beginning to address these features. The exhibit featured examples of extended-gamut displays and several new display measurement tools designed to aid the engineer, technologist, and manufacturer in the pursuit of image-quality improvement.
References
1S. Daly and T. Kunkel, Seminar M-2, “High-Dynamic-Range Imaging and Displays,” SID Seminar Lecture Notes (2015).
2J. L. Helman, “Delivering High Dynamic Range Video to Consumer Devices,” SID Symposium Digest of Technical Papers 46 (2015).
3S. Daly, T. Kunkel, X. Sun, S. Farrell, and P. Crum, “Viewer Preferences for Shadow, Diffuse, Specular, and Emissive Luminance Limits of High Dynamic Range Displays,” SID Symposium Digest of Technical Papers 44 (2013).
4International Committee for Display Metrology (ICDM), Information Display Measurements Standard (IDMS), ver. 1.03 (2012); http://www.icdm-sid.org/.
5E. H. A. Langendijk and M. Hammer, “Contrast Requirements for OLEDs and LCDs Based on Human Eye Glare,” SID International Digest of Technical Papers 41 (2010). •