Tablets Are Trending Better, Bigger, and Brighter
The key element for a great tablet has always been a truly innovative and top-performing display, and the best leading-edge tablets have always had beautiful displays. But tablet displays are challenging to produce because of their large screens, which are 3–4 times the size of smartphone screens. DisplayMate’s Ray Soneira discusses the major factors affecting tablet performance and popularity, as well as what improvements we are likely to see in the near future.
by Raymond M. Soneira
STARTING IN 2010, when Apple made the Retina display and display-quality central themes for its product marketing, displays have experienced an unprecedented renaissance in new technologies for smartphones, tablets, TVs, and even new classes of products such as wearable displays. Apple has recently given up the lead in displays – now Amazon, Google, LG, and Samsung are launching products with the best and most innovative displays, as documented in our recent Display Technology Shoot-Out article series.1 The new iPad mini Retina display, for example, recently came in a distant third place finish.
This article looks at several of the most recent mobile-device display enhancements in terms of tablets, a product category not invented by Apple, but which gained large-scale commercial success only after the introduction of the iPad in 2010. Hopefully, Apple will join the leaders again with new and innovative displays in 2014. In the meantime, here is a roster of display advances that involve tablets.
Full Color Gamut
Up until recently, most LCDs had only 55–65% of the standard sRGB/Rec.709 color gamut that is used in producing virtually all current consumer content. This resulted in subdued colors in images, videos, and photos, a limitation that stems from the traditional reduction in brightness and power efficiency as the
color gamut is increased for LCDs. This limitation can now be overcome using quantum-dot technology, which can efficiently enlarge the color gamut to greater than 100% for high ambient light and other applications. (For more about quantum dots, see the article, “The Virtues of Quantum Dots,” in the May/June 2014 issue of Information Display.) Most leading LCDs now have color gamuts greater than 85%, with the best close to 100%. This has been accomplished with a number of innovative (and costlier) approaches including using low-temperature polysilicon (LTPS) backplanes, brighter and more efficient white LEDs in backlights, and better optics and optical films. Other methods include trading some peak brightness for a larger color gamut, and, lastly, just using heftier batteries to get the job done. The most notable laggards in 2013 were the iPad mini Retina display and the Microsoft Surface 2, both with just 63% of the standard gamut. In 2014, quantum dots will play a major role in continuing to improve the color gamut of LCDs.
Improved Absolute Color Accuracy and Picture Quality
Image and picture quality, as well as color accuracy, have been steadily advancing due to improved display technology, advanced signal processing, automated factory calibration, and the motivation of increased competition. DisplayMate’s most recent lab tests and measurements show that the very best smartphones, tablets, and TVs are now comparable in accuracy to professional studio monitors. With good accuracy you will see high-fidelity presentations of your digital photos, which is especially important because you often know exactly what everything should actually look like. Another perhaps even more important advantage is that online merchandise will appear with accurate colors, so you will have a very good idea of exactly what you are buying and are less likely to return it. Potential downsides include on-screen food that will make you particularly hungry and shows, movies, and downloaded content that look so great you will find yourself watching more of it!
If you have ever wondered why some colors are way off on a display, there are many contributing factors and causes, including the color gamut, the calibrated white
point, the intensity scale, and possibly poorly implemented dynamic picture processing and color management. It is possible to accurately measure and map the absolute color accuracy and color errors for any display by using a spectroradiometer and proprietary test patterns, which we do in our Shoot-Out series. We provide both the average and maximum color errors in terms of Just Noticeable Color Differences (JNCD) and include a full chromaticity color accuracy map, which is shown in Fig. 1. Color differences less than 1 JNCD are visually indistinguishable, while values greater than 1 JNCD are visually noticeable when the two colors are touching on-screen. When the colors are not touching and are farther apart, the visual threshold for just noticing a color difference on a display is higher. Here, we will use 3 JNCD for the threshold of a visually noticeable color difference on a display.
Fig. 1: Shown is the absolute color accuracy map for the sRGB/Rec.709 reference colors.
In 2013, the most accurate display we measured was the LG OLED TV, with an average absolute color error of just 1.3 JNCD, which is visually indistinguishable from perfect. Figure 2 shows absolute color accuracy plots for the Amazon Kindle Fire HDX 7 and Google Nexus 7, and Fig. 3 clearly shows the much larger color errors for the Apple iPad mini Retina Display.
In 2014, we expect to see major improvements in absolute color accuracy resulting from the use of quantum dots as well as from improved color-management processing.
Figs. 2 and 3: Figure 2, on top, shows the Kindle Fire HDX and Nexus Absolute Color Accuracy Error Plots and Fig. 3, on bottom, shows the result for the iPad mini Retina display.
LTPS and IGZO
All LCDs and OLED displays have an internal backplane layer that has the electronic circuitry needed to control the millions of subpixels. The backplane’s performance is especially critical in high pixels per inch (ppi) displays. While most LCDs still use amorphous silicon (a-Si), many high-ppi LCDs use low-temperature polysilicon (LTPS), which has considerably higher electron mobility than a-Si, allowing the circuitry to be made much smaller. For LCDs, the electronic circuitry for every subpixel takes up precious screen area, which blocks the backlighting and decreases image brightness. LTPS results in significantly higher luminance and improved power efficiency, but also costs considerably more to manufacture.
Most high-performance smartphone displays now use LTPS, including the iPhone 5 and all mobile OLEDs (except flexibles). But most LCD tablets, monitors, and TVs still use a-Si because their larger screens would cost considerably more to manufacture using LTPS. An alternative backplane technology called IGZO (indium gallium zinc oxide), which has higher performance than a-Si but lower cost (and performance) than LTPS, was supposed to be available in 2012 for tablets, monitors, and TVs, but
major production problems have significantly delayed and limited its availability even in 2013. This will continue through 2014, as Sharp is the only major manufacturer that is beginning to ship IGZO displays.
The two highest-performance tablet displays that we recently tested, the Amazon Kindle Fire HDX 8.9 and Google Nexus 7, were the early adopters of LTPS for tablets, while the latest iPads still rely on lower performance a-Si and IGZO, which limits their brightness, color gamut, and power efficiency. For 2014, the continued problems with IGZO will benefit both LTPS and the much higher-performance metal-oxide backplanes now under development by CBRITE, which should begin arriving by late 2014.
While 2013 included a major shift to smaller 7–8-in. tablet displays from the first-generation full-size 9–10-in. displays, 2014 will see tablets growing again to include 12–13-in. tablet displays for the professional and education markets. Microsoft has the 12.0-in. Surface Pro 3, Samsung has the 12.2-in. Tab Pro, and Apple is rumored to be producing a 12.9-in. iPad. The screen resolution and ppi are also increasing, with high-end Android tablets moving up to 2560 × 1600 or Quad HD (QHD) at 2560 × 1440 with 300–340 ppi based on screen size, and current Apple iPads at 2048 × 1536 with 264–326 ppi. However, a 12.9-in. iPad would only have 198 ppi, so to enable a Retina display another higher-resolution jump to perhaps 4K, at 4096 × 3072 with 398 ppi, seems likely. High-ambient-light performance will also continue improving, with the 2013 record holders Amazon Kindle Fire HDX 8.9 with a very low 5.0% screen reflectance, and the Nokia Lumia 2520 with a very bright 684-cd/m2 display.
Also expected in 2014 are some high-resolution OLED tablets and many more LCD tablets with LTPS backplanes and quantum dots that will deliver very bright and wide-color-gamut images as mentioned above. At press time, DisplayMate had just completed a review of the OLED-based Galaxy Tab S tablets, the new OLED flagship units from Samsung.2 (For more information about the Galaxy Tab S line, see the sidebar in this article.)
The most important developments for the upcoming generations of both OLED and LCD mobile displays will come from improvements in their image and picture quality
in ambient light, which washes out screen images, resulting in reduced readability, image contrast, color saturation, and color accuracy. The key will be in enlarging the native color gamut and then dynamically changing the display’s color management and intensity scales with the measured ambient light in order
to automatically compensate for reflected glare and image washout from ambient light, as discussed in our 2014 Innovative Displays and Display Technology article online3 and the “Tablet Display Technology Shoot-Out” article in the July/August 2013 issue of Information Display. The displays and technologies that succeed in implementing this new strategy will take the lead in the next generation of mobile displays.
In 2013, the Amazon Kindle Fire HDX tablets became the top-performing tablet displays in our Display Technology Shoot-Out series, leapfrogging the competition with cutting-edge displays using quantum dots and LTPS. But with the ever continuing and impressive improvements in display technology, the OLED Samsung Galaxy Tab S has now taken the lead for the best tablet display. With display technology advancing rapidly on many different fronts, things can change again in the next generation of displays for tablets and smartphones. A strong congratulations to Samsung, but please do not rest on your laurels – and best wishes to all the manufacturers in developing the next generation of even higher-performance displays!
Up until now, tablets have been almost exclusively LCD based. There have not been any OLED tablets, with the exception of a single 7.7-in. model from Samsung launched in 2012.
Samsung is now producing the Galaxy Tab S series (see Fig. 4), with display performance widely expected to be comparable to that of the OLED Galaxy S5, which is the best smartphone display we have ever tested.
Fig. 4: Samsung’s new Galaxy Tab S tablets are OLED based, which provides excellent performance
in terms of viewing angles. Image courtesy Samsung.
Samsung provided DisplayMate with pre-release production units of the Galaxy Tab S tablets in both 10.5- and 8.4-in. form factors. Here are a few highlights of the testing results:
• Both Galaxy Tab S models offer Quad HD 2560 × 1600-pixel displays, currently the highest resolution for tablets, with 4.1 Mpixels – double the number on your HDTV. The 10.5-in. model has traditional RGB-stripe pixels with 287 pixels per inch (ppi), and the 8.4-in. model has Diamond pixels with a proprietary diagonal, symmetrical arrangement for the red and blue subpixels and subpixel rendering with 361 ppi. Both are higher than can be resolved with normal 20/20 vision at the typical viewing distances for tablets, so the displays appear perfectly sharp.
• Excellent absolute color accuracy in the Basic screen mode with an average error of just 2.1 JNCD, the best for any mobile display.
• Both of the Galaxy Tab S displays have very good to excellent screen brightness, but are not as bright as the brightest LCD tablets.
• While tablets are primarily single-viewer devices, the variation in display performance with viewing angle is still very important because single viewers
frequently hold the tablet at a variety of viewing angles – often up to 30° or more when resting it on a table or desk. While LCDs typically experience a 55% or greater decrease in brightness at a 30° viewing angle, the OLED Galaxy Tab S displays show a much smaller 21% decrease in brightness at 30°.
In summary, the Galaxy Tab S tablets are the best-performing tablet displays that we have ever tested. More testing results can be found on DisplayMate’s web site.2