For over a year, the main obsession about the upcoming 2014 iPhone models (i.e. iPhone 6) has been the size and resolution of the screen. But Apple faces other challenges for its mobile touchscreens, and how, or whether, it deals with those may have a bigger impact on the user experience than simply a bigger screen.

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Apple early on pushed high resolution displays, eventually culminating in its "Retina display" technology, and emphasized color accuracy and range in bright screens. The result was almost revelatory for end users.

+ Also on NetworkWorld:Slideshow: The iPhone 6 unknowns+

In a 2010 CNET review of the iPhone 4, the Apple smartphone that introduced the 940 x 640-pixel (326 pixels per square inch) Retina display, Kent German wrote, "Though 'stunning,' 'gorgeous,' and 'dazzling' are words we usually use to describe high-quality smartphone displays, we're not sure if they do the iPhone 4 justice. Believe us that everything about this display is fantastic, from the bold colors and graphics to the vividly clear text."

But now Apple is being matched and increasingly surpassed by its rivals, who are exploiting a range of rapidly evolving technologies for touchscreens.

A "stall" in Apple display performance

"Apple displays have simply stalled in performance over the last two years while all of the other leading manufacturers have been aggressively advancing," says Raymond Soneira, president of DisplayMate Technologies, Amherst, N.H., a company that specializes in products for calibrating and optimizing displays, including those for mobile ones. "It's not just the widely quoted small screen size, but also color gamut, color accuracy, screen resolution, brightness, screen reflectance, and display power efficiency."

In 2012, DisplayMate concluded that the then-new 4-inch iPhone 5 LCD display was state of the art. The iPhone 5 offered big improvements over the iPhone 4/4s: much lower screen reflections, much higher image contrast and screen readability in high ambient light, and a much improved and accurate color gamut and factory calibration. It also outclassed the then-new Samsung Galaxy S3, an early user of still-evolving OLED technology: the S3 screen had half the brightness of the iPhone 5's, much worse performance in ambient light, and distorted or exaggerated colors and a green caste to many images.

Yet two years later, there are dramatic differences. DisplayMate's recent review of the Samsung Galaxy S5 concluded it was the "best performing smartphone display that we have ever tested." Samsung maintained the same high definition screen and resolution as in the S4, but made big improvements across the board in maximum brightness, especially in high ambient light, color accuracy, viewing angle, power efficiency and battery life. And the S5 now offers accurate user-selectable color saturation and display calibration options.

All of these are areas that Apple, which seems to be sticking with LCD technology for now, could exploit in the new iPhones, using a range of technologies.

Bigger screens, higher resolutions

Currently, it is widely believed that on Sept. 9, Apple will unveil two iPhone models, one with the 4.7-inch and one with a 5.5-inch display. There's been largely fruitless speculation about what their pixel resolution, and pixels-per-inch (PPI) will be. Last week, long-time technology blogger John Gruber, in a lengthy post at his Daring Fireball blog, offered his conjecture that the resolution for the 4.7-inch display will be 1334 x 750 pixels and 326 PPI, and 2208 × 1242 pixels and 461 PPI for the 5.5-inch display. By comparison, the current iPhone 5c/5s display is a 4-inch (diagonal) Retina display, with 1136-by-640 resolution at 326 PPI.

Gruber arrives at these numbers by analyzing the way Apple views the user interface elements of iOS [see Apple's "iOS Human Interface Guidelines"]. Apple specifies these elements not in pixels but in virtual points, with each point on the existing Retina display represented by a 2 pixel x 2 pixel square. On a 5.5-inch Retina display, it would become a 3x3 pixel square. According to Gruber, all iPhones have had the same "point resolution" of 163 points per inch. So the Apple-specified "tappable target" -- what you touch with a fingertip -- has been consistently 44 x 44 points, and exactly the same physical size in all iPhones so far.

The bottom line: in Gruber's analysis, the 4.7-inch iPhone preserves the existing 326-pixels-per-inch, but adds more pixels on both axes, creating a 38% larger viewable area than the iPhone 5 models; the 5.5-inch iPhone's higher resolution increases the viewable area by 68% compared to the iPhone 5, with tappable points that are slightly bigger than the iPhone 5, and with 461 pixels per inch for incredibly sharp renderings.

Beyond bigger screens

But these advances will have to be accompanied by other display improvements. If anything, larger screens and higher resolutions may make shortcomings -- in color accuracy, color gamut (the subset of colors, from the full range of colors identifiable by the human eye, rendered on the display), brightness, reflectance, and power efficiency -- much more apparent.

One trend is toward color gamuts that are "wider" than the defined standard. The benefit of this wider gamut becomes apparent when it's combined with real-time color management, so the gamut can be adjusted based on the current level of ambient light, according to DisplayMate's Soneira. Daylight washes out screen colors: with a wider gamut, the display in effect compensates and produces accurate onscreen colors in those conditions.

Wider gamuts and much improved color accuracy for LCDs will come from technologies such as quantum dots, which are tiny man-made crystals that act as semiconductors -- converting incoming energy. The color of light given off by a quantum dot can be changed by readjusting the dot's size. That means colors can be tuned and mixed precisely and efficiently, resulting in very pure colors, and much less power demand, according to Soneira. One QD vendor is Nanosys.

NPD DisplaySearch predicts quantum dot penetration in smartphone thin film transistor LCDs will be 3% in 2015, rising to 26% in 2020. QD prices remain relatively high, according to the research firm: for a 5-inch smartphone screen assembly, quantum dots currently could add 20% to its cost compared to a conventional TFT LCD assembly.

Backplane changes

Another possible change is in the display backplane layer, which contains the electronics that turn the millions of individual pixels on and off. The iPhones (and other high-res displays, both LCD and OLED) use a technology called Low Temperature Polysilicon (LTPS) for this layer. LTPS replaces amorphous silicon (which is still widely used in other LCD form factors). The result is that the electronic circuitry in the LTPS backplane can be smaller, creating brighter and more power efficient displays that were also cheaper to produce.

Now, various metal oxides are emerging that could replace LTPS. One is indium gallium zinc oxide or IGZO, which has been rumored for nearly two years to be coming to the next iPhone. Oxides like IGZO perform better than amorphous silicon, and lower costs as well as lower performance compared to LTPS. But IGZO has had only a limited rollout (according to Soneira, the Apple iPad Air uses IGZO), and other metal oxides in thin film transistors have been about two years behind initial expectations, according to a September 2013 report by NPD DisplaySearch.

IGZO alternatives are ramping up, according to DisplaySearch, which predicts that "capacity for oxide TFT production is expected to grow even more rapidly [than expanding LTPS], from 3.5 million square meters to over 19 million square meters in 2016." One vendor is CBRITE, which offers a metal oxide thin film transistor technology that promises big gains in color accuracy, contrast, aperture ratio and electron mobility (which results in more power efficiency), increases useful display life by up to 20 times, and uses 10 times less power than standard LCDs. And it can be easily and cheaply integrated with existing display assemblies and manufacturing.

So far there's no public indication that Apple is yet investing in metal oxide TFT for iPhone and iPad backplanes, though the company will have spent close to $37 billion in unidentified capital projects in the last three fiscal years. For 2014, the iPhone 6 models could stick with improved LTPS: the two highest-scoring tablet displays in DisplayMate's tests are the Amazon Kindle Fire HDX 8.9 and Google Nexus 7, both of which have LTPS backplanes and both of which outclass the third tablet tested, the iPad Air.

The Amazon tablet, which uses the LCD/LTPS technology found in the iPhone 4 and 5 series, achieves "very high brightness, very low Reflectance, excellent high ambient light performance, and excellent factory calibration with the best Absolute Color Accuracy and accurate Image Contrast." It's also 30% more power efficient than the iPad Air's IGZO display, according to Soneira.

The sapphire question

Part of Apple's capital investment has been an estimated $1 billion in becoming one of the biggest synthetic sapphire producers in the world [see "How Apple's billion dollar sapphire bet will pay off"]. It's still unclear whether Apple will, or even intended, to introduce sapphire cover glass for the 2014 iPhones, creating a highly scratch-resistant touch screen. But sapphire introduces a range of other issues for the display, according to DisplayMate's Soneira.

"Other than cost, one major issue with sapphire is that it has approximately double the screen reflectance of Gorilla Glass, so it will reduce high ambient light performance by a significant factor," he says.

The iPhone screen technology is an essential part of the user experience. We'll know in a matter of days what decisions Apple has made to improve that experience in a market that's grown far more competitive since its last major advances in smartphone displays.