Smartphone Displays - AMOLED vs LCD

Future undoubtedly belongs to OLED, but for now LCD is more mature, cheaper and can be mass produced – all advantages, if you actually want to get a product to market. No wonder then that even Samsung’s own Galaxy Tab 7” tablet was outfitted with an LCD screen, while the new Samsung Wave II replaces the Super AMOLED of the original, for a Super LCD. AMOLED screens in larger sizes are cost-prohibitive, at least until real mass production is realized.

Apart from the major supply issues, the commercial AMOLED screens at first seemed to have some drawbacks such as being too reflective, which diminished their sunlight visibility. Moreover, despite the lab claims for power efficiency, battery life on smartphones with AMOLED screens was nothing to get excited about at first.

Then, in January 2010, Samsung announced the next generation of “Super" AMOLED screens. Super AMOLED is 20% brighter, 80% less reflective, and uses 20% less energy than regular AMOLED screens, thanks to having only two major components – the actual AMOLED emitting layer, and the tough but thin Gorilla Glass, sealed over it. The touchscreen coating Samsung has managed to apply as an only 0.001mm thin layer in-between, bringing the light-emitting layer closer to the glass, to show raw, vivid colors.

On the other hand, the IPS-LCD (in-plane-switching LCD) technology, has largely overcome the usual troubles with LCD screens, namely power consumption and viewing angles, plus it allows for smaller pixels, making possible the incredible resolution of the iPhone 4. The iPad and iPhone displays are mainly produced by LG, and exhibit much more contrast, compared to regular LCDs. It is probably the best the LCD world can offer, without being cost-prohibitive.

Asked about why Apple didn’t go with the emerging OLED screen technology for the iPhone 4, Steve Jobs said that the high-res IPS-LCD in the iPhone 4 is better than OLED. And he was right - at the time when millions of iPhones had to be produced, the only OLED technology that might have met Apple’s requirements is Super AMOLED. Since it belongs to Samsung, it will not be until 2011 that the Koreans would be able to mass-produce such screens. Apple approached Samsung for their AMOLED screens, in the preparations to launch the iPhone 4, but the capacity just isn't there. There are rumors that Cupertino is talking again with Samsung regarding the Koreans' new plant capacity for an eventual Super AMOLED display in the upcoming iPhone edition next summer.

Still, the more mature LCD technology managed to come up with an IPS-LCD screen for the iPhone, which hits AMOLED in a few areas where it hurts. A major advantage of Apple’s IPS-LCD is the so-called Retina Display technology, which has miniaturized pixels in order to cram a 640x960 resolution into the 3.5” display. At this resolution, only perfect vision can distinguish the individual pixels from a certain distance. That makes high-contrast situations, such as e-books and web pages look very crisp and legible.

Also, with one of the major advantages of OLED-based displays being their slender profile, Apple still managed to produce the thinnest smartphone on the market, helped by LG's slim display. Despite LCD’s need for backlighting, the advancements in power management in the IPS-LCD brought along similar battery consumption on comparable chipsets for both phones. This is not easy to be explained, until we look at one table from the dawn of OLED-based screens a few years ago:



The fact of the matter is that LCD screens draw fairly constant power, no matter what images are shown on the screen. AMOLED, in its turn, needs the most powerful current applied to pixels that are showing white. Thus, while OLED displays are up to 90%+ more efficient when the background is black, when showing pure white, OLED screens can be consuming 3 times more power than an LCD display. This is why black menu backgrounds and colorful icons are recommended in user interfaces developed for AMOLED screens.

In a recent battery endurance test, consisting of websites display on several last-gen phones, it wasn’t the Super AMOLED phones that came ahead, but rather the Motorola DROID 2 and Motorola DROID X with their last-gen LCD screens. If the test had been on a looped video, the Super AMOLED phones would have probably given up the ghost last, since websites mainly use white backgrounds.

AMOLED screens also have shorter lifespan of the blue organic diodes, compared to the green and red ones, which might result in a shorter overall lifespan of the device. Using a PenTile matrix (developed by a company, whose IP Samsung bought not long ago), is one way to remedy this shortcoming. It arranges one green subpixel with double-width red or blue ones, thus showing only two colors per pixel, instead of the usual three. Some researchers claim this effectively lowers the interpolated 480x800 resolution of the Samsung Galaxy S, to actual 392x653, making text and web pages appear more pixelated. Recent advancements of the blue diodes' lifespan, however, puts it at over 30 000 hours, which means the screen will be around for many moons after you have moved on to another phone anyway.

Both handsets use Gorilla Glass, but the OLED-based screens are more withstanding to concussion due to less layers in them, and glass elements in general. On the other hand, OLED is easily damaged by water, that is why the Gorilla Glass is sealed over the touch coating of the AMOLED layer. Not that LCD would survive much water, but we are just nitpicking here, for comparison's sake.

Actually one credible scientific research was outed recently by the specialists from DisplayMate, who tested extensively the screen types of many popular smartphones, and the results confirm what unscientific observations of the two phones have noted. 

DisplayMate’s thorough testing debunks some myths, related to OLEDs and LCDs, which might be valid in the lab, but in real production units, the results are as follows, regarding the Super AMOLED display on the Samsung Galaxy S, and the IPS-LCD Retina Display in the iPhone 4.

The color depth on the iPhone 4 appears true 24-bit (i.e. capable of 16 million colors), while due to Android 2.1's limitations, the browser and gallery in the Galaxy S are reproducing 16-bit (65536) colors. This is fixed in Froyo.

The 4” Super AMOLED screen has enough resolution at 480x800 pixels, but the picture appears less sharp, especially when reading text, because the PenTile OLEDs have only 2 sub-pixels per pixel, instead of the 3 that are used in most displays. The Super AMOLED exhibits a blueish tint when showing white, which is a factory calibration issue, and there are projects under way to boost the screen brightness and dispose of the blueish whites, such as the Voodoo project. In our sample shots inside at maximum brightness in the browser, our main page indeed looks pixelated on the Super AMOLED:



Now off to the other intriguing findings. The actual brightness of the Retina Display is much larger than most screens, the excellent 541 cd/m2. This is quite a bit more than the Galaxy S’s peak 365 cd/m2, and a very important advantage, since phone screens are usually used in a lot of ambient lighting. That brightness unfortunately degrades with 57% from a 30 degrees viewing angle on the iPhone 4, and there isn’t much that can be done about it, apart from switching to AMOLED technology. The special reflectance coating brings brightness degradation to the Samsung's phone as well, which is unusual for an AMOLED screen, but not as bad as to the IPS-LCD, as our inside samples show:



On another note, the contrast ratio on the Samsung Galaxy S is so high, that DisplayMate has marked it as "outstanding - greater than 61000:1", while the iPhone 4’s is marked as the “very good for mobile” 1117:1. Contrast ratios only matter when the ambient lighting is low, though, rarely the case with your cell phone, as the source rightfully notes.

One component where Super AMOLED excels, and is extremely important in bright sunlight, is the reflectance percentage. Samsung is advertising 4% reflectance, with the actual measured 4.4%, but this is still extremely low, and less than the iPhone’s 7%, which makes up for the brighter Retina Display, especially in broad daylight. That is why outside both screens are almost equally visible, whereas AMOLEDs previously held issues against LCDs in that respect.

Still, all screens perform pretty bad in direct sunlight, and the technologies should be constantly evolving to address that. Our own sample shots of the two screens outside at maximum brightness confirm the above mentioned observations:



The color gamut is another important eye candy – the Galaxy S produces vivid, oversaturated colors, which people immediately like, representing 138% of the gamut, whereas the iPhone 4’s colors appear washed out in comparison due to it representing only 64% of the color gamut. If you are a calibration specialist, the Super AMOLED’s colors are called gaudy, and limited color gamut preferred over the much larger one. If you are a regular user, though, the colors on the Super AMOLED will wow you, the same way oversaturated photos of the iPhone 4 can wow you over the undersaturated colors of the Nokia N8 at first. We think Samsung deliberately has let the colors stay “gaudy”, and we can’t blame them.

Without some clarity, color or contrast, one can manage, but if your screen is a power drain, you will quickly move on to a better smartphone. Battery life when talking over 3G and watching video on both handsets is comparable, over similar capacity batteries and chipsets. The reason the Super AMOLED doesn’t fare way better, as the lack of backlighting would indicate, is the improved power management of the IPS-LCD on the iPhone 4, and the above mentioned power-drain when AMOLED screens are displaying white.

We’d think that these issues are not as important as in the first OLED screens, but the actual measurements prove us wrong. With an entirely white screen at full brightness, the Galaxy S consumes 1.13 watts, and the iPhone 0.42 watts. When showing mixed colors with darker content, the consumption of both is around 0.2 watts, and at entirely black screen the Galaxy S consumes zero, while the iPhone 4 stays at 0.42 watts. No wonder the backgrounds of the menus in the Galaxy S and the Samsung Wave are black.

Therefore, to draw the line in the sand in the Super AMOLED vs IPS-LCD battle, we have to note that both are outstanding representatives of two technologies – one very mature and one fledgling. If you had to choose only by screen type, here is how the odds are stacked. Provided that you read a lot of e-books, browse the general internet (which consists mostly of pages with white background), or review a lot of office documents on your smartphone – you will probably go with IPS-LCD, capable of the amazing resolution of the Retina Display, which makes text crisp and legible.

If you are using your smartphone heavily for viewing photos and video, you can’t find anything better than the high-contrast Super AMOLED, due to the bright, oversaturated colors, which many people like. Below is a sample clip we made that plays the same YouTube clip on all handsets for comparison, and it is clear that the Super AMOLED has the upper hand when displaying video:



Display technologies are not like advances in chipset miniaturizations, where GHz increase and power consumption have been moving in inverse relationship according to Moore’s law for the last fifty years or so. Screen production methods are taking years to be perfected, tested, calibrated, and ready for mass market adoption. Thus, the current overview might be valid for your smartphone purchase for a few more years, unlike our mobile CPU article, which we will have to update, mere two months after publication. Let’s have a look at some of the wild things that are expected from the screens of the future, and judge for ourselves where technologies are heading. 

There are many technologies in the research labs that are aimed to improve the sunlight visibility and power consumption of modern displays. A few of them will even see consumer devices shipping in the near future. 

Vouching to dispose with the plague of sunlight visibility, Pixel Qi (pronounced "chee") is a hybrid technology, that acts like a regular LCD. When the backlighting is turned off, it uses external ambient light reflected from the pixels to give bright, eReader type of experience with video capabilities outside under direct sunlight. Here is a video of a 10" USB-powered prototype, compared to a regular 10" LCD. With the backlighting off, the Pixel Qi draws a tenth of the power needed to run an LCD display. Various touchscreen and digitizer options can be added on top of this hybrid display as well.

Qualcomm's Mirasol screen uses reflective technology, similar to Pixel Qi with the backlighting off. Thus, compared to LCD, it saps power only when you turn the page, not when the image is static. It is viewable in direct sunlight, as it uses refracted ambient lighting to produce an image - the more external light, the brighter the screen, opposite of LCD. This technology has been inspired by the way butterfly wings work with light, hence the butterfly shown on the prototypes, and in the logo. The advantage over E-Ink is that it can also show quite a bit of color, and play video at 30fps, and possibly more. Rumors are that we might see it in the next iterations of Amazon's Kindle. Touchscreen options can be incorporated in the Mirasol displays too.



Now off to the cool stuff. There are a few problems facing OLED-based screen's mass adoption - lifetime and cheaper scalability. For now, AMOLED screens can be produced with decent costs only in small sizes. DuPont recently introduced a printing method for AMOLED screens, where it did a 50” screen for two hours, that should last about 15 years. Other companies are hot on the heels with similar technologies, and a breakthrough is expected when they license them to big manufacturers. 

The advancements in production methods are worth pursuing since it is again OLED-based displays, where the coveted adjectives "transparent", “durable”, “flexible”, “bendable” and even “wrappable” can be applied to. Thanks to their simpler construction, and the fact that the emitting layer itself can be extremely thin, Samsung, TDK and Sony have demonstrated durable, transparent, and even rollable screens.



Now, to take a walk on the wild side, the ubiquitous nano particles can have their say in the advancements of screen technologies as well. For LCDs, they promise ultra high-definition screens, with pixel sizes eight times smaller than those of the Retina Display. For OLED-based screen, nanotechnologies research forecasts a few thousand cd/m2 brightness, compared to the 365 cd/m2 that the Galaxy S screen is achieving. What is more, a method to vertically stack layers is still in the lab stage, but holds more promises for cheaper production of large size efficient AMOLED screens than a medicine for bald men.

If we had to bet, our money would be on OLED as the technology with more development potential. Advancements in OLED’s mass production, brightness and power consumption will be happening exponentially, whereas LCD probably nears its peak. Again, if one doesn’t have a dozen smartphone screens in front of them to nitpick on their differences, the best modern displays do all you ask them to, and then some.

source: DisplayMate