A new generation of curved displays for Smartphones and TVs has just been launched. While there have been curved screens around before, they were actually just flat displays with a curved cover glass on top. This time the displays themselves are actually curved and that makes a significant difference.
Dr. Raymond Soneira of DisplayMate Technologies has made it his mission to suss out the best smartphone, tablet, HDTV, and multimedia displays from the worst with his Display Technology Shoot-Out series. Here, he digs down to the truth behind curved smartphone screens.
There is widespread misunderstanding regarding curved displays. They aren't simply a marketing gimmick as has been widely reported – we’ll show that curved screens substantially improve the display performance. In fact, curved screens are a major and very important new display technology innovation, particularly for handheld Smartphones.
Curved displays are possible now because of the availability of thin curved glass and developments in display manufacturing for both OLEDs and LCDs. But OLEDs have a significant advantage because they can be made flexible, not just on a fixed curve. In fact, the recently introduced Samsung Galaxy Round is actually manufactured as a flexible OLED display that is then bonded to a curved cover glass – and it’s identical to the flat screen OLED display in the Samsung Galaxy Note 3 that we recently tested. So the identical flat and curved displays on the Galaxy Note 3 and Galaxy Round can be directly compared and tested side-by-side, which we did using both extensive viewing tests and lab tests.
Even more interesting is that Samsung has also made that same OLED display into a completely free standing flexible display, so I was able to compare three essentially identical 5.7 inch 1080p OLED displays in their flat, curved, and flexible display formats side-by-side. This article is my analysis of curved and flexible displays for Smartphones…
First of all, the curvature on the Galaxy Round is fairly small – the left and right edges of the screen are just 2.66 mm (0.10 inch) higher than the center, so the effect is subtle, but it’s very important. The screen is curved in only one dimension – its cylindrically curved along the horizontal direction when the Smartphone is held in the standard Portrait mode. And it’s concave rather than convex – similar to a handheld magnifying mirror, which is also slightly concave, but it is curved in two dimensions rather than one. Keep this concave magnifying mirror analogy in mind…
Form Factor and Screen Privacy
The Galaxy Round and Galaxy Note 3 have similar outer dimensions because the effect of the small curvature on the overall size is small. The Round is slightly thinner but has almost identical width and height as the Note 3. It is more comfortable to hold because the hand nicely fits the entire curved back instead of just holding it along the left and right edges. It also improves screen privacy because the curvature makes it harder for other people to see the screen (unless you intentionally turn it to show them).
Screen Readability in Ambient Light
Smartphones are almost always used in significant ambient light. All display screens reflect that ambient light, which washes out the colors and image contrast, and makes it harder to read the screen. To see by how much, turn off the display but hold the Smartphone as if you’re using it and then walk around your normal environment, both indoors and outdoors. All of the reflections from lamps, ceiling lights, windows, direct and indirect indoor and outdoor sunlight, and even the reflection of your face are still there when the display is on. That requires you to increase the screen brightness to counter all of that ambient light interference. It’s so important that peak screen brightness is generally considered one of the most important display parameters by most consumers. But the higher the screen brightness the more power the display uses, and the shorter the running time on battery.
So if there is a way to reduce those screen reflections, it will not only improve the screen readability, image contrast, color accuracy, and overall picture quality, but also increase the running time on battery. One way is to reduce the screen Reflectance. It has been steadily coming down, and the Galaxy Note 3 and Galaxy Round have about 5 percent Reflectance, among lowest I have ever measured for mobile displays.
But there is a really innovative way to further cut down on screen reflections – and that is by introducing a small amount of screen curvature, like on the Galaxy Round…
Reflections from a Curved Screen
The concave screen shape on the Galaxy Round cuts down on reflections from the surrounding ambient light two ways: first, by reducing the screen’s 180 degree opening angle, which eliminates reflections from some ambient light coming from the sides. Second, from specular mirror reflections off the concave screen, because the curvature directs reflected ambient light coming from behind away from the viewer’s line of sight. This is very important because you want to minimize the amount of ambient light that is seen reflected off the screen.
Curved Screen Magnification
But the most interesting and important result of the slightly curved Galaxy Round screen is that it magnifies the sizes of all of the objects that it reflects, just like a concave mirror that I mentioned above. As it turns out, that substantially cuts down on the interference of light reflections from ambient light in three ways:
First: when you are holding a flat screen, at normal viewing distances it reflects not only your face (which is typically in shadow) but all of the light coming from directly behind and around your head, which almost always is considerably brighter than your face. Check this out right now by holding up your Smartphone with the display turned off. But with the curved Galaxy Round held at normal viewing distances, your face is magnified so that it always fills the entire screen so that you don’t see the much brighter light coming from behind you. As a result, there is typically a large reduction of reflected ambient light, both indoors and especially outdoors. This effect is the first major display improvement resulting from the curvature.
Second: because your face is being magnified horizontally by a factor of 2 or more (depending on the viewing distance), the brightness of the reflected image is also reduced (because it is being spread out by the magnification). This is equivalent to reducing the screen Reflectance and is the second major display improvement resulting from the curvature. Note that the magnified facial reflection is now so dark that you can’t use the Smartphone as a mirror for personal grooming…
Third: the curved screen on the Galaxy Round has a focal distance of about 16 inches, which is also comparable to the typical viewing distance. As you increase your viewing distance from a typical minimum of 8 inches, the horizontal magnification increases rapidly, up from a factor of 2 until it becomes effectively infinite at about 16 inches viewing distance. Because the reflection of your face is being magnified by a large factor, your facial features appear blurred and become increasingly featureless and effectively invisible. That significantly reduces the visual interference from the screen reflections. This featureless reflection is the third major display improvement resulting from the curvature.
All these effects were clearly visible by holding and comparing the Galaxy Round and Galaxy Note 3. The result is a very large improvement of the display in ambient light. This not only substantially improves screen readability, image contrast, color accuracy, and overall picture quality, but can also increase the running time on battery because the screen brightness and display power can be lowered due to the reduced interference from ambient light reflections.
On the other hand, if you aren’t looking more or less directly at your Smartphone, then the screen will be pointing in some other direction and reflecting other things. In that case your face may only partly appear in the screen reflection, or even not at all if it’s significantly angled. Anything more than a few feet away from the curved screen won’t be noticeably magnified, so the above advantages in this case will be reduced, but this also means you are paying less attention to the screen. Once you hold it back up all of the improvements mentioned above will return.
Finally, the icing on the cake was looking at the same OLED display on the Galaxy Round and Galaxy Note 3 as a completely free standing flexible display. So I was able to bend and adjust the display in any direction or way and to any degree that I wanted while comparing it to the Galaxy Round and Galaxy Note 3. Most of the time people just want to bend the flexible screen into contortionist orientations, but instead I explored the effects of subtle changes in its curvature on the reflections from all the surrounding ambient light. The curved Galaxy Round already performs extremely well when held like a Smartphone, so I explored how the three displays looked when lying down on a table. The curved Galaxy Round performed better than the flat Galaxy Note 3, but with some minor bending of the flexible display I was able to further reduce ambient light that was washing out the screen, including an annoying overhead table light. Flexible displays will have lots of applications, but a very important one is improving screen visibility by subtly bending away distracting light reflections. Curved is great but bendable is even better…
There’s more… Flexible OLED displays are manufactured using a flexible plastic substrate, so they don’t have the glossy cover glass used on virtually all existing mobile displays. That provides three more advantages: first, the screen Reflectance is lower because is doesn’t have the extra layer of cover glass. Second, the plastic bendable screen is not as glossy as the cover glass so it has a very slight matte haze finish, which cuts down on the specular mirror reflections. Third, without the cover glass the OLED display appears to be right on the surface of the screen, which is quite visually striking!
Finally, a subtle but very important technical point is that the Galaxy Round, which has the new flexible OLED display, and the Galaxy Note 3, which has the traditional rigid glass OLED display, both had virtually identical performance in both our lab tests and viewing tests (other than the enhanced optical effects resulting from the curved screen discussed above).
Curved and Flexible Display Conclusions:
Introducing a slightly curved cylindrically concave screen is a very important and major innovation in Smartphone display technology – very far from being a marketing gimmick as has been widely reported. The Galaxy Round screen curvature is very subtle, just 0.10 inches away from flat, which is similar to the slight curvature in a handheld magnifying mirror. But that small curvature is the key to a series of optical effects that result in significantly reducing interference from reflected ambient light by a large factor. It substantially improves screen readability, image contrast, color accuracy, and overall picture quality, but can also increase the running time on battery because the screen brightness and display power can be lowered due to the reduced light interference from ambient light reflections. To see this for yourself and understand the size and importance of this effect, turn off the display but hold your Smartphone as if you’re using it and then walk around your normal environment, both indoors and outdoors.
All of the performance enhancements that we have discussed were clearly visible by holding and comparing the curved screen Galaxy Round with a virtually identical flat screen Galaxy Note 3. The Galaxy Round is currently only available in Korea due to the limited production capability for curved OLED displays. Samsung has informed me that as the production volume is increased, it will be introduced in other countries.
While LCDs can now be manufactured with curved glass screens, OLEDs can be manufactured with entirely flexible screens on a plastic substrate. The Galaxy Round is actually manufactured as a flexible OLED display that is then bonded to a curved cover glass. Maintaining it as a purely flexible display provides even greater performance and functionality enhancements. Flexible displays will have lots of applications, but a very important one is improving screen visibility by subtly bending away distracting light reflections. Curved is great but bendable is even better…
It will take several years before flexible displays become incorporated into actual production Smartphones and then into entirely new products, uses, and applications. It’s clear that flexible displays will have a profound effect on the use of most display based products starting in the very near future. The display world isn’t flat anymore, and in a few years it won’t even have to be round…
About the Author
Dr. Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire, which produces video calibration, evaluation, and diagnostic products for consumers, technicians, and manufacturers. See www.displaymate.com. He is a research scientist with a career that spans physics, computer science, and television system design. Dr. Soneira obtained his Ph.D. in Theoretical Physics from Princeton University, spent 5 years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another 5 years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested, and installed color television broadcast equipment for the CBS Television Network Engineering and Development Department. He has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American. If you have any comments or questions about the article, you can contact him at email@example.com.