December 16, 2020
How do foldable screens work?
For almost as long as digital displays have existed within, so too has the idea of having a screen that bends and folds.
By Fergus Halliday
3 min read
Until recently, foldable screens have largely been confined to science fiction or closed-doors showcases at technology tradeshows like CES. Now, that's beginning to change with the arrival of mainstream foldable smartphones like the Samsung Galaxy Fold and Motorola Razr.
While their respective form-factors and hinge mechanisms may vary, there are a fundamental things that each of these devices have in common. If you're looking for a general understanding of just how foldable screens work, look no further.
Display Technologies 101
Let's start at the beginning. Normal, non-foldable, LCD (liquid crystal display) screens work absorbing and re-emitting the light projected from a secondary LED (light emitting diode) backlighting system. Essentially, there are two layers of screen here: a soft interior one and a more solid exterior one.
In terms of raw volume, LCD-LED is the dominant display technology for most gadgets — be it smartphones, smartwatches or TVs. However, in recent years, that dominance has been challenged by a new display technology: OLED.
Even if you don't know it by name, chances are you've probably encountered one in the wild before. If the screen on your current device has any sort of curvey or rounded edges, then it's probably got an OLED-based display.
Most modern flagships like the Samsung Galaxy S20 or iPhone 12 fall into this category.
OLED-based screens are different from their LCD-LED counterparts in a number of aspects such as higher contrast, faster refresh rate, saturated colors and deeper 'absolute' blacks. However, for our purposes, their most important characteristic is their flexibility.
Because LCD panels are bonded to a rigid LED backlight, they tend to break before they bend. OLED displays do not have a backlight and thus do not suffer from this limitation.
Each pixel on an OLED-based screen is able to self-emit, so there's no need for a backlight. Freed from this constraint, manufacturers of OLED screens are free to be a little more creative when it comes to form-factor.
To be clear — OLED screens can still be broken or damaged in much the same way as their LCD-LED counterparts. In the right conditions, they can be bent but they are not inherently "bendy" in the same way as way as paper.
It's also worth noting that just because you can curve a screen doesn't mean you should. Curved screens in smartphones have their fair share of detractors and the brief curved TV craze is but a distant memory for most consumers.
Nevertheless, from a design and engineering perspective, the specific structure of OLED screens gives manufacturers that little bit of extra wiggle room that makes curved and foldable screens possible.
Though they do have their drawbacks, OLED remains the secret ingredient that makes foldable screens work the way they do. Not all OLED screens are curved but all curved and foldable screens are OLED.
Let's Talk About Hinges
The other half of the engineering equation behind early foldable smartphones involves the hinges. Where every foldable smartphone relies on the same kind of OLED screens, the hinge-tech involved with each tends to vary.
Despite how mundane hinges might sound, the reality is that they're a critical part of the foldable smartphone experience. They're the mechanism that allows the OLED screen to be safety bent again and again.
Their long-term integrity matters in a very real way.
In some respects, hinge quality is one of the most useful metrics consumers have for comparing the real-world durability of the various foldable smartphones out there.
When CNET put the original Galaxy Fold to the test, it lasted 120,000 rounds before breaking. In contrast, the Motorola Razr lasted just 27,000 folds. Now, there are a bunch of rogue variables here from the difference in hinge design, form-factor and the repetitive strain incurred by the test itself. Still, despite these caveats, such results does suggest that Samsung's hinge design tends to be the more robust of the two.
If you want a foldable smartphone that lasts longer than its first encounter with the real world —given the high asking price attached to every foldable smartphone currently available, I suspect you probably do — then hinge design is a critical part of the package that can't be discounted.
If OLED screens make it possible for smartphones to fold at all, then hinges are what makes it possible to do so regularly and repeatedly without causing any damage to the screen over the long-run.