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UPDATE: 60, 90 and 120Hz explained

Smartphone screens explained: display types, resolutions and refresh rates
Smartphone display acronyms can be a little overwhelming, but if you want to know what all the numbers and abbreviations associated with smartphone screens mean, we've got you covered. Want to know what each screen type is best for and what the differences in screen resolution mean? Read on.

Resolution: 4K, 2k, FullHD, HD
Panel types: OLED, AMOLED, LCD
Micro-LED displays of the future
Refresh rates: what do 60Hz, 90Hz, and 120Hz mean?

One major consideration when we talk about smartphone screens is the resolution of the display, and as a rough guide, larger numbers are better here. Encompassed within this 'resolution' category is the size of the screen (in inches), the number of pixels (how much information it can show) and how densely those pixels are packed, referred to as Pixels Per Inch (ppi).

If you know the size of the display, you can work out how many pixels are squeezed into one square inch: that's the pixels per inch (ppi) figure, which is referred to as pixel density. You can easily calculate your phone's ppi using a pixel density calculator.

We've got the major bases covered below, but will start with HD resolution or higher, since that's what most smartphones ship with nowadays. You might also want to pay attention to the stated 'nits' too, which is a rating of brightness for displays.

HD stands for high definition. HD simply means a pixel measurement of 1280 x 720 pixels. No matter how large the screen is, as long as the pixel measurement remains at this measurement, it's an HD display.

As you can probably tell, the smaller the HD screen the higher the pixel density and, theoretically, the better the picture. So simply having an HD display doesn't mean much, as it will produce a very different image on a 5-inch screen than on a 10-inch screen (note: screen sizes are measured on the diagonal to take account of slightly different aspect ratios).

A 4.3-inch screen, for example, has a pixel density of 342 ppi. On a 4.7-inch screen, the pixel density drops to 312 ppi, but both are still HD displays. According to Apple, 300 ppi is the sweet spot, because that is roughly the point at which the human eye stops being able to discern individual pixels at a certain viewing distance (and on a certain sized screen).

Full HD
Full HD is the next step up and is currently the standard for smartphone display definition, although 2K (QHD) has been gaining traction on high-end devices since the Oppo Find 7 and LG G3, the first commercially available devices to have QHD screens.

Full HD measures 1920 x 1080 pixels. Again, the pixel density will depend on how large the screen is overall. With smartphones at the 5-inch mark, the pixel density sits around 440 ppi, while on a 5.5-inch screen that number drops to 400

QHD, Quad HD or 2K
QHD stands for Quad HD, which is four times the definition of standard HD. That means you can fit the same number of pixels as four HD displays into a QHD display of the same size. The pixel measurement for QHD is 2560 x 1440 pixels. A 5.5-inch QHD display has a pixel density of 538 ppi. For comparison, the pixel density of a 5.5-inch Full HD screen is 400 ppi.

Definitions are also often referred to by the smaller number of the pixel measurement - HD will sometimes be called 720p, Full HD gets called 1080p and so on. With QHD though, the 2K name comes from the fact that the bigger of the pixel measurements is over 2000 pixels, which can admittedly be a bit confusing (and really ought to be referred to as 2.5K, if we were being entirely accurate).

Many phones from Samsung, Motorola, Huawei, and other big-name handset manufacturers include 2K displays as the standard nowadays.

4K or Ultra HD
You can probably see where this is going. Like 2K, the 4K name comes from the larger of the two pixel measurements, which are, technically speaking, 4096 pixels in 4K and only 3840 pixels in Ultra HD. So while these two terms are often used interchangeably, they are actually a little bit different.

Ultra HD is 3860 x 2160 pixels and 4K is 4096 x 2160. Both definitions frequently get shortened to 2160p and the pixel difference is relatively marginal, but there is a difference. One of the first handsets to launch with a '4K' display was Sony's Xperia Z5 Premium, which offered Ultra HD resolution on a 5.5-inch screen.

Sony refers to this display as 4K, but it actually uses the smaller measurement of Ultra HD, not "real" 4K definition. Nevertheless, the Z5 Premium has a pixel density of 806 ppi – far beyond what many smartphones offer, and beyond what many people would say is necessary.

Resolution trends
While smartphone screens keep getting bigger, there hasn't been a race to 4K from handset makers that we might have expected a year ago. As it stands, Sony still dominates with the Xperia Z5 Premium and XZ Premium.

Instead, what has become the norm at the top of the market is a 2K panel, rather than a higher resolution option. This, among other reasons, probably has a lot to do with concerns about power, as larger, higher-resolution displays demand evermore. With smartphone battery life already a sore topic for many, handset makers seem reluctant to make that leap just yet.

However, 2018 could well be the year that we'll see more and more Ultra HD-equipped handsets going on sale, even through the mid-range.

Panel types
There are many display types used in smartphones: LCD, OLED, AMOLED, Super AMOLED, TFT, IPS and a few others that are less frequently found on smartphones nowadays, like TFT-LCD.

One of the most frequently found on mid-to-high range phones now is IPS-LCD. But what do these all mean?

LCD means Liquid Crystal Display, and its name refers to the array of liquid crystals illuminated by a backlight, and their ubiquity and relatively low-cost makes them a popular choice for smartphones and many other devices.

LCDs also tend to perform quite well in direct sunlight, as the entire display is illuminated from behind, but does suffer from potentially less accurate color representation than displays that don't require a backlight.

Within smartphones, you have both TFT and IPS displays. TFT stands for Thin Film Transistor, an advanced version of LCD that uses an active matrix (like the AM in AMOLED). Active matrix means that each pixel is attached to a transistor and capacitor individually.

The main advantage of TFT is its relatively low production cost and increased contrast when compared to traditional LCDs. The disadvantage of TFT LCDs is higher energy demands than some other LCDs, less impressive viewing angles and color reproduction. It's for these reasons, and falling costs of alternative options, that TFTs are not commonly used in smartphones anymore.

IPS stands for In-Plane Switching and it is a further improvement on TFT LCDs. It delivers better color reproduction and, most notably, improved viewing angles compared to TFT-LCDs. It does this by using two transistors for each pixel combined with a more powerful backlight. The downside is that they require more power than other types of non-LCD display. They still generally use less power than a TFT display though.

There are other acronyms you many see combined with IPS too, like IPS-NEO. In that case, it's a proprietary name for a technology created by JDI that claims to eliminate backlight leakage, but it works in the same essential way as any other IPS-LCD

AMOLED stands for Active Matrix Organic Light-Emitting Diode. While this may sound complicated it actually isn't. We already encountered the active matrix in TFT LCD technology, and OLED is simply a term for another thin-film display technology.

OLED is an organic material that, like the name implies, emits light when a current is passed through it. As opposed to LCD panels, which are back-lit, OLED displays are 'always off' unless the individual pixels are electrified.

This means that OLED displays have much purer blacks and consume less energy when black or darker colors are displayed on-screen. However, lighter-colored themes on AMOLED screens use considerably more power than an LCD using the same theme. OLED screens are also more expensive to produce than LCD.

Because the black pixels are 'off' in an OLED display, the contrast ratios are also higher compared to LCD screens. AMOLED displays have a very fast refresh rate too, but on the downside are not quite as visible in direct sunlight as backlit LCDs. Screen burn-in and diode degradation (because they are organic) are other factors to consider.

On the positive side, AMOLED screens can be made thinner than LCDs (because they don't require a backlit layer) and they can also be made flexible.

What's the difference between OLED, AMOLED and Super AMOLED

OLED stands for Organic Light Emitting Diode. An OLED display is comprised of thin sheets electroluminescent material, the main benefit of which is they produce their own light, and so don't require a backlight, cutting down on energy requirements. OLED displays are more commonly referred to as AMOLED displays when used on smartphones or TVs.

As we've already covered, the AM part of AMOLED stands for Active Matrix, which is different from a Passive Matrix OLED (P-OLED), though these are less common in smartphones.

Super AMOLED is the name given by Samsung to its displays that used to only be found in high-end models, but have now trickled down to more modestly specced devices. Like IPS LCDs, Super AMOLED improves upon the basic AMOLED premise by integrating the touch response layer into the display itself, rather than as an extra layer on top.

As a result, Super AMOLED displays handle sunlight better than AMOLED displays and also require less power. As the name implies, Super AMOLED is simply a better version of AMOLED. It's not all just marketing bluster either: Samsung's displays are regularly reviewed as some of the best around.

Retina is another marketing term, this time from Apple. A Retina display is not defined by a specific characteristic, other than that it is supposedly of sufficient resolution that the human eye can't discern pixels at a normal viewing distance and has a pixel density over 300 ppi. As we already know though, Apple doesn't measure ppi in the same way as other handset makers.

This measurement obviously changes depending on the size and resolution of the display. Apple popularized the Retina concept with the iPhone 4, which had a 960 x 640 pixel resolution on a 3.5-inch IPS LCD screen, resulting in 330 pixels per square inch (ppi).

Considering a 5.5-inch QHD display is fairly common on high-end Android phones these days and devices like the S7 Edge have 534 ppi, Apple had to ultimately capitulate on its belief that 300 ppi is plenty, which ultimately led to the iPhone 6 Plus offering a Full HD display with a pixel density of 401 ppi. The iPhone 7 and 7 Plus offer 326 ppi and 401 ppi respectively. The latest iPhones, the XS and XS Max, feature displays with 458 ppi.

Refresh rates: what do 60Hz, 90Hz, and 120Hz mean?

At the tail end of 2019, we started to see smartphone manufacturers making a big deal about 90Hz displays. OnePlus was the first to drop this technology with the OnePlus 7 Pro, and later followed it with the 7T and 7T Pro. Google was next to act, putting a 90Hz display on its Pixel 4 and Pixel 4 XL smartphones at the end of 2019 also. But what does it actually mean? Is there a difference between 60Hz (the norm for smartphones) and 90Hz? Yes, there is!

The refresh rate is the number of times in a second a TV or display refreshes its image. It's measured in hertz (Hz). A display with a 60Hz refresh rate will display changes 60 times second, and a 90Hz display will change 90 times a second. Essentially, what this means is that the higher the refresh rate, the smoother the display is for fast-moving images. Higher refresh rates mean less motion blur, and that's a big deal for smartphones on which we are often scrolling at high speeds. It also makes games look more fluid.

So far, 90Hz is the highest refresh rate we've seen on a smartphone, but Samsung is rumored to be launching the S20 series with a 120Hz refresh rate. Apple is also reported to be working on high refresh rate displays.

The downside to 90Hz (and 120Hz, if we ever get it) is that battery life suffers significantly. This is especially true on the Google Pixel 4, which already has average battery life even if you stick to 60Hz.

Displays of the future
As we're gradually seeing smartphones push into the 4K range, larger home displays for TV and computer monitors look ahead to 8K (7680×4320). In the smartphone world, 2018 could see a major breakthrough with the 'holographic' display promised by the RED Hydrogen One. While the upcoming hype machine from the cinematic camera manufacturer will 'only' have a 2K display, it's the promise of this holographic, or '4V' content that could really be revolutionary.

RED Hydrogen One holographic display: more than just marketing
Micro LEDs is a new technology still in its infancy, but it has great potential to become the next big thing on the display market. Micro-LED displays work similarly to OLED panels, but are even thinner. They consist of inorganic semiconductors, specifically a gallium-nitrite combination. Like OLEDs, these are light-emitting diodes, but they are much smaller.

Backlighting is not necessary with micro-LEDs, nor is a polarization filter required. The glass layer above the panel can also be slimmed even further. The brightness per watt, which simplifies the efficiency of the displays, is even better than that of OLED panels and beats LCD by far. With the same brightness, a micro-LED display requires only half as much energy as an OLED screen, in some cases even less. The extremely small diodes also allow higher resolutions on the same surface - a 4K smartwatch would be conceivable with micro LEDs. Last but not least, micro LED screens are not as susceptible to pixel burn-in as OLED displays.

The major disadvantage at the moment is the cost. Production is complex, there are hardly any production lines for micro-LEDs yet and the scrap rate is high. All of this is driving up costs. However, many of these difficulties are likely to be solved with mass production. Experts expect the first end customer products with micro-LED displays to be available in 2020. Apple has been investing in the technology since 2014, which is why an Apple Watch with a micro-LED display is likely to be the pioneer product.

Which display type is better?
As we have seen, each term is not restricted to one manufacturer: AMOLED is not always Samsung and Retina is not always Apple (although no one else uses the term). iPhone IPS LCD displays are currently manufactured by LG, while Samsung has built displays for the iPad. Not all Samsung devices feature AMOLED displays either. This is not simply a case of which display is better: it's a trade-off between pros and cons.

The point of all this is basically to say two things: numbers and technical data are worth considering when comparing the screens on two smartphones, but the real-world performance of these displays is more important. It's impossible to gauge a display on paper, but you really need to see it in real life to know if it is too cool or warm for you, whether you like its saturation, brightness or contrast levels, what its viewing angles are like, and so on

Knowledge base to give some idea on display - hope it is helpful 
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