Battle of display technology between AMOLED and LCD

Display technology talk may be focused on the move over to QHD displays and a potential future full of 4K smartphones and tablets, but the battle between AMOLED and LCD designs is raging as fiercely as ever. LCD brought us into the QHD age through the Vivo Xplay 3S, followed by the Oppo Find 7 and LG G3, but AMOLED is a growing threat to LCD’s dominance, as the latest Nexus 6, Moto X, and high-end Samsung devices are all packing the rival technology.

Debate still rages about which is the best and whether you should base your smartphone purchasing habits on such a technology. If you’re interested to know what all the fuss is about and how your smartphone’s display technology might be altering your viewing experience, then do read on.

It’s all in the design

The heart of the debate revolves around two quite different approaches to sub-pixel technology, designed to convert digital data into a format that you can see. These two unique implementations lead to slightly different results when it comes to accurate color reproduction, deep blacks, peak brightness levels, and battery efficiency.

LCD

LCD is the older of the two display technologies, but if anything this means that the technology has had plenty of time to mature. You’ll find LCD displays in high-end smartphones and tablets, such as the LG G3, HTC One (M8) and Sony Xperia Z line-ups. The first QHD display, found in the Oppo Find 7, was also based on LCD technology.

LCD stands for Liquid Crystal Display, although this doesn’t really give us too much of a hint about how the technology actually works. Rather, the principle is based on polarized light, which is the tricky part, and a color filter.

The color filter is simply used to determine the color of the pixel, from the standard red, green, or blue base colors. These can then be mixed together in various quantities to recreate all of the colors in between. The polarized light filters are used to control the individual brightness of each colored pixel.

 Polarization filters and the cystal adjust how much light reaches the display’s surface, while the color filter controls the spectrum the light that is seen.

Polarization blocks light through the use of a couple of filters, one in the horizontal and one in the vertical plane. Essentially, the original light source has waves oscillating in every direction, the polarization filters cut this down to just a single direction. The original light, with waves in all directions, can pass through either a horizontal or vertical filter, albeit somewhat dimmer, as it contains the components for both axis. However, horizontal polarised light cannot pass through a vertical filter, or vice versa, as it does not contain any light in the correct plane. By combining one vertical and one horizontal filter, or any two filters at 90 degrees to each other, the light can be completely blocked, which is the default state of an LCD pixel.

Two polarized filters angled at 90 degrees from each other completely block light. LCD displays use a twisted crystal to match polarity so that light can pass the second filter when desired. Source.

This is where the liquid crystal part comes in. In between the horizontal and vertical pixels there is a nematic liquid crystal that can be switched “on or off” electronically. When the crystal is on it is twisted and rotates the light around 90 degrees, meaning that the horizontally filtered light can also pass through the second vertical filter. Each pixel’s crystal is controlled by a transistor and can be switched on and off multiple times each second in order to give the appearance of different brightness levels. We can now filter light into different colors and control the individual brightness of each pixel.

The backlight that sits behind all of the pixels produces white light that is filtered for brightness and color in each pixel. The backlight can also be used to adjust the overall brightness of the display.

The Oppo Find 7 was the first smartphone to feature a QHD display, which was built from LCD technology.

As you can imagine, reproducing the right colors through all this technology can be a little tricky and LCD displays therefore can sometimes suffer from less than perfect color reproduction. There are compromises to be had when it comes to blocking a portion of the light through the first polarization filter and with potential leakage from the backlight bypassing the filtering stage.

AMOLED

AMOLED is a newer technology than LCD and its name also gives us a really good hint as to how it works. OLED is the really important part. Rather than a single backlight and lots of pixel filters, AMOLED displays use millions of individually controlled organic-LED light sources.

The AM part of the name stands for active-matrix, which simply refers to the switching transistor technology used to control each LED pixel. An active-matrix continues to drive an individual LED through a transistor even when other LEDs change states, while a passive-matrix is controlled through an X and Y axis array, meaning that you have to refresh either the row, column, or both just to change the state of a single pixel.

OLEDs are semiconductor light sources, which feature anodes and cathodes for electron flow. The brightness is determined by the electron current passing through the component. Source.

AMOLED displays are a little easier to picture than and LCD panel, although not necessarily easier to manufacturer. Rather than a backlight and grids of filters for each color, AMOLED makes use of individual tiny colored LEDs, red, green, and blue, to create a wide range of colors. The brightness of each LED pixel is controlled by the current provided through the backplane transistor, which is adjusted via the transistor’s gate voltage. The OLEDs emits light in the emissive layer between the anode and cathode, where electrons fill “holes” and give up their energy as a photon. The color of the light depends on the type of organic molecule in the emissive layer. A higher current means more electrons passing from the cathode, which results in more holes filled and more photons given off as light.

Due to the lack of filtering applied to the light sources, AMOLED displays can provide high accuracy colors, a wider contrast ratio than LCD, and can save battery through dimming individual pixels rather than leaving a backlight always-on. However, peak current draw for at high brightness levels is often higher than LCD displays. AMOLED also suffers from problems with organic-LED degradation over-time, which can lead to burn-in and/or color loss over parts of the screen.

The new Nexus 6 and Galaxy Note 4 showcase what QHD AMOLED displays can do.

For the record, Super AMOLED is just Samsung’s term for an AMOLED display which has the touch digitizer built into the display rather than in a separate layer on top of the pixels. This has the added benefit of reflecting less light and allowing for a slightly higher peak brightness, but does not change the underlying display technology.

Which one is the best?

It is impossible to judge a display based solely on the type of display technology used, as both can produce excellent results. Instead, we also have to consider the accuracy and calibration of the pixels in each display, both types can range from high to poor levels of color, white, and black reproduction.

The arrangement of pixels is a major factor that is mostly independent of the display technology type. PenTile RGBG, prevalent among AMOLED displays, “unevenly” distributes light through extra green pixels, which has had mixed reviews in the past. However, some LCD displays have adopted PenTile RGBW, with an extra white pixel, in order to allow for a higher peak brightness as the cost of some color fidelity. With investment, development, and fine-tuning over the years, some of these unorthodox pixel arrangements are showing excellent results.

AMOLED and LCD technologies are just part of the story. RGB sub-pixel layouts and sizes also determine how well a display reproduces colors.

Both display types have their own pros and cons. Resolution wise, LCD has the lead earlier this year due to its use in the first QHD smartphones. However, Samsung’s Galaxy Note 4 has closed the resolution gap. Numerous test conducted over the past few years has found that Samsung’s high-end Super AMOLED tends to reproduce colors more accurately than the best mobile LCD displays. The Galaxy Note 4 has the most accurate mobile display currently available, according to DisplayMate, although it is doubtful that the differences are hugely noticeable compared with the best LCD displays.

AMOLED is becoming an increasingly popular smartphone and tablet display technology, with good reason, but LCD won’t be going anywhere in the foreseeable future.