Understanding Brightness in AMOLED and LCD Displays
by Joshua Ho & Brandon Chester on December 22, 2014 8:00 AM EST
While we generally avoid going into deep detail when it comes to our display testing, in light of statements that seemingly contradict our testing it becomes important to contextualize our display tests. Many people are often confused by contradicting statements regarding the peak brightness of an AMOLED display, as we will state that the Samsung Galaxy Note 4’s display reaches a maximum of 462 cd/m^2, while other sites often state that the Note 4’s display reaches a maximum of 750 cd/m^2. Another commonly cited discrepancy is that we rate the Nexus 6’s display to reach a peak brightness of 258 nits, while others have rated the Nexus 6’s display to be as bright as 400 nits.
One might immediately assume that one measurement is right, and the other is false. In truth, both measurements are achievable, as we’ll soon see. Before we get into any discussion of testing methodology though, we must first understand how AMOLED and LCD displays work. Fundamentally, LCD and OLED displays are almost completely different from one another, but face similar issues and limitations. LCD is the older of the two technologies, and is fundamentally quite simple, although not quite as simple as OLED. In short, we can view an LCD display as made of a backlight, and a color filtering array which has liquid crystals that control the passage of light, along with polarizers to make sure that the filtering system works correctly.
To break this system down further, we can look at the backlight. In the case of mobile devices, the only acceptable backlight system for thickness and power efficiency reasons is the edge-lit LED, which places a line of LEDs along an edge of the display, which is then diffused through a sheet of transparent material with strategically-placed bumps in the material to create points of light via total internal reflection. For the most part, LEDs in use today are blue LEDs with yellow phosphors in order to increase efficiency, although this means that the natural white point of such a backlight is higher than 6504k and requires filtering in order to reach a calibrated white point.
Schematic diagram IPS LC display / BBCLCD
While the backlight is relatively simple, the actual color filtering is a bit more complicated, although we will avoid extensive depth in this case. In the case of IPS, the structure is generally quite simple in nature, with two electrodes in plane with each other, which is used to generate an electric field that rotates the orientation of the liquid crystals in plane with the display to dynamically alter the polarization of the light that can pass through the liquid crystal array. With a set of fixed polarizers before and after the liquid crystal array, by using the controlling TFTs to alter the voltage applied on the electrodes one can adjust individual color output on a per-pixel basis.
AMOLED is a fundamentally different approach to the problem, which uses organic emitters deposited upon a substrate. These emitters are designed to emit red, green, or blue when voltage is applied across two electrodes. Similarly, TFTs are needed to control each pixel. As one can see, AMOLED is a simpler solution, but in practice the issues with such an implementation can be quite complex.
In order to determine what picture content to use for a measurement of maximum brightness, we must turn to a measurement known as Average Picture Level (APL). This is best explained as the percentage of the display that is lit up compared to a full white display, so a display that is completely red, green, or blue would be 33% APL.
As one might already be able to guess, with AMOLED power consumption is highly dependent upon the content displayed. With a pure white image, every pixel must be lit, while with a pure black image every pixel is off. As the display typically has a maximum power use set for a mobile device, this opens up the capability for AMOLED displays to allocate more power per pixel (i.e. higher maximum luminance) when not displaying a full-white image. This is in contrast with the edge-lit LCDs used in mobile displays, which have relatively limited local-dimming capabilities. As a result, the maximum brightness of an LCD is relatively fixed, regardless of the displayed content.
In the case of the Nexus 6, we can clearly see dimishing returns after 40% APL as there is efficiency droop on AMOLED displays that are similar in nature to LED backlights. While now it’s easy to understand why it is that AMOLED can vary in maximum brightness, the question is which brightness is “correct”. While an AMOLED display can technically have a maximum brightness of 750 nits, it’s unlikely that people will look at images effectively equivalent to 1% of the display lit up with white.
In practice, it turns out that with Lollipop and almost all web pages, the average picture level is quite high. It’s increasingly rare to see cases where displayed content is below 50% APL. According to Motorola, 80% APL represents an average APL for light UIs and in light of this, it seems appropriate to test at similarly real-world APLs. Taking a look at some commonly used applications in Lollipop, we see that the APL is regularly at or above even Motorola's 80% figure. I opened some of the applications on my Nexus 6's homescreen to take screenshots of whatever they had open when they came up, and I've tabulated the results below.
| APL in % | |
| Messenger | 86 |
| Calculator | 49 |
| Settings | 84 |
| Calendar | 80 |
| Phone | 89 |
| Reddit Is Fun (Light) | 77 |
| Reddit Is Fun (Dark) | 23 |
| Chrome New Tab | 86 |
| Wikipedia | 83 |
| AnandTech | 52 |
| AnandTech Article | 81 |
| 76 |
As you can see, many of the screens in Android's interface as well as web pages and third party apps have a high APL. There are exceptions, like the Calculator application and any application with a dark theme, but the overall trend is clear. Google's new interface style also means that applications are more likely to adopt interfaces with large amounts of white than in the past.
As a result of this, we test at 100% APL in order to get an idea of perceived brightness. While there may be some need for lower APL testing, it’s important to also consider cases such as OLED aging which will lower peak brightness over time. It's also important to consider that the delta between 80% APL and 100% APL in this case is around 44 nits. This makes for about an 18% delta in brightness, which ends up being around the noticeable difference in most cases. While our testing is subject to change, in the case of brightness we currently do not see much need to dramatically alter our methodology.
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MrSpadge - Thursday, January 1, 2015 - link
Offering a dark and a light theme seems to be the best for everybody. And not that difficult to do.akdj - Sunday, January 4, 2015 - link
Just kinda sucks you've got to make that sacrifice. There's plenty of excellent apps with less than dark UIs, environments and templates for productivity.'Creative' type apps; photo and video manipulation, DAWS, NLEs, Even sketch types often have the darker interface so as to 'disappear' behind your canvas. Unfortunately, Play Store isn't the 'investment' someone into video, audio or productivity (Word processing & spread sheets), even PowerPoint/Keynote, collage creating, etc are SOL on Android. iOS is absolutely a creative's one stop shop regardless of their skill-set, gear they use to 'capture' or 'create'. IF this paradigm shifts some (I'd really enjoy the DAW, NLE & still editing options on my Note4, ESPECIALLY with four, high speed cores and a load of RAM, it seems like it would be a no brainer. dJay making its way over, VERY cool. Hopefully others will follow suit.
As cases are rare those quads are used 'in-app', music and motion manipulation and post would be perfect to take advantage of BOTH, the speed of the processors and darker UIs. I enjoy both iOS & Android because of this and always have. The 6+ as my personal, Note 4 my business phone...it's amazing how far we've seen cell comms advance over twenty years now.
Kinda a disservice to even call em 'phones' --- event 'smartphones' as today we're pocketing actual computers! With faster and higher quality 'guts' than just a couple years ago in a laptop that lasted 90 minutes off 110. NAND storage on the phones is quicker than most folks' (not us geeks with the newest, latest and baddest PCIe SSDs ;)) HDDs, and these displays, regardless of AMOLED, IPS{LC/ED}, even TFT panels on cheap, $250 computers at Wal Mart are a big step up from CRTs, and the nastiness that came with owning a bulbous display!
Hard to believe Best Buy had more CRT TVs ten years ago. And today's phones and tablets continue to push the goal posts further forward, supplemented with silicon that runs them, their 2, 3 or 5 million pixels at frame rates that, at least for the most part in parity on iOS and Android providing a fluidity we've never seen. Not even on desk or laptops. Coupled with high speed broadband and advancements in wireless, AC in particular ...as well as LTE radios make these things incredibly fast wen accessing anything online
Again, an area of the phone commonly used and whether it's Chrome, Andy's stock browser, FF or Safari...there's a whole lotta white area. Tough compromise, especially with how well these AMOLED displays have proven themselves with bettering their colors, saturation & white points, incredible pixel density and ...of course the 'blacks'. Can't beat em!
Thanks for sharing.
J
Cailey1100 - Tuesday, March 10, 2015 - link
When you said super AMOLED display, you can get better glimpse of your screen when using your phone in a direct sunlight?dusk007 - Sunday, January 4, 2015 - link
I don't see why you cannot report both. I.e. like they do on notebookcheck minimum fps and avg. fps. It works in one simple bar.Solid until 20% APL -> striped until 80% APL
That offers all the relevant information. And the high effective contrast is relevant. I do use Spotify most of the time I am running and that is the main sun use my phone gets. Spotify UI is probably below 20%. There are also various apps that allow optional dark themes. Sunlight legibility is very relevant. Just going over that characteristic is misleading.
Just like the focus on sRGB anandtech has. I read some reviews here that claim bad color accuracy when comparing a color setting that aims to provide AdobeRGB (Samsung Photomode) to sRGB. But of course Apple is always right because all they offer is one setting and that is sRGB. I am hoping the Apple bias is leaving with certain people, but still for architecture related content you guys are THE BEST. Just reviews is a bit of a weak spot.
I also said it before look at how computerbase.de does diagrams and graphs. They have done it for years but most others still use the old school images. Must be the CMS.
Wolfpup - Tuesday, December 23, 2014 - link
Seems to make sense to me given as they point out AMOLED fades, and most stuff is close to 100% anyway.erc - Monday, December 22, 2014 - link
While it's not the highest number possible, it is a more useful benchmark if you're using the information to make a buying decision since it's closer to what you can expect from real-world use.Testing with the lowest APL would get you a higher number is about as useful a metric as when hardware manufacturers optimize their drivers for common benchmarks. It produces a more favorable score, but it doesn't tell you anything about how the hardware actually performs outside of that benchmark.
grahaman27 - Monday, December 22, 2014 - link
but the concern is the fairness between AMOLED vs LCD.When you crank the brightness on LCD, every pixel is at 100% regardless of the content of the screen. This is not so on AMOLED in fairness you have to test AMOLED on a screen that utilizes every pixel... right?
Wolfpup - Tuesday, December 23, 2014 - link
Err...that's just it, it isn't fair to give a brightness number for AMOLED that it won't ever actually reach, while LCD actually can be as bright as any test shows.I've found that I have to crank up my 1 phone with an AMOLED screen to max brightness to see any details on darker scenes on shows, which I don't have to do with LCD...which is ironic given the claims about blacks on AMOLED lol
basroil - Tuesday, December 23, 2014 - link
"I've found that I have to crank up my 1 phone with an AMOLED screen to max brightness to see any details on darker scenes on shows, "Either you didn't calibrate your screen or the app you're using was made on uncalibrated devices and never bothered to distinguish between gammas
zepi - Wednesday, December 24, 2014 - link
Or he is using the AMOLED phone in a real world, not in pitch black testing chamber where the infinite contrast figures of OLED's are quoted.Try a 80% APL contrast test in sunny conditions with a modern LCD and OLED displays and you might be surprised what "infinite contrast" OLED's means in practice.