AT 101: Understanding Laptop Displays & How We Test Themby Brett Howse on July 10, 2018 8:00 AM EST
Everyone might have a different take on what they feel is the single most important factor to consider when purchasing a new laptop, but it would be hard to argue that the display quality shouldn't be near the top. There’s simply no other part of a notebook computer you’re going to use more. The good news is that display quality has improved immensely in recent years, with even some low-cost notebooks offering a good display.
For a while now we've been wanting to do some more fundamentals-driven articles – an introduction to various technologies for people who don't have 21 years of the computing industry crammed into their heads like we do – and given the importance of displays, this is a great place to start. This gives us the opportunity to cover what makes up the display stack in a notebook, how we test it, and what we’re looking for. There’s a tremendous amount of terminology which can be a bit overwhelming as well, and with the advancements in display tech, it’s worth defining the different display types and going over their strengths and weaknesses.
There’s no best display technology for everyone, although the industry has somewhat converged on just a couple of different options, but each has their own strengths and weaknesses.
Probably the easiest place to start is display resolution. This is simply the number of pixels on the display, measured horizontally, and then vertically, so a laptop with a resolution of 1920x1080 is going to have 1920 horizontal pixels and 1080 vertical. Over time, there’s been a shift away from the lower resolution displays we were used to, and of course the advantage of a higher resolution is that you can end up with a sharper image and crisper text.
Common resolutions on laptops available today are:
- 1366x768 – WXGA
- 1920x1080 – Full HD or FHD
- 2560x1440 – Quad HD or QHD
- 3840x2160 – Ultra HD or UHD, sometimes also called 4K (although originally 4K was 4096x2160)
Those are the most common, although some devices offer different aspect ratios and unique resolutions as well. If you know the display size and resolution, you can also calculate the pixel density by dividing the diagonal resolution of a display by the diagonal size of the display in inches. Since only the display size is commonly measured by the diagonal, you’d have to convert the resolution from width and height using the Pythagorean theorem. The resulting value will be the display density in pixels per inch (PPI), or how many pixels per inch of display measured on the diagonal. The higher the PPI, the sharper the display.
But as with most things, there’s a cost to moving to a high-resolution display, and it’s not just the actual cost of the panel – although that will almost always be higher as well. Because pixel density is inversely proportional to how well a panel blocks its backlight – denser panels will block more light – increasing the pixel density of a display requires ramping up the strength of the backlighting system as well. And while that’s not an issue for desktop monitors because of their constant power source, for laptops it can have a significant impact on battery life.
There’s also a cost in terms of software usability. Often, you’ll see an image such as this when comparing the different resolutions available.
If you were using your display at 100% scaling for each resolution, the image above would accurately provide you with an idea on what kind of desktop space you’d have available, but if you ran a typical 15-inch laptop with a 3840x2160 resolution at 100% display scaling, everything would be so tiny that it would be somewhat difficult to use, so Windows provides display scaling options to compensate.
Years ago, it was assumed by software developers that displays would be at or around 96 DPI (Dots per inch, which in this case is equivalent to PPI). As display technology has improved, we are now seeing many laptops offering much higher pixel densities than that. A 13.3-inch laptop with a FHD resolution is already well over the 96 PPI or a traditional display, offering 165 PPI, so Windows will scale images and text up to make it larger. However, older software can sometimes perform unexpectedly when scaling isn’t at 100%. This issue isn’t as bad as it used to be, with software being updated to address it and Windows 10 offering better scaling APIs and support, but it is likely never to be completely solved on Windows. For a deeper look into this issue specifically, check out our look at this from a couple of years back.
Subpixels on a 1920x1080 13.3-inch display - the haze is the anit-glare coating
Subpixels on a 3200x1800 13.3-inch display - no haze means it is glossy
Still, even with the drawbacks, the advantages of a higher PPI display are often worth the costs, since the image sharpness is just so much better. This is certainly a case of diminishing returns though. Although a display with a 192 PPI resolution is going to look much better than a 96 PPI one, one that is 384 PPI is going to be less noticeable compared to the first jump, and for a higher cost. Smartphones often have much higher density displays than laptops, but the display is often held much closer to the eyes than a laptop would be. Smaller laptops with very high-resolution displays are going to look great, but the extra power usage of a UHD display on a small notebook might not make it the best fit.
Inspecting the Aspect Ratio
Aspect Ratio is simply the number of horizontal pixels per vertical pixel, expressed as a ratio. The most common aspect ratio on laptops today is 16:9, so for every nine vertical pixels, there are sixteen horizontal pixels. But that wasn’t always the case.
Likely the most common aspect ratios on all computers in years past was 4:3, which was a much more square aspect ratio than we see now. When the world moved to widescreen displays, computers moved to a common aspect ratio of 16:10, but when HD televisions adopted 16:9 as their standard, there was a trend for PC makers to adopt that as well, and it has won out for the time being.
However, much of the work done on a computer often requires additional vertical space, so the loss of 16:10 was mourned by many. At this point, pretty much only Apple consistently offers notebooks with this aspect ratio. You can see the advantage of having more vertical room though when you look at most web content, which is vertical scrolling. The move to 16:9 helped when watching HD video content, and somewhat with movies, although they are generally even wider aspect ratios than 16:9, but hasn’t always been a blessing on the PC.
Luckily, we’ve seen some movement back towards taller displays on notebooks as an option for those that aren’t big fans of 16:9. One of the first was Microsoft with the Surface Pro 3, which offered a 3:2 aspect ratio. On a convertible device like the Surface Pro, the taller aspect ratio also helps when switching the from a landscape view to a portrait view, which you’d do more often with a tablet, but even on laptops, the extra height offered by a 3:2 display is very welcome for most productivity tasks. There’s a smattering of laptops out there now that have adopted 3:2, and likely more on the way.
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s.yu - Saturday, July 21, 2018 - linkSo...no mention about what non-PWM screens use for dimming? Or do high frequency PWM screens don't count as PWM? And a lot of people seem to be complaining in particular PWM of OLED screens, so why do OLED screens (at least the RGB variety) all use low frequency controllers?
Solandri - Saturday, July 28, 2018 - linkVery nice article. Some minor corrections/additions. (I'll post one at a time since the site is flagging it as spam)
Light from our sun is actually about 5800K. 6500K is the combination of sunlight and blue sky on a sunny day. It's bluer than direct sunlight because part of the red light gets scattered by the atmosphere (and sent to regions experiencing sunrise/sunset). Daylight in the shade (lit mainly by the blue sky) is closer to 9000K. That combined with the 5800K direct sunlight produces about 6500K.
Solandri - Saturday, July 28, 2018 - linkNon-RGB subpixel arrangements can offer the same viewing experience as RGB while using fewer pixels. Your eyes have the best resolution in green, not so good in red, and absolutely terrible in blue. The non-RGB subpixel layouts take advantage of this, usually by using two green subpixels for each red and blue subpixel. This results in fewer total subpixels (a "lower" resolution), but no discernible loss of resolution to your eye. Older video standards like NTSC and even newer image encoding algorithms like JPEG and MPEG do the same thing to reduce storage space by decreasing the color resolution. So this isn't something new - every TV show you've viewed growing up had its colors mangled this way, and you've never noticed it. So it's silly to suddenly pretend that non-RGB is suddenly inferior.
Solandri - Saturday, July 28, 2018 - linkAh, it was the website link for this which was flagging the comment as spam. Google "your eyes suck at blue" and you'll get the site with a graphical example of how you can completely mangle the blue channel and the picture will still look the same.
Solandri - Saturday, July 28, 2018 - linkThe oddball RGB subpixel layout often used in OLED panels can actually be better for devices which are meant to be used in both portrait and landscape orientation. The RGB subpixels are usually arranged so the relative position of red, green, and blue are the same when rotated 90 degrees (though there might be a shift of one subpixel). In contrast, the traditional RGB stripe layout is completely different when rotated 90 degrees.
Keeping the subpixel layout the same in both orientations allows you to do subpixel rendering in both orientations. Subpiexl rendering improves the apparent resolution of the screen without increasing the actual resolution. So if you're rendering a diagonal white line, instead of rendering it as (capital letters are lit, lowercase are black):
You render it as
And you've tripled the screen's apparent resolution without adding any new pixels. This trick is most often used with fonts (ClearType in Windows). But the RGB subpixel layout means apparent resolution can only be increased in one direction, and fonts designed for subpixel rendering will only work in one screen orientation. If you rotate the screen 90 degrees, suddenly the subpixels don't fall the way you expect, and your subpixel rendering breaks. Not so with the subpixel layout used in some OLED screens. If you turn the screen 90 degrees, the RGB layout remains the same, and your subpixel rendering still works.
Solandri - Saturday, July 28, 2018 - linkYou forgot to mention the RGBW subpixel layout, which I wish would die but keeps coming back. That's where they add a white subpixel to increase the apparent brightness of the screen. The R, G, and B subpixels generate those colors by blocking 66% of the light. So when you display a white pixel (R, G, and B lit), you're actually only seeing 33% of the backlight brightness. Someone came up with the idea of adding a white subpixel, so when displaying white you see 3*(33%*25%)+(100%*25%) = 50% of the backlight brightness. So the screen can appear brighter at the same power level, or the laptop will use less power at the same image brightness.
Unfortunately this comes at the cost of muting colors, since your colors are now only transmitted through 75% of the subpixels instead of 100%. And you end up with pale red, green, blue, and mustard yellows and lavender purples.
Solandri - Saturday, July 28, 2018 - linkThe solution many vendors came up with to customers complaining about poor colors is to create a mode where the white subpixel is always off. But if you do that, now your whites are generated by only letting 3*(33%*25%)+(100%*0%) = 25% of the backlight through. And now your screen will either be dimmer than RGB, or will use more power when at the same brightness as RGB. In other words, you've defeated the purpose of using RGBW subpixels in the first place. If you see a review mention the screen is RGBW, that's a big red flag and should be avoided.
HappyTechKnow - Tuesday, July 31, 2018 - link
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