Mobile Device Displays: A Few Words on Pixel Density

A lot has been written about displays on mobile devices; smartphones and portable media players rely heavily on their screens to both display information and allow interaction. A lot of consumers might consider display SIZE when thinking about their next mobile device purchase, but I'd like them to also consider another, slightly more technical, attribute: PIXEL DENSITY.

It's obvious that display sizes vary a lot in the smartphone and PMP world. One comparison of a few relatively recent devices showed a variety of sizes and aspect ratios:

When publishing the specs of devices, most often, we're given two pieces of information about the display:
• Its resolution, expressed in pixels along each side (e.g., 320x240)
• Its diagonal measurement, as in the diagram below

However, we're rarely given the statistic that has a significant effect on a screen's readability and our enjoyment in looking at it, which is the pixel density. Screens with low pixel density can have that "blocky" effect, which can make text unpleasurable to read and photos look, well, yucky.

So, higher pixel density is always good, right? Unfortunately, no. The human eye is limited in its ability to perceive resolution...more than about 300 dots per inch (dpi) and our eyes can no longer distinguish them from each other. High-quality print output is generally in the 300 dpi range. Photos generally don't need to be this high to convey good detail (see my Photo Printing Worksheet for more info).

So where does that leave us with device displays? Well, there's a pretty broad range of what people find acceptable. I consider myself a screen snob and am generally unhappy with displays below about 170 pixels per inch (ppi). However, as noted above, more isn't always better...a display with 350 ppi isn't going to be much better than one with 300 ppi, and may even be worse. The higher the pixel density, usually, the smaller (physically) fonts appear, making it more difficult to read.

We can calculate pixel density (in terms of pixels per inch) by knowing the screen resolution and diagonal size, but it requires a bit of algebra. So, I created an Excel spreadsheet with those calculations and whipped up a table of some popular smartphones, PDAs & PMPs to show the range of pixel densities found in mobile devices today.

This is simply a screen grab of that Excel sheet (here's the Excel file if you want to download it for your own use), but it illustrates how impressive some of these recent devices have gotten in terms of displays. Sony has a weird fetish for painfully high pixel densities, both on their phones and some of the subnotebooks, but RIM, which resisted higher-resolution displays for a long time, now has some Blackberries with really stunning screens.

Hopefully, this will help you think about the pixel density of the next smartphone or PMP you plan to buy, since the display is one of the biggest influences on how much many people enjoy their mobile devices.