Tue 07 Apr 2009 14:43
Understanding Portable Displays
Perhaps the most important thing about a portable computer, be it a notebook,
netbook, PDA, smartphone, or internet tablet, is what it provides you
versus what it demands from you. One of the most important things a
portable machine provides is logical screen area or screen resolution:
the amount of data it can show you on the screen at one time. But of
the most important things a portable machine requires/demands is weight:
what does it take to carry it?
Screen resolution is measured as a grid of pixels (dots) in width x height format, e.g. 1024x768 means a screen that is 1024 dots wide and 768 dots high. Weight is of course not the only thing that determines portability: size is important too, but generally larger machines are heavier and smaller ones are lighter, so weight is a good shorthand for "weight and size".
A quick way to approach the costs and benefits of a portable computer is to compute the ratio of its benefits (e.g. screen resolution) to its portability cost (e.g. weight). So a quick assessment of a portable computer is to compute its pixel to weight ratio: if the weight ratio is high, the portable computer may compare better to one that has a lower pixel to weight ratios. I've written a little tool to compute this information (in units of pixels per milligram, i.e. ppmg), at http://www.cs.toronto.edu/~jdd/screenspec.cgi.
Pixel to weight ratio isn't quite enough, though, because there are limits to human sight: a portable computer is of no use if the pixels are so small that they cannot be easily seen. "Too small" depends on the distance the screen is from one's eyes. I tend to use devices like cellphones and PDAs at a distance of 18 inches from my eyes, and laptops at 24 inches. Generally, distance multiplied by the pixels per inch of the screen is a constant. For example, for me, I'm quite comfortable with 170 ppi at 24 inches, but beyond that, I feel some eyestrain. At 18 inches, that works out to (170 x 24) / 18 = 227 ppi. In my (anecdotal) experience, many people seem comfortable with 125ppi at 24 inches and 167ppi at 18. Of course, there is much more to this than a simple ratio: tolerance for high pixel densities varies depending on what the person is doing, what fonts are being used, and many other things.
Still, a pixel to weight approach lets one compare machines in interesting ways: for example, an Apple Ipod Touch has a 3.5" 480x320 screen and weighs 115g; that's 164 ppi and a pixel to weight ratio of 1.3. This is comparable to a Nokia E90 Communicator, which has a 4" 800x352 screen and a weight of 210g; its ppi is 218 and pixel to weight ratio is 1.34. But now consider a Nokia N810 Internet tablet: its 4.13in 800x480 screen (ppi is 225) and weight of 226g gives a significantly higher pixel to weight ratio of 1.69. But with ppi around 220 vs. the Ipod's 164, either Nokia device may result in eyestrain where the Ipod Touch does not.
Now look at some notebooks. A (large and heavy) Dell Vostro 2510 notebook weighing 5.72lbs with a 15.4" WUXGA (1920x1200) screen offers 147ppi and a pixel to weight ratio of 0.9, which is (perhaps surprisingly) a higher pixel to weight ratio than a (small and light) netbook, the Dell Mini 10 with the new high-resolution 10" 1366x768 screen (ppi of 155); its weight of 2.86lbs results in a lower pixel to weight ratio of 0.8 (at a slightly higher ppi, too). Compare this to a Macbook Air: with a 13.3" 1280x800 screen, it weighs 3 lbs; its pixel to weight ratio is 0.75. Unlike the other too, though, the Macbook air has an easier-to-read ppi of 113.
Of course, this doesn't mean that one should pick portable computers based solely (or even mostly) on pixel to weight ratios, or ppi for that matter. It is merely one possibly useful way to compare portable machines, and should be at most only one criterion among many, when making a decision.