02 February 2009

Some observations on Laptop screens

Rob Galbraith recently published a review of screen quality in four select laptops. One of his conclusions is probably very unwelcome with Apple; their new 15" MacBook Pro comes last in colour accuracy; even beaten by a tiny 10" netbook.
At the top of the heap is a discontinued model from Lenovo; the T60. When this model was current, some manufactureres (not only Lenovo) tried to implement higher-quality panels in their laptops for the sake of more flexible viewing angles. Higher colour accuracy came in the same bargain, but I can't recall it as a feature reaching the marketing hype.
Anyway, the secret behind those screens were a panel type called IPS, as opposed to the cheaper and more prevalent TN. I recommend Wikipedia for reading up on the various panel technologies.
Today, IPS panels are only found in large, high-end desktop monitors such as the ColorEdge series from Eizo. They are no longer produced in laptop sizes. Well... I'm only 95% sure of that, but I have visited the websites of all the major TFT/LCD manufacturers, and the only non TN panel I have found is a 17" wide-screen from LG-Phillips. I suspect that the optional non-glare panel with the coming 17" Macbook Pro is actually this one, but that's not verified anywhere.
As far as I have been able to make out, there are currently no other 17" laptops on the market using this panel.
However, there's more to laptop screens than just panel type. Screen brightness is also important, and so is gamut (the number of colour hues discernible).
Screen brightness is mostly determined by quality and quantity of the light source(s) behind the panel. Traditionally, the backlight is provided by a cold-cathode fluorescent tube (CCFL). In laptops it's most common to use only one tube to conserve power, whilst in desktop screens one can find up to 4 tubes. One problem with CCFL lights, in particular when using just one tube, is to maintain even illumination across the whole screen area. Low-quality screens will often be darker towards the lower half and/or the corners. In Galbraith's test, by the way, there is one CCFL-based screen with two tubes. Even with two tubes, the screen suffers somewhat from low brightness. So I guess there's not much hope for further improvement with this sort of backlighting. At least not without a serious drain on the batteries.
That's where the new LED-based backlighting in eg. Apple's MacBooks or Dell's Precision series comes in. LEDs are superior both in terms of brightness and illumination evenness. It is therefore unfortunate that all the panel manufacturers have put TN panels on top of the LEDs.
I say "unfortunate" with reference to the last of my selected parameters to define a good screen; the width of the gamut. TN-panels are limited to representing red, green and blue hues with 6 bits each (ie. 26 x 26 x 26= 64 x 64 x 64 = 262,144). That's not really a big number of hues between all-black and all-white. IPS-panels, on the other hand, will display 8 bits for each of the primaries, giving you a total of 16,777,216 hues. In theory, at least. In practice the IPS panels don't achieve that wide a gamut, and the Lenovo T60 is a good example of that in Galbraith's test; displaying a considerably smaller gamut than the rivalling TN-panels. However a potentially 64 times bigger gamut than a TN panel will count for something in a good implementation. It's not without reason that the high-end desktop screens still use IPS.
The TN-panels also vary much in the width of the gamut they display in practice. Most of the panel manufacturers give these numbers as a percentage of the NTSC gamut, which is the standard colour space for TV sets. The sRGB is a smaller colour space than NTCS, but I'm not sure how Adobe's RGB relates. The figures stated by the manufacturers vary from a standard around 45% up to, and exceeding, 100%. With hardware supporting only 6-bit representation, I suppose it means that each colour is just "spread out" into a wider colour space. However it may also be the result of what someone in a forum described as "colour dithering". Apparently this is a task performed by the machine's operating system to map the real colour into something viewable on the screen. There was some usual trench-war between Mac and Windows users as to which system provided the best rendering, but I didn't learn much. And stupid as I was, I didn't even bookmark the site. :-(

Well, some technical rant, huh?

The upshot is that it is currently very difficult to purchase a laptop that is both reasonably transportable (ie. smaller that 17" screens) and reasonably well suited for image editing. Despite Galbraith's conclusion that Apple is no longer king of the heap, I would say that the 17" MacBook PRO with non-glare screen looks worth waiting for. It's a bit too big for my taste, but very light for a 17".
I would probably have to run Windows Vista on it, though.
(Grinning, ducking, and running like h...)

Anyway, I will keep looking. My goal is to decide on a laptop before summer. Meanwhile I could simply try to save up some money. As always, quality doesn't come cheap.

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