Notebook PC makers try their best, but screening out all the defects, so to speak, is not easy. Most of today's notebook PCs incorporate active-matrix thin-film transistor (TFT) technology in building their liquid crystal display (LCD) screens. TFT screens generally are brighter and more readable than their predecessor, the dual-scan passive-matrix LCD screen. But active-matrix screens, in part because of their thinness, also are more prone to arrive with dead pixels.

Aside from being the smallest element that a digital picture can be broken into, usually seen as tiny dots of coloured light arranged in a mosaic fashion, they also come with built-in transistors. And, according to one theory, that's where the problem begins.

'It gets very technical,' admits Hans Chow, IBM's technical marketing specialist for personal computing. 'But basically each pixel contains three built-in transistors. And there is always a chance that one of them may not be working, which then results in a dead pixel.

'If you have say a SuperVGA display, with a 600 x 800 pixel resolution, there is a very high possibility of this happening. So, don't even imagine what might occur with these new enhanced machines with 1024 x 768 pixels. It's almost unavoidable.' Some systems engineers cite other reasons for the phenomenon, ranging from excessive notebook usage to the inability of certain colour components to show up on screen.

Whatever the cause, Ms Hooi said after IBM replaced her screen, the offensive red dots went away, replaced by a new problem. 'Now I've got blue dots on my screen,' she said. 'Four or five of them, one big blue one and some lighter blue ones.' 'In my opinion, we have been very fair to her,' said IBM's public relations manager, William Tse.

'We have replaced her screen three times, and she is still not satisfied. Her complaint is really not a valid one, since it was only a problem of two pixels and technically there isn't anything wrong with her machine.

'At IBM we allow for up to 21 dead pixels before we will replace a screen. Given the technology that exists today, there is not much you can really do about it.

'I think that our 21-pixels standard is pretty rigid,' he adds. 'It's almost an industry standard.' Mr Tse acknowledged that the 701C had been discontinued, and said IBM planned to take no further action. But Ms Hooi said because IBM did not tell her their policy governing dead pixels, she does not feel 'obligated to accept this limitation'.

While Ms Hooi might appear unnecessarily petulant, Mr Tse is not entirely correct, either. The real problem is that there is no such thing as an industry standard for the number of dead pixels covered under warranty. Different manufacturers assign different thresholds for dead pixels. Most do not consider them a defect.

The US version of PC Magazine recently published a chart comparing latest model Pentium notebooks sold in the United States and the differences in dead pixel thresholds before they replace would screens.

Acer, NEC and Hitachi set their limits at five, Samsung at three, Hewlett-Packard at 15 and, of course, IBM at 21, while Digital, Sharp, Tatung and Toshiba all have 'no set policy'.

'Four dead pixels is probably acceptable,' said Mike McGuire, an industry expert on notebooks based at technology consultancy Dataquest in San Jose.

'This is especially true with a newer screen size and resolution. However, I'm not sure about how this applies to a discontinued product.' The location and the colour of faulty pixels usually was more significant than the absolute size or number, said K.C. Cheng, a Hong Kong service manager for Digital notebooks.

'If the dot is in a prominent position like the central area of the display or has an eye-catching colour like bright red, we usually will replace it, especially if the customer finds it irritating. Otherwise, we will explain to the customer that this is a defect which will not affect the function and performance of the laptop, and is considered acceptable.' At Compaq in Hong Kong, customer support engineer Steve Chan said his company considered six dead pixels its threshold. 'Anything under six dots is considered okay,' he said.

Compaq Hong Kong's view is that dead pixels inevitably arise in the manufacture of notebook PC screens. 'I would say that in general it hasn't been a problem for us,' said Swedee Chin, Compaq's marketing manager. 'Some dealers have raised the matter and we generally tell them that it's a manufacturing issue.' As a rule of thumb, most companies will replace LCD screens beset with dead pixels if no intentional damage was done by the user.

Apple Computer's PowerBook Duo home page has even informed visitors to the site of existing software that will help them locate dead pixels. 'Find enough, and Apple will give you a new screen,' it declares, but 'make sure they're dead pixels and not dirt'.

IBM's Mr Chow said he was heartened personally by word of on-going research, notably those involving crystalline phosphors, which has proved that portable liquid displays can be further improved. 'I think they're trying to improve the process,' he said, 'but it's quite hard. There is no perfect machine.'