Apple has been hit by a class-action lawsuit claiming the company deceived customers over the quality of the screens in the 20-inch iMacs launched last summer.

The screens are inferior in quality to those in the previous generation of 20-inch iMacs, and to those in the current generation of 24-inch iMacs, and are practically unusable for tasks like editing photographs, according to the lawsuit, filed by Kabateck Brown Kellner LLP.

"Apple is duping its customers into thinking they're buying 'new and improved' when in fact they're getting stuck with 'new and inferior'," said Kabateck Brown Kellner managing partner Brian Kabateck, in a statement. "Beneath Apple's 'good guy' image is a corporation that takes advantage of its customers."

The company pointed to claims by Apple that the 20-inch iMac's screens can display "millions of colours at all resolutions" and provides a "stunning" visual experience for uses such as watching movies, editing photos and playing games.

In fact, these claims are only accurate for the 24-inch iMac, which can display 16,777,216 colours, according to the law firm. The 20-inch system, introduced in August 2007, can only display 262,144 colours and uses a different, cheaper LCD technology called six-bit twisted nematic film (TN).

"The 20-inch iMac's TN screens have a narrower viewing angle, less colour depth, less colour accuracy and are more susceptible to washout across the screen," the law firm stated. The technology is "particularly ill-suited to editing photographs because of the display's limited colour potential and the distorting effect of the colour simulation processes," the firm said.

In 2005, Apple settled two different class-action suits covering its iPod music players, one covering the capabilities of the iPods' batteries and another claiming that the iPod Nano's screen quickly became scratched and unusable under normal use.