Camera maker Sigma is preparing its first move into the digital still camera market.

Sigma will unveil a prototype of its SD9, a single-lens-reflex and lens-changeable digital still camera on 24 February at PMA (Photo Marketing Association) 2002 in Florida, says the firm.

The SD9 will be the first digital still camera to use US-based Foveon's X3 image sensor technology. The X3 image sensor allows for more accurate and rich colour reproduction of digital still images, the two companies said.

The Sigma SD9 has a 3.5Mp (megapixel) image sensor and can produce images at a maximum resolution of 2,268x1,512 pixels. It also has IEEE1394 and USB interfaces and a video output which can switch between NTSC and PAL signals. The camera stores images on CompactFlash cards.

Existing image sensor technologies use either a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) chip, which register light levels through a layer of light-sensitive pixels. These cannot distinguish different wavelengths or colours of light and so just record monochrome images.

In order to capture colours, a filter made up of a mosaic of the three primary colours (red, green and blue), is placed over the chip's pixels. However, each pixel captures just one colour, red, green or blue.

Foveon took advantage of the fact that silicon absorbs light of different colors at different depths and built its X3 image sensor with three superimposed layers of light-sensitive pixels, one each for red, green and blue, so each pixel can capture all three colours, the statement said.

Allowing each pixel to capture three colours produces sharper images much closer to film image quality, without increasing the number of pixels, according to Foveon. And using only one chip is much cheaper than multi-chip systems using three CCDs, it said.

"The theory of the X3 technology is similar to the theory of colour films. That is why the final images captured by the SD9 get closer to film image quality," said Teruaki Kuwayama, a spokesman at Sigma. The company is targetting users of its film cameras with an interest in digital still cameras, he said.

The camera measures 152x120x79mm and weighs 809g without batteries. At the back of the camera, a 1.8in TFT LCD colour display viewer is attached. Some of Sigma's film camera lenses can be used with the SD9, Kuwayama said.