Mitsubishi has unveiled a liquid crystal display (LCD) that can be viewed from both sides. The display, which the company says is a world first, was developed initially for use in clamshell-type cellular telephone handsets and could help make such telephones thinner and lighter.

At present, many clamshell-style handsets have two displays: a large main-display facing inwards and a smaller sub-display that faces outwards that is used to display basic information when the phone is closed and the main display is out of view. Each of these displays typically consists of a glass LCD panel on which the image is shown and a backlight that sits behind the panel and projects light through it so the image can be clearly seen by viewers.

The displays are in a physically similar position inside the upper portion of the clamshell-case. However, each display can be viewed only from one side because the backlights restrict viewing from the reverse. A cross-section of this part of the phone case would reveal a four-layer sandwich of components: two backlights positioned back-to-back in the centre with their associated displays on the outer edge.

Mitsubishi's new display incorporates a conventional LCD panel with newly designed backlights constructed in a three-layer sandwich in which the display sits at the centre and the backlights are on the outer edge. The new backlights are transparent and so enable the single LCD panel at the centre to be seen from both sides even though it is in the centre of the sandwich. For viewing from the right, for example, the left-hand backlight transmits light through the panel and on through the right-hand backlight to the viewer.

Two versions

Mitsubishi has developed two variations of the reversible LCD and both were demonstrated on Tuesday at the company's research and development centre in western Japan.

The first version allows a single image to be viewed from both sides of the same panel. The image isn't adjusted depending on the viewing direction, so from one side text appears correctly and from the other side it appears reversed. A second type gets over this problem by rapidly changing the image on the display in synchronization with each back light 120 times per second so that the same image, correctly displayed, is projected in each direction 60 times per second.

The display has three modes: front view, rear view and simultaneous view from both sides.

Development of first-generation displays using the technology is nearing completion and with its unveiling on Tuesday the company is beginning to look for potential clients. In addition to mobile phones, the company anticipates that other small portable devices, such as PDAs (personal digital assistants), could also benefit from the technology.

Because the display uses only one LCD panel, it is thinner and its cost is around two-thirds that of two separate displays, said the company. The use of the reversible display also means that the sub-display on a telephone can be as large as the main display. This is advantagous for cellular telephones that incorporate digital still-camera or video-camera functions because images can be easily viewed and recorded without having to open the telephone.