Moving Images into the Future

One reason people prefer watching films in cinemas to sitting at home with a video is that 35mm-film projectors render richer colour, closer to the range of hues that the human eye can perceive. Even today, a conventional plasma-screen television set produces only 50% of the range of colour (known technically as the gamut) that the human eye can perceive. Film, by contrast, manages around 60%. Film, however, has a fundamental problem. A pristine print of a movie has rich, vibrant colour, but every time that print is projected it is degraded, and eventually it has to be replaced.

Digital cinema projectors get around this problem, but purists complain that the gamut is not as rich as film. That may soon change, though, thanks to a new digital cinema projector that uses lasers. It has been designed by a group of researchers at the Chinese Academy of Sciences in Beijing, led by Bi Yong, in collaboration with a firm called Phoebus Vision OptoElectronics Technology, and it can produce an eye-popping 166% of the gamut of the NTSC television standard used in the United States—about 80% of the range of colours that the human eye can see.

In both the 35mm projector and the digital projector, a bright source of white light provides the illumination. This is commonly a xenon lamp, in which electricity is arced through a tube filled with the gas. In the case of a 35mm projector, the image is projected through the film and onto the screen using a series of lenses. In the case of a digital projector, the white light is first filtered into blue, green and red components and then fed into an optical-processing device that makes use of micro-electro-mechanical technology to generate the image. Hundreds of thousands of tiny mirrors are employed, each of which can be tilted into one of two positions. Each mirror corresponds to a single picture element (pixel) in the projected image and acts as a beam-steering device, controlling whether that pixel is light or dark. Intermediate shades are created by tilting the mirror backwards and forwards thousands of times a second to vary the brightness of the pixel it is projecting.

The idea of using lasers as light sources for projectors dates to the 1960s, but two things have held it back. First, in those days, lasers were bulky, expensive devices, whose only role in the cinema was to threaten to cut James Bond in half. That problem has gone away as compact and inexpensive semiconductor lasers have become commercially available. The second problem, though, is that laser light can “speckle”, which is to say that when it scatters off a rough surface, a random shimmering and sparkling pattern is produced. Speckle, a consequence of the narrow range of light that a laser produces, degrades the sharpness of the projected image. It is this problem that Dr Bi and his colleagues have overcome. They use several red, green and blue lasers and feed the light from these into an optical-fibre cable to produce white light. Because the operation of each laser is independent of the others, their speckles cancel each other out. That done, the white laser light is filtered back into red, green and blue light and projected using the DLP system.

Dr Bi has, however, gone further than just eliminating speckle. He notes that if the red laser light used is made redder and the blue laser light made bluer, the gamut increases. Indeed, the theoretical maximum gamut of a laser-based system is almost 90% of what humans can experience.

The disadvantage of this increased redness and blueness is that higher-power lasers are required. But that may not matter too much. Lasers are more efficient than xenon lamps, which waste a lot of energy as heat. Dr Bi and his colleagues calculate that even with the boosted lasers their projector needs only 35% of the power required to run a normal digital projector. On top of that, lasers last for a long time, whereas xenon lamps burn out and need to be replaced periodically. Running costs should therefore be lower.

The initial cost will still be higher, though, at least to start with. When launched, Phoebus’s projector is expected to be ten to 20 times more expensive than those that use a xenon lamp. But that, no doubt, will change as the cost of lasers drops. Just as they have done with countless other areas, from telecommunications to compact-disc players, lasers could thus end up making going to the cinema even better than it is now.

Source: Economist