Tiny image projectors take a step towards the shops

17 Jul 2003

Compact personal image projectors could soon be changing the world of consumer electronics, and all at a price that's easy on the pocket. Jacqueline Hewett finds out more.

From Opto & Laser Europe July/August 2003

Have you ever wished that you could view larger versions of images stored on your digital camera without having to transfer them to a laptop or PC first? Or perhaps you would like to view pictures or documents on your mobile phone, but are frustrated by the inherently small screen size?

Although the obvious answer might be to use a larger display, there is a problem with that approach: as the display gets larger, the power consumption, cost and size of the device increase. However, a solution could be at hand in the form of a new breed of tiny image projectors that can be integrated into a handheld device or produced as a compact add-on module.

This area of research, called personal projection, was one of the highlights of the Society for Information Display (SID) annual meeting, held in Baltimore in May. Delegates at the SID show caught their first glimpse of the personal projectors that could soon be hitting the high street.

Massachusetts Institute of Technology (MIT) showed off its laser-based projector, which is currently under development and measures just 5 cm³. Also at the show was the handheld pocket projector being developed jointly by Lumileds Lighting, a Californian maker of LEDs, and Philips, the Dutch consumer electronics specialist.

Size and price matter Personal projectors need to meet consumer expectations if they are to find a market. "Any projector must take into account the main characteristics of typical handheld products," said Michael Bove, who is leading the research effort at MIT. "They must be small, inexpensive and battery-powered."

Bove and his colleagues settled on using light from a linear array of vertical-cavity surface-emitting lasers (VCSELs). The device is elegantly simple (see figure 1). A field programmable gate array takes an input video signal and uses it to modulate the light output of the VCSEL array. A rotating mirrored polygon driven by a brushless motor then provides vertical scanning to produce an image. Infrared 850 nm VCSELs are used to project images onto a phosphor screen.

After demonstrating the principle with the infrared VCSELs, the MIT team set about making a visible-light version using a linear array of fifty red 670 nm VCSELs supplied by Honeywell. One of the initial design goals was to keep the image in focus, independent of distance. To achieve this the MIT team used a plano-convex lens both to collimate the lasers and to magnify the image.

The end result projects images with the rather bizarre aspect ratio of 50x480 pixels, but Bove's team has come up with a clever way of improving on this. "Because of the way VCSELs are made, they are spaced fairly far apart," Bove told Opto & Laser Europe. "The ones we use have a 10 µm aperture but are spaced 100 µm apart, leaving dark spaces between the pixels." Bove's team exploits these spaces by applying a fast but small horizontal scan to either the VCSEL array or an optical element in front of it. This effectively means that individual VCSELs generate four adjacent pixels instead of one, resulting in an image of 200x480 pixels.

Having produced useful images updated at 60 Hz, albeit in "red-scale", Bove and his colleagues are now trying to reduce the size of their current device from 5 to 2 cm³. They also want to increase the number of VCSELs the projector uses to 160, allowing it to achieve standard video graphics array (640 x 480) resolution. Needless to say, Bove is keen to get his hands on shorter-wavelength VCSELs to improve the brightness of the projected image and ultimately make a full-colour display.

"People want to know when we are going to have green and blue [VCSELs]," he said. "But the first thing is to get better red ones. At that part of the spectrum even moving 20 nm gives you a factor-of-three improvement in brightness to a human observer."

Full-colour images Meanwhile, Lumileds and Philips have come up with a pocket projector that features a very different architecture and already delivers full-colour images. The collaboration's approach is to send light from high-power LEDs through a liquid-crystal microdisplay and a series of lenses (figure 2).

Lumileds and Philips have developed three pocket projectors to date, all of which generate colour images. The first contains a single Lumileds 5 W Luxeon white light-emitting LED; the second contains four white 5 W Luxeon LEDs; and the third contains separate red, green and blue (RGB) LEDs. The RGB version has been made into a concept device and was shown at the SID meeting.

LED-based projectors offer several advantages over their lamp-based counterparts. Firstly, LEDs have fast switching speeds - typically less than 1 µs - which means that they can handle video projection. They are also smaller, lighter and have a longer lifetime than lamps. Because they require only a low driving voltage, LEDs can be driven by a battery and produce less heat.

In contrast with the MIT team's design, the Lumileds device uses two sets of optics - the first set to collimate the light from the LED, and the second set to project and focus the image. "The device has an adjustable focus," explained Steve Paolini, director of business development at Lumileds. "The optics are configured to focus the image around 30 cm away. You can then adjust the focus manually if required." At this distance, the image would be about half the size of an A4 sheet of paper.

The last, and most expensive, component of the Lumileds device is a transmissive liquid-crystal microdisplay. The RGB prototype uses a 0.5 inch diagonal microdisplay and is wired up to project single still images. Video images are also a possibility in the future.

Paolini says that one of the biggest challenges the team has faced so far has been getting enough light out of the projector. Each LED in the RGB prototype has a flux of 120 lm, but the total flux emitted by the projector is just 4.3 lm.

The reason behind this drop is a factor called "etendue", which refers to the size of the light-collection cone of an optical part. Each component in an optical path has an acceptance angle that defines a cone through which all the light must pass. The component with the smallest etendue in a device defines the overall etendue for the whole system.

"With the exception of lasers, light sources emit isotropically and consequently have a very large emittance angle," said Paolini. "The process of collecting this light into the much narrower acceptance cone of the microdisplay is very difficult."

This problem limits the size of the illumination source, and consequently the light output of the projector. But although the brightness of the projected images is low compared with the conventional digital projectors used for business presentations (which generally have an output in excess of 1000 lm), Lumileds reckon a lower output of 5-10 lm is acceptable for the small screen-sizes used in personal projection.

Operating the projector couldn't be simpler. The interface between the camera and the projector is a memory card. "You would take your pictures, snap the memory card out of your camera and pop it into this thing, and cycle through the pictures," Paolini explained. And with dimensions of 15x9x3m, this battery-powered prototype already seems to be meeting most of the criteria demanded by consumers. But what about the cost?

A question of cost "We estimate that the pocket imager would need to retail in the electronics shops for an introductory price of around $500 [€430]," said Paolini. "We see this being introduced as a companion product for a camera or a PDA. If you're showing someone photos [from a digital camera], you don't want to cart along your laptop."

Both Bove and Paolini are optimistic that personal projectors will be making their commercial debut soon. "There is nothing from a technology point of view that prevents these things from being made today," commented Paolini. "It's more a question of whether the market is there or not. If you were trying to build a meeting-room projector, the LED technology is not yet capable of producing enough light. But in the case of the pocket imager, it's buildable with existing LED and projector technology."

Lumileds is currently in discussions with several companies interested in using LEDs in other personal projector applications, including mobile phones. Bove, too, believes that this is where the field is heading. "If these projectors - or something for the same purpose but using a different technology - are as small, cheap and efficient as we'd like, I can see them becoming a part of every PDA and laptop computer, and also many phones," he said.