Surge In Demand For Super-size TVs
Gone are the days of the humble 14 inch portable television. Lounges all over the globe are being pervaded by high-definition, super-sized displays. According to a recent report by market researcher DisplaySearch, the average display area for one technology prevalent in the large screen rear-projection television (RPTV) market – LCDs – has grown by more than a third in the past year, with 32 inch displays rated as the current top-grosser.
This surge in demand for larger screens is not going to stop any time soon. Already we are seeing 50 inch displays as the most coveted product behind electrical retailers windows and by the end of this decade those could well be replaced by 60 inch displays or larger. But there is an obstacle in the path of ever-increasing RPTV screen size: the light source. RPTVs have to work on a different principle to smaller, flat-panel displays typified by computer monitors and the like. Instead of using an LCD the same size as the screen to constitute the display, RPTVs project light through a miniature version of the image – a microdisplay – and then optically magnify it to a screen many times the microdisplay s size. In the past the light has been provided by ultrahigh-pressure (UHP) lamps, but these will be too dim to illuminate the next generation of TVs.
Thankfully, consumer electronics companies have a few new solid-state technologies to choose from. One of these is LEDs, whose small size has enabled them to be used extensively in mobile phone backlighting. While they have enjoyed some success in mid-sized displays, a step-up into the world of large RPTVs has become possible by an innovation that uses photonic lattices to enhance the light output. The other option is lasers. These are less of a direct replacement for UHP lamps, and until recently have suffered from cost and power issues, but in spite of these hindrances two companies are foretelling them as the future of light sourcing.
In 2002, a young Alexei Erchak collected his doctorate from the Massachusetts Institute of Technology having had the clever idea of using photonic lattices to extract more light from LEDs. Four years on and his established company, Luminus Devices in Woburn, MA, has successfully demonstrated the PhlatLight technology as a light source for RPTVs. Models featuring the chips, such as Samsung s 56 inch RPTV, are already beginning to appear on the consumer market.
The effectiveness of PhlatLight LEDs all hinges on the bandgap of the photonic crystal that is fabricated into the top cladding layer of the standard epitaxy material (AlInGaP for red LEDs, and GaN for green and blue). This photonic bandgap is analogous to the electronic bandgap in a semiconductor, but rather than restricting the conduction of electrons based on their energy, it is the propagation of photons that is restricted. The bandgap is tuned to include the emission wavelength of the semiconductor, without coupling to the guided modes – a major source of loss. In this way, the LEDs are free to radiate more light.
"In a PhlatLight LED the light is emitted from the surface of the chip, [whereas] in standard LEDs [some of] the light is emitted from the edge," explained John Langevin, the VP of marketing and business development at Luminus. "Surface emission allows Luminus to scale the chips without losing light and efficiency."
Available light is just one of the benefits of PhlatLight LEDs and other solid-state devices. Environmentally, they are preferable to mercury-containing discharge lamps, they can be turned on and off rapidly, and they have a large color gamut (in other words, they have a wide color spectrum). But one of the biggest advantages is lifetime: UHP lamps fail roughly every 8000 h and at $300 (€240) for a new lamp in addition to the inconvenience of calling out a qualified technician, replacement several times in a TV s 30,000 h life expectancy is an undesirable prospect. LEDs, on the other hand, can be expected to last 20,000 h, so may only need to be replaced once, if at all.
However, refining the PhlatLight technology has not been easy. "The most challenging obstacle for Luminus has been the ability to mass produce devices efficiently," said Langevin. With 10 different manufacturers having demonstrated PhlatLight in RPTVs at the Consumer Electronics Show in Las Vegas in January this year, we can expect to see a lot more from Luminus.
LEDs may be the first solid-state technology to be used in RPTVs, but Don Klein, VP of business development at Principia Lightworks in Woodland Hills, CA, considers lasers to be the light source of the future. "The reason LEDs have got any traction is because of the industry s need to replace UHP lamps, and the cost and power efficiency obstacles to a broad adoption of laser light sourcing. Principia s eVCSEL [electron-beam-pumped-vertical-cavity surface-emitting lasers] overcomes those obstacles."
VCSELs are preferable over their edge-emitting counterparts because they can be grown and tested on a single wafer, reducing manufacturing costs. Principia s eVCSELs further exploit the same benefit by using a cathode ray tube (CRT) to stimulate lasing, instead of applying a direct current. Wafers are provided by Bandwidth Semiconductor, a subsidiary of Spire, with InGaP/AlGaInP used for red lasers, ZnCdSe/ZnSSe for green and ZnSe/ZnSSe for blue.
"CRTs and their related electronics are a mature, readily available and inexpensive technology," said Klein. "There is no functional difference between an eVCSEL and a traditional VCSEL. However, eVCSELs are simpler in construction because p-n junctions are not required, as electrons are injected into the laser cavity by the CRT s scanning electron beam."
For many, the CRT will conjure unwelcome recollections of cumbersome, dusty boxes that should be consigned to the scrapheap. Unlike those of yesteryear, however, Principia s CRTs can fit into a similar width of housing as the thinner RPTVs currently on the market. They also bring a large degree of flexibility in coupling to the microdisplay and produce a better picture quality, as the ability to let the screen go completely black through modulation of the beam gives a contrast ratio of more than 10,000 to 1.
"Lasers have been the holy grail of light sources for projection displays for many years," said Klein. "In general they offer the widest colour gamut – substantially greater than phosphors or LEDs."
The earliest that customers can expect to see laser-based RPTVs is late 2007. But the head start that LEDs have does not worry Klein: "Cost is our trump card. The eVCSEL offers these performance enhancements with minimal increases in costs to consumers."
Principia is not the only manufacturer vying for the laser RPTV limelight. Just down the road in Sunnyvale, Novalux has got big ideas for its Necsel, or extended-cavity surface-emitting laser, which can achieve high power output without the need for a CRT.
This, said Novalux s VP of marketing Greg Niven, is why Necsels will prosper: "Principia is very specifically targeting RPTV – Necsel is a platform technology. It could potentially be used in many other applications that currently use lasers, such as in medical equipment."
Necsels are singlemode external-cavity lasers, which use GaInAs quantum wells in the gain region. Novalux produces the colors required by efficient frequency doubling. In principle this is a restrictive technique, but one that proves perfectly satisfactory for the red, green and blue light that a microdisplay necessitates. Nevertheless, they do maintain the characteristics that make lasers a tempting alternative to UHP lamps and LEDs: a 30,000 h lifetime (as long as the TV itself) and a color gamut that is more than double the requirements of the US standardizing body, the NTSC.
The trick up Novalux s sleeve, however, is the variety of devices aside from RPTVs that its Necsel projectors could be built into. The compactness through lack of a CRT, and the ability to scale to any required power, means that the projectors could find their way into, for instance, mobile phones in the form of "pico-projectors".
"In the short term the company is 100% focused on projection, which includes both RPTVs and mobile projection," explained Niven. "The latter could be either a 200 lm projector the size of a mobile phone, or a 20 lm projector actually embedded inside a phone."
Head to head
With three not-too-dissimilar technologies all competing for the largest share of the high-end RPTV market, it is hard to see who will eventually come out on top. Novalux s Necsels are set to be inside RPTVs by the 2007 holiday season to coincide with Principia s eVCSELs. Although Principia will have an initial price advantage by cashing in on the ubiquitous grounding of CRT technology, the mobile projection applications that Novalux are offering could prove to be a crucial venture. On the other hand, Luminus s PhlatLight LEDs will already have a year s worth of production before laser RPTVs hit the shelves. Only time will tell.