Haitz's law, pixel pitch and the 30k Display

Guest article by Bob Raikes:

The main display technologies in the early days of digital signage were projection and PDPs. However, PDPs suffered badly from „screen burn“ especially when they were run at the high brightness needed for this application and with static images. Projectors could only be used where there were low light levels.

LCD has since become the major technology for flat panel signage, but it has been difficult to make in sizes of more than around 100″ diagonal. Part of this is the difficulty in driving the display – ensuring that the voltage needed to control each dot accurately was delivered to the centre of the panel. The further from the edge, the harder the problem and some big displays actually drive the LCD as though it was really two „half height“ displays, although this is tricky and costs more.

PDP is not quite as restricted and Panasonic made and sold PDPs at around 100″ and even a small number at 150″. However, the weight and bulk of the displays in sizes above 100” made the displays very expensive and difficult to install.

The solution for bigger displays has typically been to make displays using multiple tiles using separate LEDs. In the early days, these were generally made of individual LEDs of each colour which were selected and mounted with some drive electronics on a „tile“. Multiple tiles could be put together to build large video walls and in the case of an LED failure, a single tile could be replaced. Because the LEDs were quite large, the „dots“ of the display needed to be mounted apart (they also get hot which can be a problem if they are close together).

Typical distances between the LEDs would be 10mm or 12mm, and given that LED makers have a „rule of thumb“ that viewing should not be done closer than the pitch, but in metres (i.e. 10mm pitch means a 10 metre viewing distance), that meant that LED screens were only suitable for long distance viewing. Applications were displays at sports stadia, advertising (Piccadilly Circus or the Las Vegas strip) or for concerts.

Fortunately, LEDs can be very bright, so there was a natural fit between LEDs and these applications.
However, at 10mm pitch, the size of display needed to show FullHD resolution is 868″ or 22 metres!

Around 2002, LED panel makers got down to 6mm pitch and since then, there has been an acceleration. Part of this was enabled by the development of RGB LED packages that put all three colours in a single package, making life much easier for makers.

In 2003, Barco showed its first 3mm pitch LED wall, but the cost was €108,000 per m2! Performance was getting better and better, but that was a lot of money! The size of a finished screen with FullHD was still 260″ or 6.6M diagonal. That’s a big jump up from LCDs at 100″ and left a gap (although LCDs could be tiled together to make video walls).

Over the last couple of years, there has been a big push to smaller and smaller pitches, with a number of companies claiming 1.5mm and below at the ISE event in February this year (although one claiming 1mm pitch couldn’t show us the display actually working “to highlight the black” they said!).

Now, Chinese LED maker, Unilumin has said that it has made a display with 0.8mm pitch and is aiming at 0.6mm. At the 0.6mm resolution, the company ought to be able to make a 55″ FullHD display, which means that there is now a continuum between LCDs and LEDs.

Why would you choose LED?

LEDs can have very high brightness and long life (although they can fail and need routine calibration if the visual quality is to be maintained), but they will remain very expensive for the time being. I am grateful to UK-based video wall specialist, PSCo, for advising me that for current LEDs at 1.9mm and 2.5mm, the cost per square metre is €28,300 and €14,300 respectively. That’s a stark contrast to around €2,000 to €2,500 for a square metre of LCD (around an equivalent of a 55″ monitor).

Although they wouldn’t quote us a price, I would expect the Unilumin display to be at €100,000 per sq. metre or more as, broadly, the cost of big LEDs is approximately proportional to the total number of LEDs in the display.

Nevertheless, it will be interesting to see the development of competition between the two technologies. LEDs typically follow Haitz’s law (a bit like Moore’s Law) which says that LED costs fall by a factor of 10 every decade, and the amount of light per package goes up by a factor of 20 (although we’re not too far off hitting theoretical limits). LCD prices, on the other hand fall by around a factor of 6 every decade (18% per year).

While LCDs depend for their development on huge investments in the fabs needed to make them, LED tile making is much lower tech as it can take advantage of the vast LED manufacturing capacity that is being built to supply LEDs for lighting and backlighting. Selecting the LEDs and assembling them can be done in countries such as China where costs are low. That means that the competitive environment is very different in LEDs where there are many suppliers, whereas in large LCD displays, there are only half a dozen significant panel makers. None of the big LCD panel makers is strong in the LED display tile field, at least today!

Meko Ltd. and invidis consulting GmbH are cooperating in research on the Digital Signage Market in Europe. The general manager of Meko Bob Raikes features with a monthly column on invidis.de. Bob will also be speaking at the 8th Munich Digital Signage Conference 2014 (17th/18th September). To get the most up to date news from the public display market you can register here for the Meko newsletter.