Prysm's Eco Credentials

Andrew Neale looks at the eco-credentials of Prysm's LPD technology, comparing it with other major display types. With ISE approaching fast, he also looks at some of the competitors that will be on show in Amsterdam.

One main feature of the Laser Phosphor Display (LDP) system that Prysm are keenly promoting is that the power consumption is very low compared to other technologies. At Infocomm last year, there was a live power meter showing total power consumption in Watts connected to two of the display walls on their stand. They would fluctuate according to the type of pictures being displayed on the screens.

To back up the system's green credentials, Prysm launched an Eco calculator on their website last April and we've taken an in-depth look behind the figures.

Power consumption of display systems falls into two main categories depending on the primary technology in use:
Projectors and LCD - power consumption is generally constant, and doesn't vary much according to the images shown on the display. The main power-using component, for example a projector lamp or an LCD backlight, is on constantly.
LED, OLED, Plasma and LPD - the power consumption varies according to the images shown on the display. In these cases, the light output, and therefore power consumption, of these emissive devices is modulated by the image displayed. In the case of a black image, the display is dark and relatively little power is consumed.

This makes it difficult to calculate the on-going power consumption for some types of display, unless you can do some real-world measurements with some typical content being displayed, and ideally measuring over a reasonable period of time. Some LED backlit LCD panels are now falling into the latter category, as manufacturers are using advanced techniques which modulate the output of the LEDs in localised areas of the panel to improve light output and black levels across it.

Prysm have compared the main screen technologies in a table by using a source figure for power consumption (labelled as Typical Energy Consumption), measured in Watts per square metre of screen area. They have also sensibly assumed some cooling will be required, and factored in power for cooling to be a third of display power consumption. Given that an average electricity unit cost is 15 cents per kW/Hour, the total cost of energy used is shown for each technology type, along with the percentage saved when comparing LPD with the other types.

The standard example in the first table is for a 10sqm screen area display, running 24/7 for 5 years.

At first glance, LPD naturally does very well, and it's demonstrated that it is the most economic to run by more than twice against LCD, and still more against the others.

Given the fixed parameters of time and energy cost, we looked more closely at the source power consumption figures and found a case for some adjustments to be made to this model.

Interestingly the figure of 150W/m2 Prysm shows for LPD puts its consumption above the average seen on the module power meter at the ISE show in 2010 (when an earlier generation of prototype display was demonstrated), and definitely above the "less than 100W/m2" mentioned in their March brochure. When the display is running digital graphically designed content rather than the video pictures it would be fair to say that the power consumption is likely to increase. This sort of content, more typically seen in out-of-home and digital signage applications, tends to have more areas containing higher saturated colours when compared to the video content we have seen on the system to date. More saturated colour means the lasers need to be on for longer to excite the phosphor screen brighter. Obviously it won't all be the worst case of peak white being shown, but it could be reasonable to increase the average figure to 250W/m2.

A sample of commercial grade LCD panels shows that unless you look at a model which has quite a low brightness, the consumption if anything is being understated. A more realistic average is about 400W/m2.
There is a wider variation for LED screens, with the pitch of LED pixels being a key factor in determining the performance of a display, both in terms of power consumption and picture quality. Viewing distance from a display is an important consideration when determining what pitch is suitable, and for indoor and higher quality images, 4-6mm pitch is common. Also the power consumption will vary based on the content being played, and the ambient lighting conditions will determine at what brightness the screen will be driven. The figure of 585W/m2 is probably about right as an average, but this is the figure most likely to need a proper real-world measurement in the environment in which it is being used to determine a truly accurate one.
The Plasma figure appears to be based on the Orion MPDP panels, but a further look at the latest Panasonic range shows, for example, that their monster 103” panel is much more efficient, at less than 450W/m2. With a few of the other sizes available taken into consideration, a more fair average would be 535W/m2.

LED Rear Projection Tiles can only be a reference to Christie Digital's MicroTiles product, it being a modular system like Prysm's LPD, with each display module powered by a LED light source, near throw projector engine. Christie (being the diligent engineers they are) have published conservative figures for maximum power used under full brightness. Prysm appear to have used the typical 110W per tile power consumption figure, and this is the likely source of the 880W/m2 figure. Also the typical cooling consumption figure of 330W/m2 is incorrect, as the other types have used a calculation of a third of typical energy consumption - so the 880W/m2 figure should produce a cooling figure of 290W/m2.

What actually happens in practice is that the array of MicroTiles self calibrate down to the lowest performing LED light source, and Christie's brochure refers to 30% off the typical power figure. Measurements by the factory show the power consumption to be about 70W per tile unit at their stated brightness, which means this works out at about 560W/m2.

The comparison with large format projection is rather more difficult to quantify, where any number of parameters would need to be taken into consideration. Screen surface, projection throw distance, and other environmental constraints mean that trying to quantify a general figure for comparative power consumption would be difficult without looking at specific cases, so for now that figure has been left untouched.

The second table is based on the revised figures.

Now the picture has been re-balanced, two main results can be seen. Firstly that Prysm's LPD is indeed still the most energy efficient of all the display technologies, but not by the same large margin that was previously claimed. The energy saved over running LDP against LCD is just over 37%, and in the 50-60% range for the others.
Secondly the maximum cost of energy saved by using LPD over others during the example 5 year lifetime has reduced to $30,000, or $6,000 per year.
Other parameters like overall display brightness might also be an overriding factor for some projects. Although not reflected in this energy cost analysis, brightness figures claimed by manufacturers can be notoriously difficult to compare, and to actually achieve a consistent brightness to allow energy figures to be compared accurately would really need proper controlled and independent testing. Otherwise, although a useful illustration, the Eco calculator can only be treated as a method of modelling a likely scenario.
If we take cost of ownership to include initial purchase cost and energy consumed, in the case of LCD and Plasma panels, the lifetime cost of energy is very significant, accounting for over half of the total cost (assuming purchase price per square metre is in the $10,000's range). For large screen modular based displays, like LPD, LED and MicroTiles, with the purchase price an order of magnitude greater (cost per square metre in the $100,000's), the total cost of energy amounts to 5-20% of purchase cost - rather less significant when considering the total cost of ownership.
Costs such as installation and on-going maintenance and repairs have been ignored, of course, and each technologies' strengths and weaknesses would need to be factored in when considering a true total cost of ownership.
Other emerging technologies yet to make an impact in the marketplace include laser as a primary light source, and OLED - for example the Mitsubishi Diamond Vision OLED modular display which is competitively bright, and 4mm pixel pitch. These are also claimed to have reduced power consumption when compared to existing technologies - some are claiming 50 per cent less for OLED, and it will be interesting to see what impact they will have in the power stakes when actually in production form. The Diamond Vision OLED display will be on show at ISE 2011 in Amsterdam 1-3 February on the Mitsubishi Electric stand 2A24. Prysm Inc will be displaying the LPD system on stand 11H52.

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