LED meets LCD
With the SP-F10M, Samsung has proven what many other manufacturers dismissed as being impossible - and at a competitive price, too!
LEDs are becoming more and more powerful with each new generation. While only a few years ago they were only suitable for gimmicks such as pocket projectors, they are now illuminating an increasing number of mobile phones, monitors and flat-screen TVs. And size is no obstacle to them. Pioneering devices from Sim2 and Vivitek already demonstrated last year that LEDs provide enough light capacity to adequately illuminate a home cinema screen. Since then, more and more home cinema fans have been wondering when affordable mid-range projectors with innovative lighting technology will finally be launched on the market.
Because the benefits are manifold. LEDs have a much longer life (up to 50,000 hours) and therefore do not have to be changed during the entire life of a projector. The uncertainty of the "suddenly popping" lamp and the associated added cost of several hundred euros are not an issue with LEDs. They also provide long-term stability in their brightness so that the factory information in terms of light output really is thousands of hours, and the projector does not “dim” after a few hundred hours as is the case with traditional UHP lamps. Other advantages include simpler cooling, compact dimensions, faster response times, etc., etc.
With light outputs of up to 1,000 lumens there is no discernible reason that would argue against the use of LEDs in video projectors. We have taken this view for months and have asked various vendors about their plans to use LED. To our surprise, we received only evasive responses. With the combination of LED and LCD in particular, where the "marriage" appears particularly useful, as every colour channel and each of the three LCDs would receive its own light source, we were confronted again and again with abstract excuses such as "technically infeasible", "too high light loss "etc. etc. We have never been able to understand this because our own tests have shown that the light of the Phlat light LEDs with a high lumen level is not pre-polarized and can be freely modulated by the LCDs. No one was able to give a detailed explanation regarding technical problems with LEDs perhaps because there is no real technical reason.
The suspicion quickly arises here that LEDs may not be used for "political" reasons. Perhaps because there are currently only a few manufacturers of LEDs with high lumen levels and these have to be purchased additionally and cost more than UHP lamps which are often produced by the projector companies themselves. Perhaps just because LEDs give the projectors too long a life without any additional cost. Perhaps it is simply because the development cycle and lead time for a new chassis take several years and corresponding LEDs are still too young to be taken into consideration in the last few generations. But perhaps also because other options have long since been established (e.g. laser) and these are being brought on the market first.
The selection of "conspiracies" is manifold and it is pointless speculating on why many manufacturers treat LED technology in such a shabby manner. But there are exceptions. One of the world's largest LED supporters is Samsung. The Korean electronics giant is known for implementing LEDs in millions of televisions and displays. And Samsung has also been building home cinema and business projectors for many years. It is therefore not surprising that Samsung has launched one of the world's first 3 LED projectors on the market. The SP-F10M model was introduced a few weeks ago.
Much more surprising is the choice of projection technology. Although usually found in the DLP camp, Samsung relied on LCD technology for the SP-F10M. It is the first full 3 LED/3 LCD projector in the world. It may be the case that the choice of projection technology also has political reasons. The existence of the SP-F10M alone has proven that it is not impossible to construct an LED/LCD projector and launch it on the market in large quantities.
The technical features clearly represent the advantages of LED as expected. Samsung advertises a lifetime of 30,000 to 50,000 hours guaranteed for a period of three years. Therefore, you do not have to worry about the light source failing. What is surprising is the power of the light. The manufacturer says 1,000 lumens. Should this really be achieved, it would set new standards for LED projectors and there would be no reason to hide from various UHP lamp competitors. And certainly not when the extremely powerful colour yield of LED projectors is taken into consideration. Splitting the light into three coloured (LED) light sources with very pure spectral properties ensures a particularly sharp coloured bright image display, which cannot be achieved by many presentation DLP projectors in this form.
More surprising and particularly pleasing was the price. At around € 980, the projector is in the lower class price range for projectors. It must be said, however, that this is not a device optimised for use in the home cinema, but a presentation projector, and in this market segment there is fierce price competition, so that taking the XGA resolution into consideration it belongs rather to the "more expensive" category.
But even without optimisation for home cinema, the model is highly interesting for our technical tests. In this Special, we show how the interior of the world's first full size 3LED/3LCD projector is constructed. Did the engineers have to use magic in order to make this combination possible, or is the construction surprisingly simple? At the forefront, the price would imply the latter...
Back to top
Light path of an LCD projector
Before we begin with the Samsung SP-F10M, we will again call the technical structure of a 3 LCD projector to mind. As the name suggests, three separate LCDs are installed in one projector, one for each primary colour.
These are illuminated like a slide and modulate the brightness of the primary colours (red, green and blue) on a pixel basis and thereby the colour mixing. In conventional projectors with UHP lamps, the white light from the light source has first to be divided into its primary colours by semi-permeable mirrors and directed to the corresponding LCD panel.
Through a glass prism, the three individual basic images are reunited / transferred and finally projected onto the screen by the projection optics.
With LED technology, the dividing of colours by mirrors part is not necessary because, instead of a single light source with white light (UHP lamp), three LEDs are used, with one of the three primary colours (red, green and blue) respectively. In other words, each LCD has its own coloured LED lighting, therefore three-LED.
The dividing of white light by semi-permeable mirrors is not necessary
That is the theory, but what about practical use? To illustrate this, we opened the Samsung SP-F10M as usual. After removing the cover, the view is not exactly spectacular.
As with any LCD projector, the light path is completely encapsulated and not much can be seen. What is unusual however is that the top cover is made of aluminium.
We looked at this more closely and besides the LED lighting we came across a second surprise. The first "complete" dust protection through a completely encapsulated cooling path of the LCDs! What sounds like science fiction has been implemented as brilliantly as it is clever.
The first dust-proof LCD cooling
Let us first of all call the conventional LCD technology to mind once again. LCDs and their associated polarisation filters are extremely sensitive to heat; even a momentary overheating can lead to permanent and expensive damage. Efficient cooling is therefore essential, but is hampered by a second fact. Since the LCDs are illuminated like a slide, they have no back and cannot be cooled passively by heat sinks. Because both sides must remain visually free, the cooling can only be by an (invisible) air flow which is generated by strong fans.
The problem is the dust. To date, projectors suck in the necessary air from the outside (room air) (yellow arrows). In order not to stuff the sensitive LCDs with dust in the long term, the air is usually drawn in by as fine a filter as possible that should leave the unwanted dust "outside".
Filters as fine as possible should keep the dust away
Good systems provide largely reliable dust protection, and yet there is still some risk. A single speck of dust on one of the three LCDs is enough to leave irritating colour spots that are visible in dark scenes (on black). In addition, the filters have to be thoroughly cleaned on a regular basis by the user. If this maintenance is not performed, not only the dust risk, but also the cooling capacity is reduced, as less cool air gets trough the clogged filter, the cooling of the LCDs worsens due to less circulating air and the risk of a shortened life span thus increases.
This technical fact is often denounced by producers of competing technologies, LCOS (SXRD/D-ILA) and DLP. This is because the reflective mode of operation of LCOS and DLP makes it possible to fully encapsulate the actual light path and to cool the image chips from the outside, one-sided. In terms of dust protection, these technologies had in fact a clear advantage to date, that is, until the Samsung SP-F10M. Because its light path is also completely enclosed making it filter-free and maintenance-free. And that brings us back to our “projector dissection”.
The light path is partially covered by a thermally conductive cover (picture above). Removing the cover shows that it is part of a sophisticated air channel. A medium-sized fan is located in a small area on the bottom left, as can be seen in the picture below.
This fan is obviously responsible for transporting the cooling air necessary for the LCDs from below to the panels, as further dismantling displays.
In the picture above we see the air inlet (arrow) to the heart of the projector, the three LCDs (green arrow). The fan is mounted directly on this small air inlet.
The fan supplies the small air channel with cooling air, but is sealed from the outside in metal casing.
That’s nothing special, but what is interesting is if you locate where the “used” and heated air is now. Instead of pumping the air out of the device, it is sucked directly from the fan back into the metal housing under the metal cover (first picture) and re-pumped into the intake port of the LCDs.
The enclosed air path of the Samsung SP-F10M
The result is a fully enclosed air path in which the cooling air rotates in a circle (picture above). Thanks to this encapsulation, no dust can penetrate into the inside or outside, and sensitivity to nicotine is also reduced.
But one question has yet to be answered. How is the necessary cooling effect achieved with such a short circuit and the direct re-use of the air? After all, the air heats up more and more with each revolution... The cover and the fan chamber now come into play here. Both are made of thermally conductive aluminium and have cooling fins on the outside.
A “second” external fan that leads the heat from the metal directly out of the device is placed beside these cooling fins in the chassis. The air in the circuit is thus cooled passively.
- The air is trapped in the circuit and warms up with each revolution past the LCDs.
- The air releases its heat to the aluminium housing of the channel, the metal heats up.
- The aluminium air channel in turn is cooled by an external fan.
Thanks to this heat chain, the engineers succeeded in guaranteeing a fully enclosed and thus dust-free cooling circuit for the LCDs. The system is simple and ingenious. Taking the relatively high light output of the projector (1,000 lumens) into consideration, it quickly becomes clear that an encapsulated cooling path could be easily incorporated in a home cinema projector, especially as home cinema chassis are generally larger and would thus allow more space for the passive cooling of the aluminium channel from the outside. Therefore, we can only hope that other engineers start using this system, and build the first 100% dust-free LCD home cinema projector.
Back to top
The LED cooling
Let us stay with the cooling for a moment. In addition to LCDs, the LEDs need powerful cooling. Very few people know that high performance LEDs also react very sensitively to overheating and transform a large part of the energy into heat instead of light, even if in so doing they have a substantially lower loss than conventional UHP lamps. Cooling is therefore a must.
But they have one important advantage. Whereas, due to their flask structure similar to that of the LCDs, conventional UHP lamps can only be cooled by a strong air flow. This can be achieved in part passively from the rear side in the case of LCDs. This is because the LCDs are mounted on a copper plate and can therefore emit their heat on one side similar to a computer processor.
The Samsung engineers used this cleverly. They flanged so called “heat pipes” directly onto the back of the copper plate, which guide the heat like small pipelines away from the LEDs.
These heat pipes (green arrows) open into passive heat sinks (yellow arrow), which are in turn coupled with medium-sized active fans (red arrow).
There are three of these constructions located in the projector and each was placed in the corners of the chassis. This makes it possible for the heat to be emitted directly from the device.
The cooling system is generally very well thought out and offers maximum dust protection despite the projector’s compact dimensions. And now to the volume. For a presentation projector, the SP-F10M is pleasantly quiet (26dB in eco mode), but the current reference values for the home cinema area are not reached. We think, however, that with further optimisation measures, and above all, a larger chassis, almost inaudible ventilation is possible. A fully encapsulated air path around the LCDs and water cooling for the LEDs as with the Sim2 high end projector (as identified in our detailed review) would be no compromise here. On the whole, the sum of its features (complete dust protection, maintenance-free, low volume), make the cooling system of the Samsung LED newcomer, however, one of the best we have been able to examine to date.
3LED/3LCD light path in detail
We have kept the construction of the light path for the highlight of this Special. How complicated is the marriage between LEDs and LCDs? The answer: not at all complicated, but amazingly easy!
For the sake of explanation, let us once again take a look at the beginning of a conventional LCD light path with UHP light source.
The light of the UHP lamp is captured by corresponding lenses at the beginning of the light path and homogenised, that is spread over the cross section of the light path by an integrator level (arrow) consisting of many small square mini lenses. This is exactly the same with the 3-LED projector except that we are dealing with three light sources here, and thus the light has to be homogenised three times through the lenses.
Needless to say, the lenses are different in detail and adapted to the small LEDs, but the system is largely identical to the conventional light sources, as can be seen in the pictures.
The remainder of the light path is even simpler than a conventional projector. The dichroic mirrors to split the white light are not necessary. Instead, three simple mirrors just direct the light to the LCDs. Strikingly clever are the three corresponding intermediary lenses located in a plastic frame, the height of which can be adjusted by means of a screw.
With them, it is possible to focus the LED light exactly on the LCDs and to maximise light output and minimise shading effects in production. From here, the LED light penetrates to the usual polarisation filters, which are placed before the LCDs. These polarisation filters also have no major technical differences to conventional UHP projectors. However, they are of course manufactured such as to be optimised to the narrow-band spectral properties of the LEDs.
Back to top
From here on, there are simply no further differences to other LCD projectors, a glass prism combines the three individual images and the optics project the colour image onto the screen.
The LCDs are not special models either; on the contrary. Latest generation Epson D7 panels with XGA resolution are used. Substitution by D7 Full HD panels, as currently used in high-quality home cinema models, should easily be possible. At this point, the suitability of the home cinema LED/LCD combination is clear.
The electronics and power supply of the LEDs remain to be examined. Although these function with a very low voltage of only approx. 4V, they have higher amperage. Up to 30 amps are conducted through the copper cables at full brightness to ensure a suitably high power at low voltage.
The power control of the LEDs posed the toughest obstacle for the first LED projectors. In conjunction with the DLP technology, the LEDs must be turned on and off very quickly in order to replace a colour wheel and prevent the rainbow effect. This is made possible by a highly clocked control board from Osram. It can switch the LEDs on/off in the millisecond range, also called "pulsing". The drawback, however, is an electrical hum which can interfere with the movie experience.
Samsung made life easier for itself with the 3LCD variant. Since all three primary colours can light up simultaneously here (and not sequentially as in a DLP projector), pulsing is not necessarily a must. The three LCDs of the SP-F10M are supplied with the power required by three individual switch-mode power supplies.
Clearly colour-coded, the orderly power supply of the LEDs
Here, the LEDs are not clocked, but supplied with a steady continuous current. This naturally limits any dimming possibilities, as this cannot be conducted with pulse width modulation (on/off cycling), but only with voltage. In other words, the brightness of the LEDs cannot be controlled as precisely, which can be noticeable with adaptive lighting control. This feature thus works inaccurately in the projector. It should be noted though that the F10M has an eco mode like a UHP projector, in which the LEDs are driven less, and thus consume less electricity, the cooling fan can be operated with reduced (quieter) speed, and the life expectancy is increased from 30,000 to 50.000 hours.
That leaves the signal plate of the projector. Normally it is not directly related to the light source used, but in the case of the Samsung SPF10 it's different. On closer inspection, we noticed three light sensors, marked with arrows in the picture.
Apparently, the electronics control the brightness of the LEDs with the help of the photodiodes, from which extra small light channels run to the diodes.
We can only speculate about the reasons for the brightness control. It is possible that in this way the projector determines if one of the LEDs loses brightness during their long working life of many thousands of hours, and the loss is compensated by adjusting the voltage (or the other two are adjusted) in order to prevent colour shifts even at the advanced age of the projector. Or the diodes monitor the brightness in the "adaptive light" mode in order to monitor an even dimming of the three LEDs, thereby preventing colour variations in the dynamic mode when the amount of light is adjusted to the image scene. For it really is more difficult to dim three light sources of different colours by exactly the same factor as it is for a white light source. Every deviation would be noticeable in variations in colour temperature.
Back to top
Conclusion
That is our technical overview of the first "real" 3LED/3LCD projector in the world. Even if the Samsung is "just" a presentation projector with XGA resolution, it makes what is currently possible in terms of the technical fusion of LED and LCD on the market abundantly clear.
The project combines the advantages of both technologies in an impressively simple way. The mounting flexibility and steadiness of an LCD projector with the longevity, intensity and compactness of LED lighting, and all of that with an impressive light output and at a price that, in view of the new development, is more than fair.
We view the Samsung SP-F10M as a technical example which illustrates that LED projectors are technically feasible in the home cinema right now, and have the potential to replace the conventional UHP lamp technology with all its disadvantages (working life, cooling, replacement price, colour temperature) in the short-term if a manufacturer is prepared to perform the necessary development work.
The device provides new impulses in dust protection too, which will hopefully appeal to future generations of home cinema projectors. Thanks to encapsulated cooling paths, LCD technology can finally catch up on DLP and LCOS when it comes to dust protection.
And all of this in usual price ranges, because the new Samsung SP-F10M proves the following: LED projectors do not have to be more expensive than conventional UHP models, on the contrary. For the same or a slightly higher purchase price, the LED version offers a price advantage of up to €5,000 (!!) including its enormous life of up to 50,000 hours, because a conventional projector consumes about 15 bulbs in this time period.
There has never been a projector that was designed so economically with a long, maintenance-free life for such a low purchase price as this new 3LCD/3LED model. We hope it is not an only child but makes it to home cinema systems. For there is one option engineers no longer have, and that is, excuses. Because the contrary has been achieved.
5 July 2010 Ekkehart Schmitt, Karsten Becker Back to top
|
