How does Panasonic's Smooth Screen technology Really Work?
Ever since the well-known Japanese conglomerate Panasonic has been offering home cinema projectors with LCD, they have been equipping them with their own "Smooth Screen" technology. This serves to reduce the marked pixel structure of digital projectors. And this technology has divided the opinion of home cinema fans for as long as it has been in existence. One group considers it not only effective but as absolutely necessary for an LCD projector, while others think that the projection focus is adversely affected.
But what is right? Does smooth screen really compensate a typical LCD shortcoming, or is it just a cheap defocusing trick? To answer this question, we explain in detail the technology behind it in this Know-how Special, since in fact there are many false "myths" about smooth screen...
1. Background and Problem: "Screen Door Effect"
"Screen door” is actually a term that has nothing to do with electronics, it simply means: "Fly screen". And fly screen is a simple and demonstrative paraphrase of a typical display phenomenon common to digital video projectors.
Be it flat screens, computer monitors, mobile displays or projectors, in our digital age, the image is composed of innumerable single pixels. In the case of conventional PAL, these are just over 400,000 in number. For current Full HD there are over two million. Ultra-modern cinema projectors (digital cinema) quadruple this resolution to almost 9 million pixels, and thus enable a high sharpness of image even with distances beyond 10m.
The amazing thing about all this is bean-counting of pixels is that each of these millions of pixels can be individually controlled in brightness and colour, and up to 120x per second! With a full HD projector, this means that up to 240 million pixels per second are processed for the image production of our video picture. This is an enormous figure which illustrates the high volume of data involved.
Being able to control each pixel individually requires individual control lines. In other words, each of the millions of image pixels has its own power line with which it is allocated the relevant voltage depending on the desired brightness. And these keywords help us to reach the cause of the screen door effect.
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1.1 The Rivals: DLP vs. LCOS vs. LCD
Currently, three main projection technologies are vying for consumers’ attention. And in terms of pixel structure, they have come up with very different quality features.
- DLP
DLP technology was the leader for a long time where an image with as few pixels as possible was concerned. For the DMD chip is not illuminated but the hundreds of thousands of pixels are modulated by small mirrors.
This means that the small mirror can be controlled from behind, i.e. the power lines are located on the back. Thus they are not in the light path and the individual pixels can move closer together.
And still they cannot flush. A small residual gap remains between the pixels. The reason for this is simple. Each of the small mirrors of the DMD chip is tilted to the light modulation. This tilting movement requires a little leeway, a safe distance as it were to the neighbouring pixels.
Pixel structure in the real exposure
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In effect, we have (depending on the chip version and resolution) a fill rate of between 80% and 90% do. This means that 80% to 90% of the image actually contains picture information and the black dividing lines between the pixels make up only 10% to 20% of the screen. These are already very good values which ensure that the pixel structure is barely discernible even from short viewing distances and the screen door effect becomes invisible. The light output is higher because the black lines barely obscure the picture.
- LCoS
It looks even better with LCoS technology which is available in the form of D-ILA (JVC) and SXRD (Sony) for home cinema projectors. This is also a reflecting, mirroring technology but is LCD-based.
The individual LCD chambers have a reflective-coated electrode, and the brightness modulation is carried out by the overlying layer of crystals. This technology has the added advantage over the DLPs that there are no mechanical movements involved, allowing the individual pixels to move even closer together.
This sophisticated system results in a fill rate of over 90%! This means that only less than 10% of the viewing area is composed of black lines.
Barely visible and photographable pixel structure
The pixel structure is invisible accordingly. You really have to go up very close to the screen to recognise the latter. The high fill factor allows for high light output and an analogue looking image without losing its sharpness.
- LCD
LCD technology is the clear loser in terms of the screen door effect. This is due to its transmissive screened functioning. An LCD chip is illuminated like a little slide.
Thus, it is not possible to place the conductor paths “behind” the pixels. They are always in the light path and are illuminated. In this way, they are automatically part of the image.
Pronounced pixel structure
Since you can not shrink these conductor paths as you desire, their share of the total area of the screen is large. The black lines make up an astounding approximate 50% (!!) of an LCD projector image. The pixel structure is clear accordingly.
The black lines make the individual pixels clear so that a moderate viewing distance must be kept from the picture. In addition, the black mesh that fills half the screen will cost about 50% of the image brightness.
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The distinct pixel structure is a significant drawback for many home cinema enthusiasts, for many even a knock-out criterion, although the phenomenon has been relativised in the full HD resolution era and is not nearly as noticeable as it was in the age of 720p.
Can we do something about the distinct pixel structure? The possibilities are very limited for home users. If a projector has very high quality and finely adjustable optics, you can try to minimally defocus the image so that the edges of the pixels blur slightly. However, the danger with this approach is high that the pixels will blur into each other and thus clear dividing lines become impossible. The sharpness of the image will suffer, meaning that you have taken another disadvantage on board.
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2. Improving the Fill Rate with Smooth Screen
The Panasonic engineers have also given thought to pixel structure and have, as mentioned, developed the so-called "Smooth Screen" technology. The aim of this technology is to improve the filling rate in the image and reduce the dividing lines between the individual pixels. Thus the large gap between LCD and DLP and LCOS should be closed.
But how does the Panasonic smooth screen technology work and what does it do? There are a lot of misconceptions in this area, as we keep hearing in conversations with home cinema enthusiasts, or read in various forums.
The opinion is widespread that smooth screen is an additional lens in the optical path that slightly defocuses the pixel structure of LCD panel. Accordingly, there is the opinion that although the smooth screen reduces pixel structure, it costs sharpness. We could not substantiate any of these statements in our various projector tests because the detail display of our test candidates was always on the same high level as that of many competitors without smooth screen. This discrepancy led us to investigate how smooth screen technology really works. Panasonic was happy to provide information.
2.1 Technology and functioning of smooth screen
Before we start, we would like to make one point clear. The commonly held view that smooth screen technology defocuses the image is not correct. Panasonic too recognised that you can not solve a technical defect (screen door) by replacing it with another (blur).
The aim of the engineers was therefore to reduce the dividing lines between the pixels without blurring the sharp demarcation of pixels to each other. In other words, we want to enlarge the pixels and simultaneously reduce the lines. How is this possible in a common light path? Logically speaking, it should not work. But the Panasonic engineers were very resourceful and found an interesting solution...
Let us make a little excursion into nature to explain the technology. Here, we find a mineral called calcite or calcspar. It looks a bit like a transparent crystal, but when it is used as a magnifying glass, we observe a distinctive property.
As shown in the picture above, the mineral stone "doubles" the writing that passes it. This effect is aptly called "birefringence". How does this come about? It should in no way be confused with the different angles of refraction of different wavelengths (colours) because, as you can see in the pictures, there are no colour shifts.
What causes two different refraction angles? As is often the case, the answer is complex and is explained by the polarisation of light. Light is composed of numerous photons of certain wavelengths which are also “polarised” with a different vectorial orientation.
The special feature of the calcite is that its angle of refraction varies depending on the polarisation of the incoming light. Horizontally polarised light is deflected at a different angle than vertically polarised light.
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This material is nothing more than a "polarisation point”. The entire LCD-based image generation system is based on the polarisation of light, and the Panasonic engineers have used this to their advantage. Looking at a single image pixel, we can illustrate the potential benefits of birefringence.
Through a small optical layer with birefringent properties, you can copy a pixel minimally displaced in one direction and thus obtain two pixels which mostly overlap. The pixel is “extended” upwards in this example.
If you now add a second and third birefringent lens, you can reflect and move the pixels around various axes until you have increased the surface evenly in all four directions, like a kaleidoscope.
If you look at the photo with the calcite stone above more closely, you will have noticed that this does not impair sharpness but creates two different equally sharp images. This ensures that the pixels are not blurred at the edges as is the case with defocusing.
We will now illustrate how the birefringence system is used in the case of Panasonic smooth screen technology.
1) A picture pixel produced by the LCDs reaches the first level of birefringence. In accordance with the polarisation of light, a part of its light is passed but another part is moved to the upper right.
First level: make two pixels out of one
2) The “pixel couple” reaches the second special coating of the birefringence. The original pixel is then shifted to the right, while the copy from the first round of the shift is just passing the coating. Now we have a pair of pixels shifted to the right.
Second level: a step to the right for pixel 1
3) The pair of pixels is doubled on the last level and shifted to the left this time. As a result, we have quadrupled and re-aligned the original pixel.
Third level: the four copies together produce a magnified pixel
The lower drawing shows the whole process again at a glance. Needles to say, in practice the angles are chosen so that the pixels are superimposed in such a way as to fill the black borders. The surface area is not quadrupled.
The Smooth Screen light path in three stages
This complex structure makes it possible to increase the individual image pixels so that they approximate the edge of the neighbouring pixel without compromising sharpness. The dividing lines remain clearly defined, and minimally lose brightness sloping towards the edges. The light output cannot, of course, be improved compared with a conventional LCD projector without Smooth Screen. The brightness is just spread more evenly. Back to top
3. Smooth Screen in Practical Test: PT-AE4000 vs. conventional 3LCD projectors
That was the technical background, but how well does the technology actually work? According to the engineers’ plan, the screen door effect would be reduced in the event of an optimum result without affecting the sharpness. To determine this objectively, we compared conventional LCD projectors with the Panasonic PT-AE4000 and conducted a little experiment...
3.1 Pixel structure
We first examined the pixel structure of a "normal" full HD 3LCD projector without any additional optics. As expected, the individual pixels could be identified very clearly.
The native pixel structure of 3LCD projectors
If you look closely or magnify, you will see that the area of the black line actually does make up about 50% (!) of the screen.
The relationship between pixel size and spacing is 50:50
Sobering as this result may seem at first glance, one thing is guaranteed for certain. Small details are (with appropriate signal processing) clearly distinguished from each other which is conducive to clarity.
Now let us look at the same area through the "eye of a Panasonic PT-AE4000” with smooth screen technology.
The pixel structure is greatly reduced with Smooth Screen
The pixel structure really is significantly reduced. There are no gaps between the pixels. It is interesting that the individual squares are still recognisable however. There is still a separation.
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3.2 Pixel sharpness
The main task of reducing the pixel structure on surfaces has already been successfully fulfilled after this first result. Other observations have not been made, but the question is: are we sure we haven’t brought a loss of sharpness on board? To this end, we looked at the sharpness of a conventional 3LCD projector.
We deliberately used the menu screenshot to avoid any influence from the signal source / processing and so on. This test is exclusively about the optical properties!
As you can see, the writing is sharply defined and meets high quality standards. The individual pixels are also clearly more differentiable even with white lines. Even reasonably priced models such as a Sanyo PLV-Z700 show this.
This photo also makes the problem of lack of convergence clear, which is often the case with lower priced models. A poor convergence (covering of primary colours) affects the sharpness considerably.
Then we tried the Panasonic menu - with a surprisingly good result. The writing is also very sharply defined here, but the annoying pixel structure is gone.
The sharpness of the edges is just as good with Smooth Screen
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Up to this point, the thesis that Smooth Screen costs image sharpness has been refuted. But the test is not enough. We then looked at the image sharpness of the smallest possible picture detail, a single pixel!
We began with the traditional 3LCD projectors from the high price range (above). Insofar as the convergence allows, the pixels are sharp, shown without tail or curves. And now the Panasonic in comparison:
The checkerboard pattern is more easily recognised with Smooth Screen
There is nothing wrong with the pixel resolution here either. The tiny dots are also sharp and well defined. Most DLP projectors are no better in this discipline!
The pixel sharpness is optimal. You can even recognize their square shape 
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3.3 Experiment: defocusing as an alternative?
At this point, we opted to perform a little experiment. Can you come close to the Smooth Screen effect with a minimal defocus? To investigate this, we defocused the image of a projector with high quality optics to the point where the pixel structure was similar to that of the Panasonic.
Defocused at the top, bottom "real smooth screen"
Amazingly, the comparison is almost identical. Now our conventional 3LCD projector shows a reduced but still well-separated pixel structure in places. The intermediate result was easy to reach. But the disillusion is just around the corner.
Pixel structure: hurray!
Sharpness: oh oh!
Now the difference becomes clear. Even the slight defocusing is sufficient to bring the detail display completely off balance. Defocusing is no alternative to Smooth Screen technology, and the results are not comparable.
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3.4 Pixel overlapping
Although our previous tests already indicated that, in optical terms, Smooth Screen does not cost sharpness, they were still not sufficient! For our test images all had one thing in common, a black background! This means that neighbouring pixels were always turned off. Thus, the sharpness of a pixel can be analysed, but not whether it might be overlapping into its neighbouring pixels. And yet it is precisely overlaps that can severely reduce the detail display. If, for example, small elements of colour run together, their clear separation is no longer guaranteed.
In this last discipline, we thus investigated the clear separation of adjacent pixels. This can best be realised with straight dividing lines. First, the normal "Joe Bloggs” LCD projector.
As you can see, the pixels are clearly separated. Only a small edge can be seen that is attributable to the unavoidable minimal convergence displacement of 3-chip projectors. Even without the black background, small details are separated neatly from each other improving sharpness. Now let us consider the PT-AE4000 with Smooth Screen.
Here too, you can see a clear separation of the colours in an extreme close up. The lines are sharp and defined with good contrast.
It should just be noted that, due to the reduced “safety zone” of the pixels (absence of black dividing lines), convergence shifts result in overlaps faster than without Smooth Screen. Our test specimen showed, e.g. half a pixel offset of blue which becomes evident with a corresponding change in colour.
Violet transition
No results from Smooth Screen but the convergence of the 3-chip projector!
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These reduced convergence tolerances however, affect not only the smooth screen technology but every 3-chip projector with a high fill rate, including DLP, or LCOS projectors. It would therefore be wrong to speak of a disadvantage caused by Smooth Screen.
The image impression is strikingly similar to Smooth Screen: LCoS / SXRD with a 90% fill rate
The comparison photo above of a Sony VW15 shows clearly that separation and sharpness and fill rate of an LCOS projector are on an almost identical level as with an LCD projector with Smooth Screen.
4. Conclusion
Our comprehensive Test Special shows that through a complex system of various birefringence lenses, smooth screen technology fulfils the task set down by its manufacturer of substantially increasing the fill rate of a 3LCD projector without compromising sharpness.
The area of pixels is magnified to the borders of neighbouring pixels without blurring the edges or provoking overlaps. The visual sharpness remains intact even at full HD resolution.
A higher fill rate and shorter pixel distances are thus often circumscribed as "more film-like” or "natural”, which is also accurate because in nature we are not dealing with individual pixels or black-rimmed detail transitions.
If this circumstance can be presented that simply and objectively, why have there been heated discussions about it for years? This can be explained by the other "image look". All projectors with a high fill rate, in particular LCOS models or devices with Smooth Screen technology, lose the clear dividing lines between image details due to barely existent pixel distances. Stated conversely, a classic 3LCD projector with a fill rate of 50% always separates the small details with black lines that actually attain the half-pixel width. With increased image widths in particular these black borders and thus contrast increases are interpreted as additional sharpness by the eye.
 
Both images are equally sharp and yet the visual impression is different
However, this sharpness impression is not generated by an improved detail display but by a certain "screening". In addition, a large pixel distance forgives variations in the convergence with a lot of image content more than a little one does. On the negative side, the pixel structure is more evident on the screen.
The question of whose image characteristics appeal most to whom does not depend on the quality of the technology applied or assumed deficits, but only on personal preferences. For this reason, we have not made any recommendation on what is considered “better". The objective result of this Special is that a lot of "myths and rumours" were untrue. Smooth Screen technology reduces the pixel spacing and thus screen door significantly without provoking any loss in actual sharpness!
Your Cine4Home Team
Ekkehart Schmitt
www.Cine4Home.com
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