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January 19, 2023

Like Flat Panels, 8K Projectors are hard to make, too!

Recently, we looked at some of the challenges to making 8K OLED TVs and why there are few models and those available are in the premium segments of the market. 8K projectors from JVC and Delta/Digital Projection exist, but there are also challenges in reducing the cost to make them more widely available.

The average resolution of projectors has lagged behind most direct-view displays. According to projection market analyst, PMA, 30% of the market is still 4:3 aspect ratio at just SVGA (800 x 600) and XGA (1024 x 768) resolutions! Those resolutions were superseded in the flat panel markets many years ago.

There are several reasons for this, and we wanted to dig into them.

Light Sources, Imagers, and Optics

Projectors consist of imagers, light sources, and optics. In terms of light sources, whether bulbs, LEDs, or lasers, there is no real technical difference in the resolution that can be addressed. Having said that, light sources such as LED and laser cost more than lamps, so units with those light sources are likely to be at the higher end of the market, with higher resolutions.

There can also be differences in the optics. It’s no secret that creating good optics is a real challenge – but it is especially so if you need to be able to create good optics at a low cost. We’ll come back to that topic.

Imagers are the Main Challenge

There are three kinds of volume imagers in the market. There are two reflective technologies, DMD/DLP from TI and LCOS from various vendors, including JVC with its D-ILA and Sony with its SXRD. The third technology is HTPS LCD, and Epson is the key supplier of that technology.

While DMD/DLP and LCOS share the idea of modulating the light reflecting off the device to create the image, the technologies are very different. The DMD/DLP technology uses physically pivoting micromirrors to reflect light into an absorber or out through a lens to produce darker or lighter pixels. The LCOS technology uses an LCD to control the reflection of light from the surface of the device. Behind the imaging surfaces, both technologies are based on semiconductor manufacturing technologies.

The use of chip-making technologies means that the cost of the device is precisely correlated to the area/size of the imager device. If you go up in area because the resolution rises, the price also increases. To switch from FullHD to 8K, if you keep the pixel size the same, the area will go up by a factor of 16. That would make the chip price, at the very least, 16 times more expensive. It would also make the optics more costly because of the increase in ‘etendue’ or the area covered by the image that needs to be processed through the optical system.

Shrink the Pixels

One alternative is to shrink the size of the pixels. That’s extremely challenging. In the case of the DLP, TI has shrunk the size of pixels considerably down to around 5.4μmequivalent to around 4,500 PPI. (for comparison, the 4K OLED on the Sony Xperia 1 IV, the highest resolution smartphone, is 642ppi). Making an 8K chip with the same pixel size would mean a chip of around 2″ diagonal. That would be prohibitively expensive not only because of the area but because it is hard to make a chip of that size without defects so the manufacturing yield would be reduced. Currently, the largest DLP chip made is 1.38″ for cinema and high-end applications.

Chip-making processes are limited by the size of the photomask (or reticle) that can be used to make a single image. Each chip has to be made with a single reticle exposure (so no tiling & stitching) as the joins between the different tiles would be visible.

Another factor is that developing new chips is expensive for R&D and non-recurring engineering (NRE) charges for the semiconductor processes. Spreading those costs over a high volume of chips can make each one economical, but if the market is relatively small, the cost of each chip becomes prohibitively high.

Similar challenges and cost calculations apply to the LCOS chips – although they can cost less than DMD chips.

Transmissive LCDs

Epson uses transmissive LCDs – rather than reflecting off a chip surface, the light from the light source passes through the LCD just like a direct-view LCD with a high-powered backlight. However, the LCDs are very small. As we mentioned in our article on 8K OLEDs, a challenge for LCDs is the ‘aperture ratio’. That number is the proportion of the area of the pixel that is transparent. Ideally, you would get this as close as possible to 100%. However, part of the area of the pixel is taken up by a mask, which keeps the pixels separate and hides the areas where the conductors are, and part is blocked by the transistor that controls the pixel.

Improvements in polysilicon LCD apertures

Making the transistor as small as possible is a critical part of making this kind of LCD, so the material used is polysilicon. This type of silicon is much more efficient than the amorphous silicon or oxides used to make the transistors in most direct-view LCDs in notebooks, monitors, and TVs. Epson has developed a process called High Temperature Polysilicon (HTPS) to make the backplanes for their imagers. HTPS is even better than the low-temperature polysilicon (LTPS) used in the high-definition LCDs used to make smartphones. However, making HTPS devices requires high fabrication temperatures so the LCDs are manufactured on quartz substrates rather than the lower-cost glass used in direct-view LCDs. That makes them significantly more expensive than glass-based devices.

Epson has developed a range of technologies to help improve the effective aperture ratio, including microlenses to focus the light from the light source through the central part of the pixel where it will not be blocked. The firm has developed double lenses from an original single lens to improve efficiency further. Epson is reported to have shown an Ultra HD HTPS prototype chip, but as far as the writer knows, it has never come to market.

Microlenses improve the transmission of polysilicon LCDs. Source:Epson

Move the Pixels

The solution that has been developed by all of the device makers in the three different technologies is to exploit the fact that the human visual system has persistence. That is to say, if the eye is exposed to light, it continues to register an image even after the light has gone away. By moving the imager during the period of a motion image frame, the resolution can be increased. A number of ways have been tried to achieve this, but the main ones are to move the imager or image diagonally or orthogonally. The slight vibration is often referred to as ‘wobbulation’ or pixel shifting.

Typically, this motion is achieved using a very small motion motor synchronized with the image to get the right pixels into the right position. Still, it has also been suggested that this could be done using a liquid crystal layer to shift the image according to its polarization. https://ieeexplore.ieee.org/document/9438653

When the industry moved to 4K/UltraHD, TI used a DMD with 2716 x 1528 micromirrors and moved it diagonally to create the 4K version. Digital Projection was the first firm to make an 8K projector (the Insight for professional use) and used three of TI’s 4K 1.38″ digital cinema grade ‘dark chip’ devices with vertical and horizontal wobbulation and the same concept is used in the JVC DLA-NZ8 projector that won the Value Electronics projector shoot-out in December 2022 ( see article HERE). The Samsung 8K UST projector launched at CES in January 2023, like the Hisense 8K Laser TV, uses a 1.3″ DLP chipset with image shifting.

Optics

Finally, the optics in the projector need to be very good to maintain a sharp image in the final picture. Even when 4K arrived, many lenses had to be completely redesigned. The lenses previously used and optimized for light transmission, cost, and bulk were not good enough for that resolution. The author was shown the results of testing of lenses on 4K projectors conducted by a major distributor of projectors some years ago when 4K projectors arrived, and the results were very, very variable from the best to the worst. Moving up to 8K increases the challenge another step, and sadly, there’s no Moore’s law for optics – although the best lenses have got a lot sharper in recent years. And the challenges get more interesting when you move to ultra-shortthrow optics typical of Laser TV configurations where maintaining sharpness over the full screen becomes a concern.

Finally, all projectors need a surface to project the image on. Sometimes a white wall will suffice, but for the best results with an ultra-shorthrow projector, an ambient light rejection screen is needed as well – and they are not inexpensive.

As we noted at the top, 8K projectors existing and can offer amazing image quality, but cost reducing these offering will be a challenge going forward.

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