LCD 3D Projection Technology
This blog discusses the use of Liquid Crystal Display (LCD) projection technology for 3D. This is the oldest of the micro-display technologies with the basic idea dating back to 1968 with the first commercial projectors appearing about 2 decades ago. However, it has only been about a decade since micro-display projectors started to gain a solid market position against the long dominate CRT based projectors for video/home theater applications.
LCD front projectors have been popular, first for business class projectors then for home theater, based in part in their modest price. DLP has been, and remains, the main competitor in the business projector market, while LCoS as well as DLP projectors have been strong competitors in the mid-priced home theater market. More on the history of LCD projectors can be found on Wikipedia – HERE.
The focus of this blog is on home theater projectors and all that follows should be taken in that context.
Note that while LCoS (Liquid Crystal on Silicon) as discussed in a previous blog, is related to LCD technology, traditional LCD micro-display chips are less complex, easier to manufacturer and lend themselves to use in less expensive projectors. Over the past several years just about all LCD micro-display chips used for home theater oriented projectors have been manufactured by either Epson or Sony. More recently Sony has been gradually reducing the price for their entry-level LCoS (SXRD) projectors to the point that today Epson is not only the largest manufacturer of LCD projectors but also the dominate supplier of LCD micro-display chips used by other LCD projector manufacturers. All LCD projectors use 3 LCD micro-display chips with one for each of the primary colors (i.e., red, green and blue). Projectors using this technology are referred to by Epson as “3LCD”. With LCD technology the liquid crystal micro-displays are of a transmissive design where the light from the projection lamp is split into 3 individual beams that then passes through each of the liquid crystal micro-display panels (unlike DLP and LCoS where the light reflects from their micro-display panels).
In the most generic form LCDs are created by placing a liquid crystal layer between two linear polarizing filters with the orientation of one polarizing filter rotated 90 degrees relative to the other filter. In its inactive state light will not pass through the display panel due to the different orientation of the polarizers. However, the liquid crystal layer, when a voltage is applied, has the ability to rotate the orientation of the light as is passes through. This provides the basis that allows for control over each pixel on the display to transition between more-or-less opaque and transparent or various states in between. The electronics that controls each individual pixel is placed on the micro-display chip itself and is located in the space between the display area of adjacent pixels. Epson refers to the technology used for the LCD micro-display technology in their projectors as High Temperature Poly-Silicon (HTPS). More info on Epson 3LCD projection can be found HERE.
While LCD projectors traditionally lacked the level of performance that could be achieved by the better DLP and LCoS based models, much progress has been made over the past 3 to 4 years in the performance of the better home theater oriented LCD projectors. LCD projectors still suffer from a lower “fill factor” as compared to DLP and LCoS. The term “fill factor” refers to the percentage of the overall display area of the chip that is actually occupied by the pixels being displayed (as compared to the unused space between the pixels). In the case of DLP and LCoS a 90%+ fill factor is achieved while LCD displays typically have fill factor of 60%+ to 70%+ (see illustration below).
When viewed up close this relatively low fill factor creates a “screen door effect” (SDE) which is more obvious with LCD projectors that have 720p and lower resolutions. With the modern 1080p LCD projectors the SDE is not visible to most viewers when seated at a typical viewing distance of 1.5 times the screen width, or more. When viewed up close the image from Epson LCD projectors clearly displays the pixel structure and the black space between pixels. However, Panasonic uses an additional optical element within the light engine of their LCD projectors to provide what they call “smooth screen” which modifies the projected image to reduce the visible pixel structure. More information on this Panasonic smooth screen technology can be found HERE.
Earlier LCD projectors generally suffered from lower contrast images, with higher black levels, as compared to LCoS and DLP based projectors. More recent higher-end LCD models from both Epson and Panasonic now have native on/off contrast ratios similar to mid-level DLP models and entry-level LCoS models. However, most LCD home theater oriented projectors now incorporate a dynamic iris to improve the on/off contrast ratio which can improve the overall perceived performance of the projector. Some dynamic iris implementations do at times display undesirable side effects such as visible pumping of the projected black levels or a noisy mechanical iris mechanism. By comparison the better LCoS implementations are able to achieve excellent on/off contrast ratios without the use a dynamic iris or with less aggressive action from the dynamic iris.
LCD and 3D
The first 3D capable projectors using LCD technology have just recently appeared with the first models to begin shipping being the Panasonic PT-AE7000 and the entry-level Epson Home Cinema 3010. Both of these new models have been discussed by Art here at Projector Reviews and his initial review of the PT-AE7000 and a full review of the Epson 3010 are available by clicking on these above links. Epson is supplying the new generation of LCD micro-display chips used both in their own projectors as well as those from Panasonic. The Panasonic is a higher-end model that presumably uses the same, or similar, LCD micro-display chips that Epson will be using in their soon to be released higher end models Home Cinema 5010 and Pro Cinema 6010.
The major technical challenge in using LCD technology for displaying active 3D images, is how to accommodate the response time inherent with this technology. This limitation of LCD technology applies to both flat panel LCD 3D TVs as well as to LCD 3D projectors. Since active 3D systems work by alternately displaying the images intended for the right and left eyes, the display must be able to display the new image frame without retaining any (for very little) traces of the previous image. Liquid crystals cannot instantaneously change between different states (e.g, between transparent and opaque or between light grey and dark grey). If the response time required by the LCD to complete the required transition between is too long then the results will be 3D crosstalk, or ghosting. The approach taken by such manufacturers as Samsung and Sony, for their first generation of LCD flat panel 3D TVs was to refresh the display 240 times per second (240 Hz) with every other frame being a black frame. This was done in an attempt to remove the previous image from the display before the next image was displayed. This also allowed time for the liquid crystal lenses of the active shutter 3D glasses to switch between their clear and opaque states. The approach presents each eye with 60 video frames per second. While this technique reduced to 3D crosstalk to generally acceptable levels for many viewers, the response time for some such LCD 3D TVs (or so-called LED 3D TVs – actually LCD with LED backlight), was still too long to fully eliminate visible 3D crosstalk. The drawback of inserting a black frame between each active video frame is a reduced image brightness since each eye individually is now only seeing every 4th frame (i.e., the other 3 are two black frames plus the video frame intended for the other eye which is being blocked from view by the active shutter glasses). Thus the viewer starts out with a 75% light loss plus the additional light loss in going through the 3D glasses. The net result with such 3D TVs is the overall image brightness, as seen by the viewer, in 3D modes is perhaps only 15% as bright as viewing the same TV for standard 2D video (i.e., without wearing the 3D glasses). It can be noted that Sony’s first generation of LCoS 3D projectors, introduced in late 2010, also used a similar approach of using an overall 240 Hz refresh rate with a black frame inserted between each active video frame.
The Epson/Panasonic approach taken for their first generation of LCD projectors, is similar to what is described above for LCD flat panel 3D TVs. However, the refresh rate has been increased to 480 Hz and rather than simply alternating between black frames and active video frames, the Epson/Panasonic approach is to repeat each active video frame 3 times followed by one black frame. Thus the sequence is 3 repeated left active frames, one black frame, 3 repeated right active frames, one black frame, etc. See the following illustration of the video frame sequence used with these Epson and Panasonic LCD projectors that use a 480 Hz refresh rate.
The net effect is that each eye still is only presented with 60 unique active video frames per second with each one being 3 x 1/480 sec. = 1/160 sec. long. Projectors using this approach are displaying the active frames for each eye for 3 out of 8 consecutive 1/480 sec. intervals, or 37.5% of the time, rather than 25% of the time as in the previous example. Thus there will be less light loss than with the previously described approach used by the 240Hz LCD flat panels 3D TVs and certain 240 Hz 3D LCoS projectors. With this Epson/Panasonic approach the real question becomes: do the LCD micro-display chips have a short enough response time to fully transition between left and right sequential active frames during the 1/480 sec. where the black frame is being inserted. Initial reports have been that 3D crosstalk on these new Panasonic and Epson projectors is visible at times as is also the case with LCoS based 3D projectors (and much less so with DLP 3D projectors).
I hope in future blogs to do some comparisons on the level of 3D crosstalk achieved by some different types of 3D projectors.
Finally, just a quick update on the upcoming 2012 models JVC 3D projectors. All JVC projectors use LCoS technology sold under the JVC trade name DILA. A nice color brochure is now available from the JVC UK consumer division web site – HERE. JVC offers both a consumer version (i.e., X30, X70, X90) and a profession version (i.e., RS45, RS55, RS65) that are virtually identical. The first of these new JVC 3D projectors are expected to start showing up at dealers in the United States during December.