Liquid crystal shutter glasses
Liquid crystal shutter glasses (also called LC shutter glasses or active shutter glasses.) are glasses used in conjunction with a display screen to create the illusion of a three dimensional image, an example of stereoscopy. Each eye’s glass contains a liquid crystal layer which has the property of becoming dark when voltage is applied, being otherwise transparent. The glasses are controlled by an infrared, radio frequency, DLP-Link or Bluetooth transmitter that sends a timing signal that allows the glasses to alternately darken over one eye, and then the other, in synchronization with the refresh rate of the screen. Meanwhile, the display alternately displays different perspectives for each eye, using a technique called Alternate-frame sequencing, which achieves the desired effect of each eye seeing only the image intended for it.
LC shutter glasses mostly eliminate “ghosting” which is a problem with other 3D display technologies such as RealD 3D, or Dual projector setups. Moreover, unlike red/cyan colour filter 3D glasses, LC shutter glasses are colour neutral enabling 3D viewing in the full colour spectrum.
Flicker can be noticeable except at very high refresh rates, as each eye is effectively receiving only half of the monitor’s actual refresh rate. Modern LC glasses however generally work in higher refresh rates and mostly eliminate this problem.
LC shutter glasses are shutting out light half of the time; moreover, they are slightly dark even when letting light through, because they arepolarized. This gives an effect similar to watching TV with sunglasses on, which causes a darker picture perceived by the viewer. However, this effect can produce a higher perceived display contrast when paired with LCD displays because of the reduction in backlight bleed.
Frame rate has to be double that of an ordinary stream to get an equivalent result. All equipment in the chain has to be able to process frames at double rate; in essence this doubles the hardware requirements of the equipment. This is especially noticeable when the image stream is interactively generated in real time by 3D hardware on computers.
In addition, shutter glasses tend to be much more expensive than other forms of stereoscopic glasses. Whereas most anaglyph,ChromaDepth, and polarized 3D glasses can be purchased at very low prices (less than US$1 as of 2010, with anaglyph filters being the least expensive), shutter glasses feature far more advanced technology and usually sell for three orders of magnitude higher than paper anaglyphs and two orders over paper ChromaDepth and polarized glasses, with most models selling for well over US$100, particularly for the standard wireless models.
Shutter glasses are also matched to the TV so it’s not possible to take your shutter glasses to a friend’s house if he owns a different brand 3DTV. However, efforts are being made to create a Universal 3D Shutter Glass.
LC glasses providers
There are many sources of low-cost 3D glasses. IO glasses are the most common glasses in this category. XpanD 3D is a manufacturer of shutter glasses, with over 1000 cinemas currently using XpanD glasses. With the release of this technology to the home-viewer market as of 2009, many other manufacturers are now developing their own LC shutter glasses, such as Panasonic, Samsung, and Sony.
Nvidia makes a 3D Vision kit for the PC; it comes with 3D shutter glasses, a transmitter, and special graphics driver software. A certified 120 Hz monitor is required to use the 3D Vision; ordinary flat panel monitors run at 60 Hz.
Liquid crystal shutter glasses were first invented by Stephen McAllister of Evans and Sutherland Computer Corporation in the mid-1970s. The prototype had the LCDs mounted to a small cardboard box using duct tape. The glasses were never commercialized due to ghosting, but E&S was a very early adopter of third-party glasses such as the StereoGraphics CrystalEyes in the mid-1980s.
In 2007, Texas Instruments introduced stereo 3-D capable DLP solutions to its OEMs, and Samsung and Mitsubishi introduced the first 3-D ready televisions. These solutions utilize the inherent speed advantage of the Digital Micro-mirror Device (DMD) to sequentially generate the left and right views required for stereoscopic imaging.
DLP 3-D technology uses the SmoothPicture algorithm, which compacts two L/R views into a single frame by using a checkerboard pattern, only requiring a standard 1080p60 resolution for stereoscopic transmission to the TV. The device re-generates two independent views for the left and right eyes and interpolates the missing pixels in each frame using the quincunx sampling algorithm. The claimed advantage of this solution is increased spatial resolution, unlike other methods which cut vertical or horizontal resolution in half.
A synchronization signal is then generated for LC shutter glasses worn by the viewer, using either a standard VESA Stereo plug to connect wired glasses or wireless emitters, or brief flashes of light on the viewing screen during the blanking interval (DLP Link). The LCD shutter glasses process the signal and control the shutter for each eye to ensure that the correct left and right views are presented to the correct eye.
Plasma display panels are inherently high-speed devices as well, since they use pulse-width modulation to maintain the brightness of individual pixels, making them compatible with sequential method involving shutter glasses. Modern panels feature pixel driving frequency of up to 600 Hz and allow 10 bit to 12 bit color precision with 1024 to 4096 gradations of brightness for each subpixel.
Samsung Electronics launched 3D ready PDP TVs in 2008, a “PAVV Cannes 450” in Korea and PNAx450 in the UK and the US. The sets utilize the same checkerboard pattern compression scheme as their DLP TVs, though only at the native resolution of 1360×768 pixels and not at HDTV standard 720p, making them only usable with a PC.
Matsushita Electric( Panasonic) prototyped the “3D Full-HD Plasma Theater System” on CES 2008. The system is a combination of a 103-inch PDP TV, a Blu-ray Disc player and shutter glasses. The new system transmits 1080i60 interlaced images for both right and left eyes, and the video is stored on 50-Gbyte Blu-ray using the MPEG-4 AVC/H.264 compression Multiview Video Coding extension.
Liquid crystal displays have traditionally been slow to change from one polarization state to another. Users of early 1990s laptops are familiar with the smearing and blurring that occurs when something moves too fast for the LCD to keep up. This smearing can result in a completely unviewable image when using shutter glasses.
LCD technology is not usually rated by frames per second but rather the time it takes to transition from darkness to brightness and back to darkness, in milliseconds. In order to achieve an equivalent minimum refresh rate of 120 Hz, an LCD must be able to transition at a speed of not more than 8.33 ms. However, each frame is displayed for at most 8.33ms, and minimizing the response is key. For example: if it takes 8.33ms for the LCD to transition to the desired image, and a sequential black/white image is shown, the 8.33ms which should be displaying “white” will begin at black, and after 8.33ms finally achieve white. Similarly, the next 8.33ms which should be displaying “black” will begin as white, and after 8.33ms finally achieve black.
However, because pixel transition speed has become a strong selling point of LCD monitors, marketing hype has unfortunately obscured these speed-of-transition specifications with what some consider tortuous qualifying statements that make inadequate technology appear to be better than it really is (see PMPO for another example of such marketing distortions). While the average person attempting to buy a high quality LCD for normal home use might not notice these minor performance differences, a slowly transitioning LCD can have a severely negative impact on usability with shutter glasses. For stereoscopic applications, it is important that the LCD be truly capable of what is being claimed.