LCD shutter glasses

LCD shutter glasses are glasses used in conjunction with computers or TV screens to create the illusion of a three dimensional image, an example of stereoscopy. Glass containing liquid crystal and a polarizing filter has the property that it becomes dark when voltage is applied, but otherwise is transparent. A pair of eyeglasses can be made using this material and connected to a computer video card. The video card alternately darkens over one eye, and then the other, in synchronization with the refresh rate of the monitor, while the monitor alternately displays different perspectives for each eye. This is called Alternate-frame sequencing. At sufficiently high refresh rates, the viewer's visual system does not notice the flickering, each eye receives a different image, and the effect is achieved.

The problems associated are the cost for the additional equipment (typically up to US$100), and that the flickering can be noticeable if the refresh rate is not sufficiently high, as each eye is effectively receiving only half of the monitor's actual refresh rate. As with other single image methods the brightness is considerably diminished.

Another possible effect is "ghosting". Since the opaque phase of the LC still permits a small amount of light to transmit, some users experience secondary "ghost" images from the alternate channel. This effect can be exacerbated by persistence effects in the phosphors of the CRT, causing images to "bleed" over into the other channel.

Until recently, the method only worked with CRT monitors; some modern flat-panel monitors now support high enough refresh rates to work with some LC shutter systems.

Another potential use of shutter glasses is to allow private viewing of a publicly observable screen (e.g. a laptop on a train). The display could flicker between the normal display and the normal display's photographic negative image. To the unaided eye this screen would appear a blank grey. The screen, viewed via shutter glasses blocking out the negative image, would appear to show only the private image. Fact|date=March 2007

Usage limitations

Because shutter glasses require a screen refresh rate twice that of a normal display, extremely high performance graphics technology is required to support it. Most inexpensive products advertised as being shutter-glasses compatible are only capable of 120Hz refresh, equivalent to the industry minimum of 60 frames per second. But on a standard CRT, 60 Hz is slightly too slow and the flickering is often readily apparent when looking away and the screen is in the periphery of the visual field. Long-term viewing of 60 Hz refresh on a CRT can lead to headaches and eye strain.Fact|date=February 2008

The slightly visible CRT flicker at 60 Hz diminishes as the refresh is increased, with 85 Hz commonly being a commonly preferred choice for standard CRTs. However, the equivalent shutter-glasses refresh rate would be 170 Hz, which almost no equipment is capable of achieving. Typically 150 Hz is as high as most video cards will go, which is equivalent to 75 Hz on a normal CRT.

Compatible DLP hardware

DLP projectors are commonly available in two designs. The less expensive is referred to as a "one-chip" DLP and uses a single light-modulating DMD that only reflects varying shades of gray. To achieve a color display, a transparent color wheel with different colored regions spins in the light path. The more expensive version is a "three-chip" DLP with three separate DMD arrays, each with its own dedicated red, green, and blue color filter. While the three-chip DLP is ideal for use with shutter-glasses, a three-chip DLP can easily cost more than US$4000, pushing it well out of reach of the computer hobbyist.

A one-chip DLP can be used with shutter-glasses, but with limitations. The rotational rate of the color wheel is often two to three revolutions per video frame, up to about 85 Hz, with the DMD flashing multiple patterns of color that seem to blend together to the observer's eye. When a one-chip DLP is pressed into service running at a framerate of 120 Hz or more, the rotational rate of the color wheel must be limited to prevent damage to projector.

In normal service at 85 Hz, a 3x color wheel rotates 255 times per second, or 15,300 RPM. If this same one-chip projector were to operate at full speed at 120 Hz, the color wheel would be spinning at 21,600 RPM, far outside the design limits for the projector. Rather than rotating at the normal three times per frame, the wheel must drop back to turn only two revolutions per frame (14,400 RPM) to stay within safe operational parameters. This results in a visibly degraded image with incorrect colors and/or incorrect brightness.

Compatible LCD hardware

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 because pixel transition speed has become a strong selling point of LCD monitors, marketing hype has unfortunately obscured these speed-of-transition specifications with tortuous qualifying statements that make inadequate technology appear to be better than it really is (see PMPO for an example 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.