Deployed projection technologies

1.4 Deployed projection technologies

The transmissive LCD is a digitally addressed analogue modulated technology that uses a LCD crystal to modulate the light polarization at each pixel location. Light source is either a backlit panel or a lamp. The LCD has analogue-like properties that can vary the intensity of light at each pixel based on how much the pixel’s crystal is twisted. As the crystal twists, the light’s polarity is changed. The intensity changes are then realized by using polarizing filters in the light path. Each LCD panel handles one colour signal; the projectors employ three panels to handle RGB colour images.

The LCD technology is found in small to medium brightness front projectors. The brightness can be as high as 6K lumens for some models. Typical large venue resolutions are SXGA (1 280  1 024), although a new model with 1 080  1 920 resolution has been announced. The projectors using transmissive LCDs update the entire image at once. These projectors can accept and display interlaced content as they contain the appropriate processing circuitry as part of the projector electronics.

The DMD, also called digital light polarization (DLP), is a binary reflective technology that uses pulse width modulation to achieve an analogue-like representation of brightness at each pixel location. Each pixel is created by a mirror mounted on a movable post that can be toggled to reflect light either onto the screen or into a light dump. The entire image is loaded into a frame buffer and each mirror is then modulated based on the brightness value of the pixel. The fraction of time the mirror is in the on position is directly proportional to the brightness of the addressed pixel. Each device containing an array of mirrors handles one colour component. Projectors employ three devices to handle RGB colour images. The DMD is widely deployed in very high brightness front and rear-view projectors because of its high tolerance to heat and light. DLP projectors update the entire image at once and require interlaced content to be processed prior to display. Some models can accept and display interlaced content as they include appropriate processing circuitry as part of the projector electronics.

The highest quality DMD projectors, including those installed in digital cinema applications, are currently SXGA (1 280  1 024) resolution; projectors based on a higher resolution DMD began shipping in the 4th quarter of 2003. The new models have 2 048  1 080 pixels. While LSDI pixel rates may range up to 130 megapixels/s (1 920  1 280  60 fps), the DMD devices themselves can handle up to 200 Megapixels/s. The DLP cinema projector models that are on the market, and newer models that have been announced, that are intended for digital cinema applications, cannot accept and display interlaced content. The manufacturers instead supply optional add-on units that offer a de-interlacing function. These units typically convert 1 080  1 920i 25/30 fps signals into a progressive format that is then delivered into the projector via a digital video interface (DVI) interface. Projectors based on DMD devices are available that can handle xenon arc light sources up to 6-7 kW, and can provide light output as high as 20 k lumens.

Reflective LCD displays, commonly known as liquid crystal on silicon (LCoS) and digital image light amplifier (D-ILA), use a mirrored substrate with an LCD structure to modulate the light. The reflective LCD’s tend to be less efficient than the transmissive LCD or the DMD. D-ILA and LCoS displays can be manufactured with higher pixel densities and higher fill factors than the DMD, but suffer from the same temperature and heat capacity issues as the transmissive LCD technology.

Projectors using reflective LCD devices are on the market for use on screens up to 10 m, and prototypes for larger screen venues have been shown. Resolutions for the D-ILA are as high as QXGA (2 058  1 536) and a demonstration of a 3 840  2 048 projector has been conducted. These projectors all update the entire image simultaneously. Current D-ILA projector models can accept and display interlaced content as they include appropriate processing circuitry within the projector.

1.5 Technology in development

The L-CRT is a device that is essentially a CRT, without phosphor, bonded onto a lasing crystal. The crystal emits a laser beam with intensity proportional to the intensity of the exciting electron beam. The image is formed using a raster scan across the faceplate at an extremely high scan and refresh rate. The e-beam cannot linger on the crystal due to localized heat build up. The device is limited in its power handling capabilities due to limitations in the size of the lasing crystal. Each LCRT operates at one colour so three devices are required to supply RGB colour images. The LCRT can suffer from the distortions inherent in three tube CRTs.

The grating light valve (GLV) is a device that uses continuous beam lasers modulated by an electronic diffraction grating. A vertical line of pixels is displayed simultaneously and this is scanned horizontally creating a ribbon of light that sweeps across the screen. The GLV is an example of a modified raster scan device. In a commercial projector each GLV device would modulate one laser beam; three devices, each illuminated by an R, G or B laser beam, are employed to provide full colour images.