Digital Projectors

The field of image projection has seen huge changes over the past few years, fueled both by the opening up of new application areas and technological developments. Foremost amongst the former has been the boom in the popularity of DVDs and the “home theater” phenomenon that has accompanied it.

Modern-day projectors convert a digital signal from a computer, DVD/blueray player, or media streaming device, the projecting it onto a screen using a light source.

There are two main types of projector: LCD and DLP.

LCD

With LCD projectors, the light must first of all be split into the three primary colours of red, green and blue. This is done using dichroic (double-refracting) mirrors. These are produced by coating glass with metallic oxides inside a vacuum furnace. Different metallic oxides reflect only a specific part of the light spectrum, while allowing all other light components to pass through. Two dichroic mirrors arranged in series thus split white light into red, green and blue light.

The image coming from the computer or streaming device is displayed on a small LCD panels inside the projector, one for each primary colour. The light from each LCD panel is then recombined in a dichroic cube and beamed through a lens onto the screen.

Inside LCD projector

DLP

DLP (digital light processor) projectors  work a bit differently from the LCD type. A  Digital Micromirror Device (DMD) panel has micromirrors mounted on tiny hinges that enable them to tilt either toward the light source (ON) or away from it (OFF), creating either a light or dark pixel respectively on the projection surface.

DLP Projector

The bit-streamed image code entering the processor from the input source directs each micromirror to switch on and off up to several thousand times a second. A mirror that’s switched on more frequently than off reflects a light grey pixel; a micromirror that’s switched off more frequently reflects a darker grey pixel. In this way, the micromirrors in a DLP projection system can reflect pixels in up to 1,024 shades of grey to convert the video or graphic signal entering the DMD into a highly detailed greyscale image.

While all this is happening, the white light generated by the lamp passes through a colour wheel as it travels to the mirros on the DMD panel. The colour wheel filters the light into red, green, and blue, from which a single-chip DLP projection system can create at least 16.7 million colours.