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Projector Technology Explained: How Video Projectors Work

Thursday, November 19, 2015

The basic function of an image projector is to project an image onto a surface by shining a bright light through a small transparent lens. The idea behind an image projector has been around for as far back as the 1500's. However, it was during the 1800's when the first types of image projectors appeared such as the Magic Lantern and the Opaque Projector. The Magic Lantern used a curved in mirror in the back of an Argand lamp to project an image, while the opaque projector or episcope used a combination of mirrors and prisms to project an image.

Types of Projectors
The 1960's saw the advent of the slide projector and overhead projector, which used among other components motorized filter wheels, an optical beam shutter and scanning galvo mirror systems, as well as an electric incandescent light bulb, a reflector and a condenser lens to direct the light.

The way a slide projector and overhead projector work is by using a very powerful halogen lamp as the light source to project an image through a transparent slide via a condenser lens, which then enlarges the image. To avoid burning the delicate slide and lens, a high-flow blower is needed as well as a heat absorbing glass placed in between the condenser lens and slide.

How an LCD video Projector work
Between the years 1980's and 2000's, the first liquid-crystal display (LCD), digital-light processing (DLP), liquid-crystal on silicon (LCOS) and laser video projectors were conceived and brought to market.

The basic technology behind an LCD video projector consists of an LCD, a high CRI (color-rendering index) LED light source, a high-intensity discharge lamp (HID lamp) and ballast with a condenser and collector Fresnel lens. LCD video projectors work by sending a beam of white light through three glass color mirrors that are specially shaped to reflect red, blue, and green wavelengths. Each beam of colored light is then fed to an LCD panel, which receives an electrical signal that tells it how to arrange the pixels in the display to create the image.


How a DLP video projector works
The digital-light processing (DLP)video projector uses an optical semiconductor chip which has on its surface thousands upon thousands of microscopic mirrors arranged in a rectangular array. Each mirror on this chip can be independently adjusted closer or further from the light source to create a dark or light pixel. Color is fed to the chip by a beam of light that passes through a spinning color wheel before it reaches the chip. After color reaches the chip, the image is fed through the lens and onto the projection screen.

How a LCOS video projector works
The liquid-crystal on silicon (LCOS) video projector combines LCD's liquid crystals and DLP's micro-display technology by applying liquid crystals to a reflective mirror layer. As the liquid crystals open and close, the light is either reflected from the mirror below, or blocked to balance the light, thus creating the image. LCOS video projectors typically use three memory chips, one each to balance light in the red, green, and blue channels to deliver them simultaneously. LCOS video projectors deliver very high image resolution and a typical LCOS projector costs upwards £3,000. Quality LCD and DLP projectors can be bought for around £200 and £600.

How a Laser projector works
Laser video projectors use solid-state lasers (essentially small LEDs), which are specifically designed to create the required color wavelengths without the need of filtering required with LCD, LCOS and DLP video projectors. Laser video projectors are are "the creme de la creme" of video image projection as they offer a lot of benefits such as less heat generated, better power consumption and life span (around 29,000 hours compared to the 3,500 hours a typical LCD projector will last). Laser projectors are priced upwards £6000.

The video projector technology has many great applications such as the IMAX Dome, which is a projector which projects images of celestial objects onto the dome in a planetarium. This plentarium projector technology has also expanded to home planetarium projectors such as the Sega Toys Homestar Planetarium (pictured).

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