Imaging technologies explained: DLP projectors

Once upon a projector

We could start our journey in the history of projectors with magic lanterns and camera obscura, but really the origins of color projectors as we know them today go back to the 1950s.

It started with CRT (Cathode-Ray Tube) type of projectors using three cathode-ray tubes, one for each of the primary colors (red, green and blue), to project the video images onto a screen. Typically, these projectors also had three separate lenses for the different color tubes.

Couple decades later, in the mid-80s, LCD, or liquid crystal display, projectors made their first appearances. And the introduction of DLP projectors, which stands for digital light processing, took place in the 90s. By now DLP and LCD have replaced most CRT projectors.

There are still other display technologies that have found their way in the projector manufacturing industry like LCoS. Nevertheless, for this article, we’ll focus on the two most common ones, which are still DLP and LCD technology.

Next to the changes to these newer imaging technologies, the evolution in projection also included a switch from lamp to LED and laser light sources. If you want to know more about the differences between lamp and laser, here’s another article that might be interesting.

How do LCD projectors work?

LCD (Liquid Crystal Display) is a transmissive imaging technology. It means that the three colored beams (red, green and blue) formed by the light source pass through three LCD imagers, one for each primary color. These LCD cells are made out of a number of pixels and contain a liquid crystal layer, which is controlled via local electrical signals.

The signals affect the pixel in the layer and changes the polarization of the light. This change of polarization allows more or less light to pass through the polarizers placed at either side of the LCD module and this then forms a frame of the digital image.

Almost all LCD projectors have a three-chip architecture, which means that after the frames are formed by the polarizers, the three frames, one per color, are then combined into one through a color combination prism, and then projected through the projection lens onto the screen.

How do DLP projectors work?

A DLP (Digital Light Processing) projector, on the other hand, directs the red, green and blue light to one or more Digital Micromirror Devices (DMDs). As its name suggests, a DMD is covered with millions of microscopic mirrors and each of those tiny mirrors corresponds to the individual pixels of the image.

While projecting the image, the micro-mirrors alternate between their on and off position. This means they change their orientation to either reflect the light towards the projector’s lens and onto the projection screen or the light is reflected away from the main lens into a light-absorbing area. It’s why we call DLP a reflective imaging technology. The mirrors’ ultrafast continuous on/off alternation defines the brightness and color of the corresponding pixels and forms the projected image.

In a single-chip DLP projector, each primary color is directed to the imager sequentially, and the image for each color is formed separately. Because it happens so fast our brain is able to blend the images into a full-color result. 3DLP, like the 3LCD projectors, use three imagers which will form the red, green and blue image simultaneously.

Barco has a history in offering both LCD and DLP projectors. Today our portfolio mainly focusses on the DLP imaging technology. 

The advantages of DLP technology

Shut the screen door

The Screen Door Effect refers to the visible lines or boundaries around the individual pixels on the screen or wall which can be seen when watching an LCD projection image up close. The grid effect is caused by the non-transparent lines which surround the pixel electrodes and carry the electrical signals in LCD imagers. The effect can be especially distracting on high-res videos or fast-moving images. 

The lines are in fact wasted pixel space on the imager. The micro-mirrors on a DMD only have a small inter-pixel gap and can be placed closer together. As a result, the DMDs have a higher pixel fill factor, produce a smoother picture quality and are capable of higher native resolutions.

Another one bites the dust

The imagers in DLP projectors typically have a sealed DMD chip and cooling is done through the back of each imager device without hindering the optical path. This means that the DMD is shielded from the cooling air and dust particles. It makes sure your viewers will see a clear and dust-free image.

And it also reduces the time and money required for upkeep of your projector. Most LCD projectors typically do not have sealed optics, are more sensitive for dust build-up and require periodical replacement or cleaning of the air filters to avoid dust spots on the projected image.

Long live the DMD

In addition, DLP projectors are considered more durable with guaranteed image uniformity, color accuracy and light output over time. Not only because the design prevents dust build-up but also because DMDs are micromechanical structures. They don’t change or fade over time.

The organic character of the liquid crystal modules, on the other hand, makes the LCD imagers sensitive to light, blue light in specific. Meaning that after long exposure, their brightness and color uniformity will degrade, leading to a reduced image quality and performance.

Conclusion

All imaging technologies have of course their own pros and cons and the ideal choice always depends on several factors.

While LCD projectors can be considered for their lower initial acquisition cost, in a lot of cases DLP will offer you a better experience in the long run. The DLP Digital Light Processing projectors will be able to maintain their premium image quality over time, lowering your TCO. The difference becomes even more relevant when switching to higher resolutions, higher brightness levels or multi-channel set-ups. For these reasons, the DLP models are typically used for mid- and high-end applications such as large venue projections by the rental industry and immersive experiences in the fixed install market.

Are you looking for a new projector and you want to find out which imaging technology fits your needs? Check the complete Barco projection portfolio or simply reach out and we’ll be glad to help you with more information.