In the previous posts we focused on the signals and resolutions of projectors. In this post we will turn more to how those images are projected and the light sources being used.
Many people get confused between different types of information about projection. Terms such as LCD or DLP are not referring to the same information as lamp types or laser technology. So let’s break this down into two areas: display technology and light sources.
Display technologies have changed over the years, but there are still some old technologies used for home theaters. If you have ever seen a projector with three separate lenses that appear to project three different colors then you know what I mean. What manufacturers realized is that if you split the projection signal into three different colors and project them separately, you will get a brighter and higher detailed image. This has held true as the technology advanced to newer projection methods such as LCD and DLP. While the advances in light sources and cooling technologies have made it possible to get brighter and brighter images from a single image, the best resolution vs. brightness is still achieved by splitting this up into three images.
So why don’t you see three lenses any longer? In order to simplify the use of the projectors and lower the cost of multiple lenses, the three images are now combined internally in projectors through the use of mirrors and prisms. The result is a single image coming from the lens, but internally the projector may have a single source, or three different sources. This difference is commonly referred to as “3-chip” or “single chip” technology.
The most common of the displays today are LCD (Liquid Crystal Display) and DLP (Digital Light Processing). LCD has been around for quite some time, and display devices were once made that sat on top of overhead projectors. An LCD image device similar to your laptop display or your flat screen TV was laid on top of the glass and the light from the projector would go through it the same way it would the transparencies.
This idea actually sparked a revolution to improve the brightness of overhead projectors, which would later assist in the growth in brightness of LCD projectors. Once this technology was improved to where the LCD imaging device could be shrunk down to a small size, and the brightness of the lamps could reach a high enough level, combined with cooling technologies to keep the glass in the LCD from shattering, LCD projectors were born. Early projectors were the size of some of the larger projectors on the market today (and cost about the same) but the brightness and resolutions were at a level of what you commonly find on lower end projectors today. As these areas of technology grew, the projectors shrunk in size, became brighter, and resolutions increased, but the difference in single vs. three chip still holds true today, and brightness levels are often limited on single chip compared to three chip.
Then along came TI (Texas Instruments) with an idea that the images could be projected with a more fluid approach by mounting thousands of tiny mirrors onto a single display device, where each mirror would adjust to reflect a pixel. This was dubbed the DMD (Digital Micro-mirror Device). Early models (and even some today) would have failures in those mirrors, causing individual pixels to be stuck in a single color. Over time the reliability has improved, but as the saying goes “you get what you pay for”. Cheaper single chip DLP projectors may still often have these issues after only a year or so of use. The higher level chips are far more reliable, and are used in the vast majority of cinemas today. Colors are reproduced in one of two ways: either using three different colored light sources, or by projecting through a color wheel that is synchronized with the movement of the mirrors on the chip. This often has what is commonly referred to as a “rainbow effect” where the eye perceives a rainbow type of distraction when viewing the image.
When TI first produced the technology it was licensed to a single company, Digital Projection. This had the effect of it being slower in wide acceptance, and also slowed the advancement in how the use of the technology. Other manufacturers began the use of the technology, which has caused the advancement in its use and abilities. Typically a three chip DLP projector can produce a far brighter image than an LCD projector, and will usually produce a more blended image where the pixels are concerned.
One other technology to note is LCoS (or Liquid Crystal on Silicon). This is less common and generally only available from two manufacturers, JVC and Sony. Each has their own marketed name for the product, D-ILA from JVC, and SXRD from Sony. LCoS is essentially a highbrid between LCD and DLP, using reflective mirrors behind the LCD chip.
None of these technologies would be possible without a light source. Brightness in projection is often referred to in ANSI (American National Standards Institute) lumens. It is important to be sure that the rating you are looking at is referred to this way as it is a universal standard put out by a non-profit organization in order to provide an even playing field. Some manufacturers will skirt this definition from time to time in an effort to improve how the projector is perceived.
In Part IV we will look at lamp technologies that are used to project these images.
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