The rear-projection TV (also referred to as an RPTV), with its lower pricing yet high performance, bridges the affordable screen size gap between large screen TV and separate video projector/screen setup. The following article outlines the technology and basic elements of a rear-projection television.
Definition Of Rear-Projection
The actual term "rear-projection" comes from the fact that the image is projected and reflected onto the screen from behind the screen, unlike traditional video and film projection in which the projector itself is placed in front of the screen, such as in a movie theater.
The Elements Of Rear-Projection Television
There are five basic elements in the construction of a rear-projection television. First, there is the type of projector technology used to produce a video image. Second, the type of lens assembly used to magnify the projected image. Third, the necessity to employ a mirror to reflect the projected image. Fourth, the screen upon which the reflected image is presented. Fifth, the sealed box that contains all of the previous elements.
Video Projection Technology
Currently, there are three major basic types of projection technology commonly used in rear-projection televisions in today: CRT, LCD, and DLP. There are also variations of these three types that are not widely used at this time (including D-ILA and LCOS). However, for the purposes of this article, I have chosen to present an overview of the three major video projection technologies that have been used in rear projection televisions.
The CRT (Cathode Ray Tube) Projection System
NOTE: Although the CRT rear projection TV option is no longer available for general consumer use, some older units are still in operation and may still be available as used items, which is why it is being covered in this section.
When rear-projection televisions first arrived on the scene, television technology was based on the Cathode Ray Tube (CRT). In a CRT-based rear-projection television, three small CRTs (one for each primary color), coupled with a light magnifying lens, projects a color image onto a mirror and is then reflected onto a screen. With the proper video processing circuitry, CRT size, and lens combination, a CRTs can produce excellent high resolution images.
Advantages Of CRT Video Projection
1. A CRT is basically a large vacuum tube in which an electron beam, emanating from a single point in the neck of the tube, scans the face of the tube very rapidly, which, in turn lights up phosphors on the tube's surface in order to create an image. A CRT produced image is not limited to a fixed pixel field, as are other rear projection types (to be discussed, in more detail, later in this article). This makes the CRT-based video projection possibly the best option where the flexibility of displaying variable resolutions is the main consideration.
2. CRT-based rear-projection technology can produce the blackest blacks of all projection types as well as the full range of color, and brightness, giving a CRT projector the ability to produce the most film-like images of projectors for home use (up to this point).
3. CRT-based rear-projection televisions, since they are based on technology that has been around for decades, is less expensive to manufacture. Thus, prices of CRT-based rear-projection televisions are several hundred dollars less than their DLP and LCD rear-projection counterparts (screen size and features being equal).
Limitations Of CRT Video Projection
1. A CRT projection television is rather large. A CRT-based projection television can take up a lot of space, in terms of cabinet depth, not only in comparison to Plasma and LCD flat panel sets, both also in comparison to DLP and LCD rear-projection units.
2. A CRT-based rear-projection television has to be converged properly for best image reproduction. Since the modern CRT-based rear-projection television houses three projection tubes (red, green, and blue), the tubes need to be aligned so that the projected image does not exhibit color halos and the colors are mixed correctly. If one projection tube fails, all three have to be replaced in order to provide the correct color and brightness balance.
3. Although all projection televisions generate some heat, this is a challenge in CRT-based projection sets, due to the fact that three separate projection tubes must be cooled and kept at an even temperature. This is accomplished by liquid gels developed for this purpose. Although very rare, the liquid gels casings can leak, thus causing the television to malfunction.
4. CRT projection televisions are more susceptible to image burn-in from continuously displayed station logos and window box lines resulting from extensive display of 4x3 programming on 16x9 CRT projection sets.