University of South Carolina

PHYSICS AND THE VISUAL ARTS
Notes on LESSON 14


Things to remember from Lesson 14.


Projectors

Most projectors have the following 7 parts in common:
Some projectors can get away without using a fan (an overhead projector with the lamp at the top, for example) and some movie projectors do not have a heat filter. But most projectors will have all of these items.

In most modern projectors the lamp comes with a built-in reflector. That reflector is a cold mirror, that is, it reflects the visible light but passes the infrared radiation out the back. In the Carrousel projector that was shown in the lecture, the light from the lamp was reflected into the projector's optical path by a cold mirror. You saw that it reflected the green light, but passed some of the light from the red laser pointer because the cutoff between the visible and the infrared overlapped into the red.

You also saw the necessity for the condenser lens. The condenser lens directs light from the lamp through the slide and onto the back of the projector lens. In doing so it provides for even, edge to edge illumination of the slide and guarantees that there will be no image of the lamp filament on the screen.

LCD projectors are similar to the Carrousel except that they have a tiny LCD screen instead of a slide. It is this LCD that is imaged on the viewing screen. DLP projectors use a microelectromechanical system to control the light to the screen. You can get more information about them at the DLP home page. In recent years many movie theaters have replaced their film projectors with digital projectors utilizing the DLP devices. I think the digital movies are better than the film images. In addition, the digital techniques afford the ability to show movies in 3-D that are far better than what could previously be attained. There is also a boon to the distributors as they no longer have to ship huge reels of film to each theater resulting is a big saving. Of course, that saving comes only after the expense of replacing thousands of older projectors that are still in use.

When you think of television, you may think of the bulky sets that contain a cathode ray tube (CRT) with a fluorescent screen in the front. A moving electron beam (the cathode ray)excites the phosphor and generates the light and, hence, the image on the screen. However, with the developments in the newer thin displays and in HDTV, the CRTs are disappearing from the stores. Thin LCD screens are used in laptop computers, monitors for desktop computers, and in televisions, especially the HDTVs.Initially, the size of the LCD screens was rather limited, but by 2008 LCD televisions are available in sizes up to 52 inches diagonal. Larger, relatively thin displays employ the radiation from a plasma to excite phosphors in tiny cells to generate the pixels. Large screen TV sets using this technology are referred to as plasma screens or plasma TVs. Other large screen TVs use a built-in DLP projector to create the images.

Really big displays are referred to as jumbotrons. These include the giant advertizing displays like those found in Times Square as well as the giant screens in stadiums and arenas all over the country. These jumbo screens use LEDs for their light sources. In part, they are made possible by the relatively recent invention of blue LEDs.

Many consumer videocameras use a single chip, CCD or CMOS, for their sensor. Higher quality cameras (broadcast cameras) have three chips. The incoming light is separated into three parts; red, green, and blue. This separation is accomplished using a prism block that contains several prisms and interference layers between the filters that act as dichroic filters. That is, at some of the interfaces, one color of light is reflected and the rest is transmitted. You saw this demonstrated in the lecture.

In the United States, the television system that has been in use for many years is known as NTSC. It is also referred to as SDTV, for standard television. The recent development of high definition television, or HDTV, is growing in use as the nation adjusts to the new TV standards. Some comparisions between the two systems are given in the table below.

StandardsSDTV (analog) HDTV (digital)
Aspect ratio4 x 316 x 9
Total lines5251125
Active lines4861080
Maximum resolution720 x 4861920 x 1080
Audio2-channel stereoSurround sound

There are 18 different digital options within the US DTV/HDTV standards that have been approved. Three of these seem to be dominant. They are known as 1080p, 1080i, and 720p. (The p stands for progressive scan and the i stands for interlaced scan.) A comparison is given in the table below.

Active lines
per picture		Pixels per
line 		FormatFrame rate Scan
method
1080192016 x 924, 30, & 60Progressive
1080192016 x 924, 30, & 60Interlaced
720128016 x 924, 30, & 60Progressive


Compact discs (CDs) are recorded and played back with diode lasers. The standards for music CDs and the CDs used for computer memory storage are based on using infrared lasers with a wavelength of 780 nm. DVDs used to deliver movies are designed to hold more information on the disc as thus need a laser with a smaller wavelength. The diode lasers used for the DVDs have wavelengths in the red at 640 to 650 nm. [Your text may say green laser, but you will rember that 640 nm is in the red, not in the green.]

Recently, to accommodate HDTV, newer formats and technologies of disc storage have been developed. Both the Blu-ray and HD-DVD systems have been used by major movie studios, but in early 2008 there was a shift with more of the studios lining up behind Blu-ray. Eventually the HD-DVD format did not survive.

With the switch to HDTV and the capability of digital systems, the manufacturers are already designing 3-D plasma and DLP television sets. Current generation LCD sets are not fast enought for eye-sequential operation. Thus the consumer will soon face a choice of 2-D only, 3-D ready, or full 3-D HDTV sets. (3-D ready sets will require an add-on decoder box and/or 3-D glasses.) The viewing glasses will incorporate LCD shutters to control which image gets to which eye. The shutters will switch on an infrared signal from the HDTV. Sales estimates for 2008 are for more than 1.8 million 3-D ready or 3-D HDTVs.

This link will connect you to Cybercollege index page. From there you can find a wealth of information about television.
Basic information on CDs and CD burners. More on CDs and a more technical article on audio compact discs.
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Last Modified: 08/13/08