Extra Class question of the day: Optical devices

September 14, 2012 Dan KB6NU

Cathode-ray tubes (CRTs) used to the be most common type of display. They were not only used in television sets, but also computer terminals. They have an electron gun which shoots electrons onto a screen which then glows where the electron hits the screen. By sweeping this “beam” both horizontally and vertically, you can display an image on the screen.

To sweep the beam across the CRT, you deflect it by passing it though a set of plates. Varying the voltage will change the angle at which the beam is deflected. Electrostatic deflection is the type of CRT deflection that is better when high-frequency waveforms are to be displayed on the screen. (E6D13)

To accelerate the electron towards the screen, you apply a relatively high anode voltage to it. The higher the voltage, the brighter the CRT will glow. You don’t want to make that voltage too high, however. Exceeding the anode voltage specification can cause a cathode ray tube (CRT) to generate X-rays. (E6D02)

A spot on the CRT screen will glow even after the beam moves onto another spot. Cathode ray tube (CRT) persistence is the length of time the image remains on the screen after the beam is turned off. (E6D01) This characteristic is useful in many different applications.

A more modern type of display is the liquid-crystal display. A liquid-crystal display (LCD) is a display using a crystalline liquid which, in conjunction with polarizing filters, becomes opaque when voltage is applied. (E6D05) The principle advantage of liquid-crystal display (LCD) devices over other types of display devices is that they consume less power. (E6D15)

Unlike the CRT or LCD display, which transform electrical signals into an image, a charge-coupled device is used to transform an image into electrical signals. A charge-coupled device (CCD) samples an analog signal and passes it in stages from the input to the output. (E6D03) One of the things a charge-coupled device (CCD) does in a modern video camera is that it stores photogenerated charges as signals corresponding to pixels. (E6D04)One thing that is NOT true of a charge-coupled device (CCD) is that it is commonly used as an analog-to-digital converter. (E6D14)

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