Can you tell me about CRT TVs?

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Offline thedoc

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Can you tell me about CRT TVs?
« on: 08/01/2015 19:30:01 »
Brooke Strahley  asked the Naked Scientists:
I am Brooke, a Pre-AP Chemistry student at Brewster High School. In my Chemistry class we are writing a research paper about different types of TVs. I was assigned CRT TVs and I am aware that you know quite a bit about them. I was wondering if you could help me understand the topic more by answering these two questions:

I understand that the use of the phosphor screen is imperative to displaying the image on the monitor but what I don't understand is why phosphorous does that. Why wouldn't another element work instead of phosphorous.

The electron guns shootout electron beams to form an image on the screen. How though do the electron guns know where to shoot? Is that part of the impulse that is transmitted?

Thank you for your time,

What do you think?
« Last Edit: 08/01/2015 19:30:01 by _system »


Offline evan_au

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Re: Can you tell me about CRT TVs?
« Reply #1 on: 09/01/2015 08:42:47 »
How do the electron guns know where to shoot?
The path of the electron beam is steered by some magnetic coils in a "raster" pattern  - across the screen and down the screen (just like someone reading a book). These are labelled as the deflection coils on a CRT screen .

The timing of this raster scan is synchronized between the transmitter and receiver - when the two sometimes got out of synchronization, the picture displayed a very annoying "rolling" motion: up, down or sideways.
Why wouldn't another element work instead of phosphorous?
Historically, Phosphorus was renowned for its ability to glow in chemical reactions.

However, there are many chemicals that will produce light when struck by electrons in a CRT screen. Many of them don't contain phosphorus at all.

When an electron strikes the "phosphor", it kicks an electron to a higher energy level. In most substances, it will fall back in nanoseconds, triggering a vibration (heat) or emitting a photon.

In a CRT screen, you want the photons to be emitted progressively over a  period of about 10-15ms, ready for the next raster scan. You want a substance where a higher energy level is "metastable", ie it will not immediately emit its photon. For a color TV, you want a selection of substances which can emit red, green or blue light (which add together to make a wide variety of colors, including white).

So I think the term "Phosphor" is purely historical legacy.