Understanding Visual Displays: CRTs, LCDs, and Beyond - A Comprehensive Guide

[TCM]

Hi.

I'm doing an A-level coursework (Uk sort of 17/18-year old level thing) on visual displays, and I'm confused.

Can anyone clarify the physics of the phosphor on the screen of a CRT, to which energy level the electron is absorbed and why, why the electron decays, why the electron is emitted again and with how much energy, how to relate this to the energy it has before it's absorbed, etc.
And how are the deflecting coils arranged? What is their orientation and do they coil around the tube or around in a circle next to the tube or something else? Is the cathode heated or is the PD between cathode and anode enough to get the electrons to move?

Also, can you explain the exact nature of the liquid crystals in LCDs, because this is very confusing to me? I seem to have arrived at the conclusion that there are such a thing as Smectic C Chiral Nematic liquid crystals between the glass plates, but Smectic and Nematic seem to be contradictory. What do these terms mean, which ones am I dealing with, what is the "director" which lines the molecules, how does it work, do the molecules start facing with their longest bit parallel to the glass and then when they line up, have their longest bit perpendicular to the glass, and is this what Smectic C means (I know that they twist when influenced by the "director", but only in the plane that is parrallel to the glass, and I'm talking about whether they move in a plane which is perpendicular to the glass - is this "director" an electric field or an electric current or something else?)?

Sorry if this is confusing, but it reflects my state of mind right now

Thanks very much in advance for any help you may give me.

If this should go in any other forum, I'm sorry, and I don't mind if it's moved.

EDIT: oh, and I'm also going to go on to talk about projection TV, holograms, and then briefly go over any other interesting futuristic display ideas I can find. So if anybody can help with that, it would be much appreciated :D.

 

[Andrew Mason]

Can anyone clarify the physics of the phosphor on the screen of a CRT, to which energy level the electron is absorbed and why, why the electron decays, why the electron is emitted again and with how much energy, how to relate this to the energy it has before it's absorbed, etc.

I am not sure I understand what you are asking. An electron is accelerated by the potential difference between the cathode and anode. It is focused and directed by magnets and the electrons crash into a phosphor coated screen, exciting the phosphor atoms and causing them to give off radiation, mainly light.

And how are the deflecting coils arranged? What is their orientation and do they coil around the tube or around in a circle next to the tube or something else? Is the cathode heated or is the PD between cathode and anode enough to get the electrons to move?

The cathode is heated. There are many good sources that will explain how a cathode ray tube works. see, for example:
http://en.wikipedia.org/wiki/Cathode_ray_tube

AM

 

[thepatientmental]

Hello!

I understand that you are working on a coursework about visual displays and you have some questions about the physics behind CRTs and LCDs. I am happy to clarify some of your confusion and provide some information on other display technologies as well.

Firstly, let's start with CRTs. The screen of a CRT (cathode ray tube) is coated with a layer of phosphor, which is a material that emits light when struck by electrons. The electron gun at the back of the CRT tube emits a beam of electrons that are accelerated towards the screen. The energy level of the electrons depends on the voltage applied to the electron gun. When the electrons hit the phosphor, they transfer their energy to the phosphor atoms, causing them to become excited and emit light. The specific energy level at which this happens depends on the type of phosphor used.

Now, to answer your question about the decay of the electrons. After the electrons transfer their energy to the phosphor atoms, they lose some of their energy and become decelerated. This is because of the electric field created by the positively charged anode at the front of the CRT tube. The electrons then travel back to the electron gun to be accelerated again, creating a continuous stream of electrons hitting the phosphor and producing the image we see on the screen.

The deflecting coils in a CRT are arranged in a specific pattern, usually in a spiral shape around the tube. These coils produce a magnetic field that can deflect the electron beam, allowing it to scan across the screen and create the image. The orientation of the coils depends on the specific design of the CRT.

Moving on to LCDs, the liquid crystals used in these displays are a type of material that can change its optical properties when an electric field is applied. The liquid crystals used in LCDs are typically a type called twisted nematic (TN) liquid crystals. This means that the molecules of the liquid crystals are twisted in a specific direction, known as the "director". When an electric field is applied, the molecules align with the direction of the electric field, causing the light passing through the liquid crystals to be either blocked or allowed to pass through, creating the image on the screen.

To answer your question about the different types of liquid crystals, smectic and nematic are two different phases of liquid crystals. Smectic crystals are layered and have a more ordered structure compared to nematic crystals, which are more randomly oriented. In