MadSci Network: Physics

Re: How to project a tv image without using expensive electronic equipment

Date: Wed Feb 23 13:38:10 2000
Posted By: Todd Jamison, Staff, Image Science, Observera, Inc.
Area of science: Physics
ID: 951240387.Ph


Unfortunately, not even lenses and mirrors can make this magic work! :-)  

I don't believe that what you are describing can be done.  The reason is not 
so much an issue of the physics of projection as it is the amount of light 
energy generated by a TV.  TV's work by exciting a phosphor on the back side 
of the screen with an electron beam.  Electron beams were known as "cathode 
rays" - hence the name Cathode Ray Tube or CRT for a TV or monitor.  The 
excited phosphor glows for a short time when excited by the electrons, 
giving off a small amount of emitted light as photons.  These photons are 
focused by the lense in your eye and impinge on your retina, which sends a 
signal to your brain - and you get to watch "Friends".  

Unfortunately, as light gets farther from its source, it gets weaker, 
because the light is emitted in all directions and so at larger distances 
from the source, there are fewer photons to impact per unit area.  In order 
to project light onto a surface at some distance and have it viewable, the 
light needs to be very strong.  That is why overhead projectors and Liquid 
Crystal projectors have such powerful light sources behind them.  They also 
collimate the light (make all of the photons travel in the same direction) 
very near the bulb, so that most of the energy is used in the projection and 
not wasted.  The light coming from a conventional TV screen is much too weak 
and omnidirectional (i.e., is not collimated - goes off in all directions) 
to be useful for projection at any reasonable distance (e.g., greater than a 
few millimeters), regardless of lenses and mirrors.  

The least expensive method I know of is using one of the LCD type projectors 
that you can lay on top of a standard overhead projector.  These aren't 
cheap, but they may get you what you need.  Good luck with your project.  

[note added by MadSci Admin: There actually exist projection systems
which use 3 CRTs, each one made so that it is capable of generating
much more light than a standard TV screen.  Each CRT has only one
color of phosphor on it, which is one reason why it can generate
more light than a standard TV (no space wasted on the inside surface).]

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