Light, Particles, Waves, Polarization, Liquid Crystal, Twisted Nematic Effect, LCD, CRT, Pixel and Thin-Film Transistors

Posted by PITHOCRATES - June 13th, 2012

Technology 101

Light is Electromagnetic Radiation consisting of a Vertical Electric Wave and a Horizontal Magnetic Wave

The sun lights our day.  And artificial sources of light do the same for our nights.  We see light.  Light rays behave both like a wave.  And like particles.  Complicated stuff.  And fascinating.  The kind of stuff that physicists pass their day thinking about.  But we’re not going to delve into that complicated stuff.  Just the waves.

Light is electromagnetic radiation.   A light wave has an electric portion.  And a magnetic portion.  Or an electric field.  And a magnetic field.  In other words a light ray is made up of two waves.  Think of the up-and-down sinusoidal wave you see on an oscilloscope.  When you look at the scope this waveform moves from left to right.  It rises up on an angle.  Then bends and curves down at the peak amplitude of the wave until it is moving down at an angle.  Then it bends and curves up at the bottom of the wave and rises up again on an angle.  Again and again.  Now imagine this waveform existing in the open air just as it appears on the scope.  Turn the wave 90 degrees so it is moving towards you.  Going up and down.  Now imagine a second wave doing the exact same thing.  Coming towards you going up and down.  Now rotate that second wave 90 degrees so it’s going side-to-side as it approaches you.  Now you have one wave coming at you going up-and-down.  And a second wave coming at you going side-to-side.  This is a light wave.  The up-and-down wave is the electric field.  And the side-to-side wave is the magnetic field.

Because of these properties we can do things with light.  For example, if you’re driving into the sun there will be a blinding glare coming off of the road.  It’s annoying.  And rather dangerous.  This glare is a reflected light wave.  And it so happens that these reflected waves are also magnetic waves.  The side-to-side waves.  Which brings us to something interesting we can do about this glare.

When you Apply an Electric Field to a Liquid Crystal the Molecules Orientate themselves as they would be in a Crystal

A polarized lens is a light filter.  It will only pass light that is oriented in the same polarization.  Think of a picket fence.  Built from vertical slats of wood (or pickets) with some open space in between.  Imagine bouncing a ball like a sinusoidal wave directed at the open spaces between the pickets.  The ball will pass through.  Now rotate a part of that fence 90 degrees.  So the pickets run horizontally.  And bounce the ball towards the fence.  As the ball descends on an angle towards one of the vertical spaces it may clear the top edge of one picket but will bounce off of the top edge of the next picket down.  When the spaces of the pickets are oriented 90 degrees out of phase with the plane of the bouncing ball it block the ball from passing through.

This is how a polarized lens works.  It only passes light waves that are in phase with the lens.  And blocks the waves that are out of phase with the lens.  Which is a very handy property to have when you’re driving into the sun.  Because you can wear polarized sunglasses that pass only the up-and-down waves.  And block the side-to-side waves.  If you have two pairs of polarized sunglasses you can experiment like a physicist.  Hold up both pairs in the same orientation.  Like wearing two pairs of sunglasses.  You’ll be able to see through them.  Glare-free.  Now if you slowly rotate one lens 90 degrees you’ll see the lenses turning opaque.  Blocking more and more light until they block all light.  Both the up-and-down waves.  And the side-to-side waves.

This experiment in physics was interesting.  But it’s what we can do with this knowledge that is fascinating.  When we combine it with an interesting material.  That has the properties of a liquid.  And a crystal.  We call this material a liquid crystal (LC).    If we carefully select the type of liquid crystal material and carefully apply a voltage we can make this LC do what we did by rotating one polarized lens in front of another.  And the big breakthrough that made liquid crystal displays (LCD) possible was the twisted nematic effect (TN-effect).

In Color Displays a Cluster of Three LCDs (Red, Green and Blue) form a Pixel

An LCD is made in layers.  The first layer you look through is a polarizing filter with a vertical axis.  This lets only the up-and-down waves through.  The second layer is a glass with a conducting oxide substrate forming electrodes.  These form one side of the electrodes that set up an electric field.  This is a thin film that is both electrically conductive and transparent.  In a clock or a calculator we could apply this in the layout of sequential 7-segment displays.  Where to display a number we apply a voltage to the appropriate segments.  The third layer is the twisted nematic liquid crystal.  The fourth layer is a glass with another transparent conducting oxide substrate film. This is the other side of the electrodes that set up the electric field.  But this is a common electrode without any patterns in it.  The fifth layer is another polarizing lens.  Only this one has a horizontal axis to pass only the side-to-side waves.  The sixth layer is either a reflective surface to bounce back the light that reaches it.  Or it’s a light source to send light from the back out through the front of the LCD display.

The key for making the LCD work is the TN-effect.  In the un-energized state the crystals form a helical structure.  Like the twisting pairs of DNA.  This helix twists 90 degrees.  When light enters it this crystal structure rotates the light waves 90 degrees.  Allowing this light wave to pass through the second polarized lens.  And reflect back out.  Imagine a calculator with an LCD display switched off (or without any batteries).  All you will see is a blank display.  When the electrodes are energized, though, the crystals untwist and align in phase with the light entering it.  So the light passes through the crystal without rotating.  And is blocked by the second polarized lens.  By energizing the appropriate segments of the 7-segment display the light waves the LC blocks form the numbers we see.  By varying the voltage across the electrodes you get the same effect we saw when we rotated one pair of polarized sunglasses behind another pair.  We block varying amounts of light through the crystal.  Which modulates the intensity of the passed-through light.

LCDs work with small voltages.  Better yet, they need no current other than the backlight and the electronics that drive them.  Making them ideal for battery-powered devices.  Like lap-top computers.  Tablets.  Smartphones.  And even for things that plug into electrical outlets.  Like televisions.  Which produce very little heat unlike the cathode ray tubes (CRT) they replaced.  In color displays they use the same principle.  Only a cluster of three LCDs (red, blue and green) form a pixel.  We place a color filter in front of the LCD to pass only that color’s wavelength of light.  And each pixel has a thin-film transistor (TFT) added to that transparent conducting oxide substrate film (in an active display matrix).  A TFT varies the amount of light passed through an LCD.  Combinations of intensity of each pixel create all the different colors.  And the different colors of the pixels produce the images we see.

LCD technology has come a long way thanks to a lot of profit-seeking people.  For imagine what would happen if the CRT manufacturers had successfully lobbied government to protect their industry by hurting the LCD industry.  It’s happened throughout time.  Rent-seekers who tried to buck change and innovation.  So they could still profit on last year’s technology.  By blocking new technology from reaching the people.  Forcing them instead to pay higher prices for their inferior products.  Thankfully the profit-seekers dominate the LCD market.  Which is why we have laptops, tablets and smartphones.  And will soon no doubt have even better devices.  Because these profit-seekers are always working on something newer.  And better.  While the rent-seekers hang onto the past.  Frozen in time.  Using the power of government to protect jobs that build products the people would rather not buy.  Causing economic stagnation.  And a lower standard of living.

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