everyday-lifepolarizationvisible-light
Have a look at this youtube video (and some extra footage here). Here I will post a gif showing in short what it is about:
It shows that if you remove the polarizing filter from a pc's monitor and you place it on a pair of glasses, you obtain a monitor which looks white (not showing the actual output of the pc) unless you look at it throught the polarizing glasses.
Why does this happen?
Is this technology dependent? (will it happen for LCD, OLED, plasma, CRT,…)?
First you consult a CIE 1964 Supplementary Standard Colorimetric Observer chart, and look up the CIE color matching function values for the wavelength you want:
For your desired wavelength:
┌─────┬───────────────────────────────┬─────────────────────────────┐
│ λ │ CIE color matching functions │ Chromacity coordinates │
│ nm │ X │ Y │ Z │ x │ y │ z │
├─────┼──────────┼──────────┼─────────┼─────────┼─────────┼─────────┤
│ 455 │ 0.342957 │ 0.106256 │ 1.90070 │ 0.14594 │ 0.04522 │ 0.80884 │
└─────┴──────────┴──────────┴─────────┴─────────┴─────────┴─────────┘
Note: The chromacity coordinates are simply calculated from the CIE color matching functions:
x = X / (X+Y+Z)
y = Y / (X+Y+Z)
z = Z / (Z+Y+Z)
Given that:
(X+Y+Z) = 0.342257+0.106256+1.90070 = 2.349913
We can calculate:
x = 0.342257 / 2.349913 = 0.145945
y = 0.106256 / 2.349913 = 0.045217
z = 1.900700 / 2.349913 = 0.808838
You have a color specified using two different color spaces:
We can also add a third color space: xyY
x = x = 0.145945
y = y = 0.045217
Y = y = 0.106256
We now have the color specified in 3 different color spaces:
So you've converted a wavelength of pure monochromatic emitted light into a XYZ color. Now we want to convert that to RGB.
XYZ, xyz, and xyY are absolute color spaces that describe colors using absolute physics.
Meanwhile, every practical color spaces that people use:
depends on some whitepoint. The colors are then described as being relative to that whitepoint.
For example,
But there is no such color as white. How do you define white? The color of sunlight?
Some people use the white of their (horribly orange) incandescent bulbs to mean white. Some people use the color of their florescent lights. There is no absolute physical definition of white - white is in our brains.
We have to pick a white. Really it's you who has to pick a white. And there are plenty of whites to choose from:
(FL8)I will pick a white for you. The same white that sRGB uses:
D65 (which has a color close to 6500K, but not quite because of the Earth's atmosphere), has a color of:
With that, you can convert your XYZ into Lab (or Luv) - a color-space equally capable of expressing all theoretical colors. And now we have a 4th color space representation of our 445 nm monochromatic emission of light:
Lab (and Luv) are color spaces that are relative to some white-point. Even though you were forced to pick an arbitrary white-point, you can still represent every possible color.
RGB is not like that. With RGB:
If you specify an RGB color of (255, 0, 0), you are saying you want "just red". But there is no definition of red. There is no such thing as "red", "green", or "blue". The rainbow is continuous, and doesn't come with an arrow saying:
This is red
And again this means we have to pick three pick three primary colors. You have to pick your three primary colors to say what "red", "green", and "blue" are. And again you have many different definitions of Red,Green,Blue to choose from:
I'll pick for you. I'll pick these three colors:
Those were also the primaries chosen for by an international committee in 1996.
They created a standard that said everyone should use:
And they called that standard sRGB - and you can see these four points plotted out on a chromacity diagram:
sRGB Chromacity Diagram (D65 & red,green,blue primaries)
Now that we have chosen our
we can now convert you XYZ color into RGB, using the sRGB choices for "red", "green", "blue", and "white":
/*
The matix values in the next step depend on location of RGB in the XYZ color space.
These constants are for
Observer: 2°
Illuminant: D65
RGB Working Space: sRGB
*/
r = X * 3.2404542 + Y * -1.5371385 + Z * -0.4985314;
g = X * -0.9692660 + Y * 1.8760108 + Z * 0.0415560;
b = X * 0.0556434 + Y * -0.2040259 + Z * 1.0572252;
Giving you your RGB of:
Unfortunately:
So we have to round:
I wanted to point out that nearly everyone uses sRGB as the standard. It's a general standard for all digital cameras, for JPEGs on the Internet, and computer monitors. The goal is that every one of these devices agree on:
And those places outside the triangle on the sRGB chromacity diagram are still all valid colors; your monitor just can't display them.
And the very outside edge of the curve (called the locus) is the location of different pure frequencies of monochromatic light. That is where your pure 445nm monochromatic light source would be:
The LCD panel consists of elements shown in the figure below. The unpolarized light from backlight panel travels through polarizer, after which the light is linearly polarized. TFT panel controls the voltage on the liquid crystal, voltage applied will cause the liquid crystals to "twist" and thus rotate the polarization of the light. Light then passes the color filter and another polarizer. This polarizer is orthogonal to the first one in normal case, and thus light will pass it. If LC does not rotate the light, it will be blocked by the polarizer.
If the polarizers are parallel, the behavior is reversed, light that has not been rotated by LC will pass the stack and pixel is seen lit, and vice versa.
Best Answer
The video you show is of a liquid crystal display (LCD) monitor, also known as 'TFT'. The 'trick' depends on the specific design characteristics of these monitors (described below) and will not generally work with other types of displays such as LED and PLASMA displays.
All liquid crystal displays (LCD) operate on the principle of being able to 'twist' polarized light as it passes through a 'nematic' liquid crystal. The orientation of each liquid crystal in a display is governed by an electric field applied to a transparent electrode, through an array of thin-film transistors (TFT). The liquid crystal is normally 'sandwiched' between two polarizing filters at 90 degrees to each other. Polarized light enters the back of the liquid crystal from the back-lit LED. When the nematic crystal is not energised, it 'twists' the polarized light by 90 degrees so that it passes through the second polarizing filter. Wnen an electric field is applied to the liquid crystal, the light does not get twisted so gets blocked by the second polarizing filter.
See this video for an animation of the nematic liquid crystal: https://www.youtube.com/watch?v=Bf3547WB5qs
For more detail on the internal workings, including a'tear down' of an LCD-TFT monitor see this video: http://www.engineerguy.com/videos/video-lcd.htm
By taking out the second polarizing filter and placing them on a pair of glasses, the display appears 'invisible' (white) to the naked eye because ALL the from the LED backlight that passes through the first polarizing filter gets through the TFT section to the naked eye, regardless of it's orientation (polarization) so the naked eye sees it as 'white'. It's not until the second polarizing filter is applied to 'filter' the light from specific pixels which have 'twisted' their light (with respect to the other pixels) that we can distinguish between the pixels.
Note that the 'trick' is not really very secure if intended to prevent 'evesdropping', sinces anyone with polarizing glasses (including polaroid sunglasses) will be able to read the display.