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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:

The official HTML5 Icon

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,...)?

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    $\begingroup$ For anyone interested, this is Brusspup's video youtube.com/watch?v=zL_HAmWQTgA $\endgroup$
    – QCD_IS_GOOD
    Dec 28, 2014 at 11:32
  • $\begingroup$ @JoshuaLin I did not know the source, thanks for the tip! $\endgroup$
    – glS
    Dec 28, 2014 at 11:34
  • $\begingroup$ Have you done a Google search for how LCD monitors work? $\endgroup$
    – Kyle Kanos
    Dec 28, 2014 at 12:12
  • $\begingroup$ @KyleKanos well yes, but I thought it would have been nice to have this cool effect discussed here! $\endgroup$
    – glS
    Dec 28, 2014 at 13:17

1 Answer 1

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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

enter image description here

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.

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    $\begingroup$ TFT is not synonymous with LCD. There are TFT LCDs (e.g. computer monitors) and LCDs that are not TFT (e.g. pocket calculators). $\endgroup$
    – ntoskrnl
    Dec 28, 2014 at 16:11
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    $\begingroup$ Does this also apply to IPS (not TN) LCD monitors? $\endgroup$
    – March Ho
    Dec 28, 2014 at 17:49
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    $\begingroup$ @MarchHo TFT is the technique of using thin film transistors to individually-address single pixels; it's not directly related to IPS/TN. Active matrix IPS and TN displays are both types of TFT displays. IPS is very similar to TN except, unlike the diagram above, the voltage is applied across two electrodes on one side of the liquid crystals (In-Plane Switching), rather than on either side of the layer, so this trick would work with an IPS display also. $\endgroup$
    – Michael D. M. Dryden
    Dec 28, 2014 at 18:46
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    $\begingroup$ Isn't this hugely energy inefficient? I mean afaik, crystals only can twist light really lightly, so most of the light would be filtered? $\endgroup$
    – HopefullyHelpful
    May 21, 2018 at 13:38

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