Polarize Me

Amazing Secret Monitor! (How To)
Christopher Webb explains what's going on.

A standard laptop display is nothing more than a common LCD panel. It consists of a grid of subpixels with a back-light behind them and an overlayed colour filter with red, green and blue “windows” for the light to shine through. Three subpixels therefore make up one pixel.

LCD Computer Display Construction

Those subpixels are made using Liquid Crystals. These crystals sit in a state where they aren’t quite solid but aren’t quite liquid either. Whereas solids stay together by themselves and liquids flow easily, these crystals can be in a number of possible substates or “phases”. The important one here is the nematic phase.
In the nematic phase the crystals act like a liquid. Their molecules can move around but they all more-or-less point in the same direction, so they’re more organised than a normal liquid like water.
The interesting thing about the nematic phase is that the crystals can react when a voltage is applied. LCD panels use this trick to turn individual pixels on and off, but that on its own is not enough - there is another layer in front of the colour filter and this is the polarizing filter.
The polarizing filter is needed to filter out the light passing through the liquid crystal pixel. This means that only the light vibrating on a particular plane will pass through the filter and reach your eyes. If the filter were to be removed then the screen would be white and it would be impossible to distinguish what was meant to be displayed.
With this new-found knowledge you could dismantle the LCD panel, remove the polarizing filter and reassemble it. Now no one will be able to see what’s on your screen. So how do you see? Simple; you cut the polarizing filter to shape and fit it into the frames of a pair of glasses. Assuming you’ve done it correctly you’ll be able to see the image on your screen perfectly, while everyone else will see nothing but white.
BUT, there is still a risk: Someone else may have a pair of glasses that allow them to see the screen as well. Some sunglasses have a polarizing filter and could be used to view your display.