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I basically know how TFT' displays work. They have on both sides a polarizing foil, in 90 degrees with the crystals in the middle modifying which light particles should pass through or not. The light is emitted by a backlight in the display. So far so good.

What I try to find out or understand is, whether it would be possible somehow - by modification of the TFT / the 2 polarising foils / or by adding a new polarizing foil / or by any other means or filters that the backlight can be removed and replaced by a mirror.

Basically what I am trying to find out if its possible to build - in theory - a TFT display that is not backlight but gets the light from the front (from the direction where the viewer is).

As I see, polarized light that gets into the TFT display from the front should be able to be reflected at the back from a mirror no? Normally the polarized light is reflected by the mirror and passes back the same way throught the TFT it came from.

Or am I wrong? Any help appreciated to untie the knot in my head :)

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There are plenty of front-lit LCDs. Either way, you need to recognize that backlit displays pass light thru just once, while putting a mirror behind means double-pass, to you'll want half as much polarization rotation applied (per pass). – Carl Witthoft Feb 26 '14 at 17:01
This is how simple LCD displays work (Casio watch or Nokia 3210). – gigacyan Feb 26 '14 at 17:03

Optical systems not involving magnetic fields are symmetric. So, if the display passes light in one direction, it will pass light in the other. Putting a mirror at the back of the TFT and lighting it from the front is therefore equivalent, expect that some light will be attenuated on the way in as pointed out by @CarlWitthoft in the comments.

As a matter of fact, many LCDs work in this way. One example of this type of LCD is a wristwatch as pointed out in my answer to this question.

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The electrical impulses used to render a display on a TFT device such as you describe do so by modulating the THICKNESS or THINNESS of the liquid crystals. Depending on the thickness of the liquid crystal, different colors are displayed because different wavelengths of polarized light are given different phase rotations through the liquid crystal, decoded by the linear polarizer at the front.

There are some easy experiments you can do with the quarter wave, half wave plates that were once supplied with the Berkley Physics Waves text. One of those demonstrates how total extinction (crossed polarizers) results from sandwiching a quarter wave plate between a linear polarizer and a mirror. It all goes dark. I'm not saying the entire display will go completely dark if the backlight is replaced by a mirror; just most of it.

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