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I'd like to make a "magic mirror", something very similar to this https://blogs.windows.com/msedgedev/2016/05/31/magic-mirror-hosted-web-app/, but much smaller.

Basically, I have a closed box (no light can enter), with the electronic board inside, and a semi reflective mirror on the top (screen under the semi reflective mirror).

I am currently using an OLED display like this http://www.buydisplay.com/default/oled-display/2-4-inch because I thought it was the only solution to have black background, so that we only see the white characters which seem to float behind the mirror.

But, actually, I found out that there exist cheap lcd displays with a black background. I'm not sure to understand because I thought that lcd were working with a backlight, so that I can't understand how the background can be black (which is not light !).

If it exists, would you explain me how it works, and if I could obtain the same effect that white characters seem to float (no rectangular backlight under the mirror) ? That would be great because OLED screens are so expensive.

Thank you

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  • \$\begingroup\$ Well, I can display white text on a black background on my computer monitor. Any colour LCD display can have a black image displayed on it. Is that what you're asking? \$\endgroup\$ – uint128_t Jun 3 '16 at 4:51
  • \$\begingroup\$ By "black background" they probably mean set the lcd to "black", so it'd be dark but not full black (I'm sure you've seen that feint backlight bleedthrough on LCDs, however, looking throung the mirror, you're unlikely to be able to te;l the difference between true black and LCD black anyway, I wpuldn't worry about it \$\endgroup\$ – Sam Jun 3 '16 at 5:08
  • \$\begingroup\$ Monochrome backlit LCDs are are often available in white on black versions at least in OEM quantities though popular units are easy to get. Having a monochrome display will give your project better contrast than a full colour display showing black. Here is one oppod.com/products/69.html \$\endgroup\$ – KalleMP Jun 3 '16 at 7:36
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These LCDs have a blacklight, and are black when undriven, turning the LCD elements (pixels/segments) on makes them transparent so that the backlight shines through.

This is the way a normal LCD monitor, or cellphone, display works.

Black is not fully black, if run it in a dark room you'll see that some light leaks through.

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You can turn most simple LCDs into white on black by flipping the polarizer sheet. This link shows the construction of an LCD.

Here's the relevant picture:

enter image description here

If you can disassemble the LCD and get to the polarizing film (marked F in the picture) you can flip it either horizontally or verticallly then reassemble the LCD. With the polarizeer flipped, black and white will be inverted.

I found this out accidentally many years ago when I disassembled and reassembled an LCD calculator. I had to do it all over again to make it display black numbers on a white back ground like it originally did.

Not all cheap LCDs can be reassembled this way for the simple reason that you have to destroy the unit to get it apart. But there are displays that can be taken apart and put back together, and in those cases you can invert the colors.

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    \$\begingroup\$ This flip works when the polarisers are at 45 degrees so you can flip the one polariser over and the polarisers are then crossed. In a black on white display they are normally both at the same angle and in a light on black display they start off crossed. Some displays use variants or circular polarisation and then the orientation does not matter, but flipping one polariser front to back can reverse the bark and light sometimes. \$\endgroup\$ – KalleMP Jun 3 '16 at 7:40

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