Black coloured region of smartphone screens

Question

We have all used backlit screen. Indeed, you must be reading my question in one of them. Everyone of us must have encountered, for multiple times, the texts or the images with regions of hexadecimal colour code #000000 or "BLACK". My question is, do these are regions where we have (almost) no light coming out of the screen? Will it mean a completely black screen (or near-black screen) would emit almost no light out of it?
I couldn't hold my curiosity back and so I opened this- -on my screen and turned off the lights of my room. On the contrary of what I was expecting it to be like, there was enough light coming out of my screen to make the neighborhood of my phone visible. 
If my theorising that there should be almost no light coming out of screen was right, then why did I get observation of such kind. Or is it that I misinterpreted my observation and compared the emitted intensity with a false 'standard' (by which I mean, is it that I saw what I was expecting but I misinterpreted that visibility to quite a large amount of light being emitted)? 
Could you please help me figure out where I went wrong or was I actually right?

noah · Answer

LCDs in general work by using some white backlight and liquid crystals in front of it. They change the polarizing direction of the light in such a way that a polarizing filter in front of it will only let a certain amount of light through. If you want the screen to be black, all colors will have their polarizers orthogonal to the polarization direction of the light coming through the crystals.
Now, the amount actually coming through will depend on the quality of the polarizer and the accuracy of the polarization direction induced by the crystal, so there will usually be some leakage.
Compare that to OLEDs, which work without a backlight and by turning on the individual pixels as needed. They will go completely black.

Black coloured region of smartphone screens

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