I understand the principle of operation of LCD panels, but I am wondering why the specific output angle of polarization was chosen over, say, horizontal or vertical.
Positioning the polarizers and alignment layer in a TN display at 45° and 135° can help improve viewing angle performance. By doing this, the angles of best contrast are placed in the display's horizontal plane.
This has to do with the asymmetric shape the liquid crystals assume in a TN display.
From: 3M Optics 101.
While all the videos are informative (if a bit repetitious) the last one, "Optics 101 - Original Flash Animations" cuts to the chase. I'd recommend watching from the beginning, but the answer being sought occurs around the 13 minute mark, "Section 15: Twisted Nematic Displays: TN Orientation"
My guess was that it may be due to sunglasses being polarized horizontal (or vertical), and would then block the LCD light (unless polarized at a different angle).
I did a guick google and found this post:
Are all sunglasses polaized the same way?
Yes, unless they were manufactured incorrectly. The glare they are trying to eliminate is usually reflected from horizontal surfaces (road surface or water) at a shallow angle so the reflected light is horizontally polarized. Polarized sunglasses are vertically polarized to reduce the glare.
Which sounds reasonable enough to me. Any takers?
When light reflects off of water it is horizontally polarized ( w ). All "anti-glare" polarized sunglasses are vertically polarized to block that light. (If there were polarized at any other angle at least some of that "glare" would leak through).
Most LCD panels are designed so that the light exiting the panel is also vertically polarized, so they look as bright as possible even to someone wearing polarized sunglasses. Alas, such displays are unusable -- they look completely black -- when rotated 90 degrees either way and then viewed through polarized sunglasses.
Some some LCD panels are designed so that light is polarized at 45 degrees ( a ). I speculate that is so the same display is usable in both "portrait" and "landscape" mode, even though it makes them half as bright.
Recently I've dug out my polarized sunglasses for driving and noticed a number of interesting phenomenon. First, I assume that the sunglasses are vertically polarized because most glare experienced when driving is from horizontal objects - car roofs, wet roads, lakes, etc - making the reflected light horizontally polarized, so vertical polarization removes this glare.
Based on this assumption, my digital watch must also be vertically polarized so that it is its brightest when my arm is horizontal and I'm wearing sunglasses; when I rotate my arm 90 degrees so that it is vertical the watch display goes completely black, as expected. However, the LCD displays in my car (Citroen C4) are polarized at 45 degrees; those in the central console (speedometer and stereo) are polarized 45 degrees clockwise from vertical and the display above the steering wheel (the rev counter) is polarized 45 degrees anti-clockwise from vertical. This means the displays on the left (central console, right-hand drive car) go black when I tilt my head to the left (ie, towards the displays) and conversely the display above the steering wheel goes black when I tilt my head to the right when wearing the sunglasses. Nevertheless, both displays are equally readable when my eyes/glasses are horizontal, although they are not at their brightest.
So why are they rotated 45 degrees? I suspect it is because the displays can be read equally when wearing vertically polarized glasses whether rotated clockwise or anti-clockwise, but I do not know why they are not vertically polarized like my digital watch. As suggested above by David Carey, it may be because the displays can be mounted horizontally or vertically without the risk of appearing totally black when wearing polarized sunglasses, which is vitally important when the display is the speedometer of a car!
Slight correction to the posts above. When light encounters non-absorbing surfaces, a fraction of the incident light is transmitted through the surface and the remaining fraction is reflected. The fraction that is transmitted/reflected depends on the polarisation of the incident light and properties of the surface. For dielectric surfaces (water, glass etc) light that polarised in the plane of the surface (s-wave) is (generally) preferentially reflected, whereas light that polarised perpendicular to this (p-wave) is preferentially transmitted. Light from the sun is mostly unpolarised containing equal amounts of s and p wave light. Sunlight reflected from a surface becomes partially polarised in the plane of the reflecting surface. If this surface is horizontal the light will be partially polarised horizontally. The polaroid in sunglasses (should) be orientated to admit only light with a vertically polarised component - that is, the polaroid is orientated to filter out the polarised reflection from horizontal dielectric surfaces. This property eliminates the glare from water, for example, so that fishermen can see the fish below, or so that a driver is not blinded by reflections from a wet road.
If you have some polarised sunglassed, try tilting your head while looking at a rainbow.