Andrew Lorimer, September 2017
Twisted nematic (TN) displays are known for their poor viewing angles and contrast ratios, and IPS monitors are so cheap these days that there is really no reason to buy a TN monitor. But I noticed something interesting when rotating a TN monitor, and the physics behind it is quite neat.
Most ergonomic guidelines from the last two decades suggest that the top of the monitor should be at or slightly below eye level [1]. This has the following benefits:
This introduces a problem, though, because TN monitors have a roughly logarithmic dependence of brightness on viewing angle which is not favourable towards large screens with an inherent vertical gradient in viewing angle [5]. It is impossible for a stationary single-point observer to see any two points on a flat surface at the same angle. This results in some well-known visual effects like parallax on telescopic sights and barrel distortion in camera lenses.
For example, if a 30 cm high display is placed at 50 cm away from an observer (about an arm's length) and with the top of the display at eye level as recommended, the bottom of the display will be viewed at an elevation of -31 degrees whilst the top will be viewed at 0 degrees (from basic trigonometry). Roughly interpolating from Figure 6 in the previously referenced paper, this would result in the bottom row of pixels being about 10% as bright as the top row. Clearly this is not feasible.
Therefore, the pixels in a TN monitor must be somehow tilted towards the viewer, with the bottom tilted more than the top, to correct for this viewing angle gradient and make each pixel's emissions normal to the eyeball. This is achieved through an optical compensation film which encompasses various methods generally based on a polarisation gradient across the display. Sharp engineers termed this a gradual refraction polariser [6]. A model and characterisation of optical parameters for optical compensation films was shown in 2003 [7].
The polarisation gradient imparted by the film effectively tunes the viewing angle for each row of pixels such that the display appears uniform when viewed from the recommended position. When the monitor is inverted, the whole display is perceived as about 10% of its full brightness. And when rotated into portrait orientation these monitors are almost impossible to use because they have both a horizontal and vertical perceived brightness gradient.