An inherent weakness of Twisted Neumatic (TN) LCD displays which dominate the LCD HDTV market is changes to brightness, contrast and color saturation when viewed off axis. In comparison, plasma displays deliver very close to full 180 degree viewing angles with very little change in the picture quality.

The picture degradation when viewed off axis is a result of a reversal of contrast with bright LCD sub-pixels (three colors to each pixel) decreasing brightness with viewing angle and initially dimmer sub-pixels brightening. The red, green, and blue sub-pixels vary independently leading to color shifts.

Most consumers take the viewing angle published by LCD HDTV manufacturers’ specifications as he maximum angle at which the LCD display can be viewed and still see an acceptable picture quality. In fact, display manufacturers use numerous measurement techniques for measuring viewing angle making accurate comparison impossible.

The viewing angle is measured from one direction to the opposite, giving a maximum of 180 degrees for a flat, one-sided screen. If manufacturers specify both horizontal and vertical viewing angles, it is simply reflecting a difference in behaviour between the horizontal and vertical axis.

Most commonly, the viewing angle quoted is the angle at which the contrast ratio is 5 to 10 percent of that when viewed head-on (at 90 degrees to plane of LCD display). One could call this the minimal acceptable viewing condition.

The majority of consumers would most likely find pictures with 10 to 15 percent of the head-on contrast unacceptable. In reality, image differences are noticeable at shallow angles of 15 to 30 degrees from on axis and are reasonably profound by 45 degrees.

PVA (Patterned Vertical Alignment) LCD technology gives a better off axis viewing angle performance. However, the bright star for improvement is IPS (In-Plane-Switching) LCD display technology which has become far more affordable in comparison to TN LCD and has genuinely wider viewing angles. With each generation of LCD HDTV manufacturers have continued to improve off axis viewing performance through techniques such as the sub-pixel cell depth reduction and aperture ratio widening.

A relevant, recent report by DisplayMate Technologies and Insight Media revealed that the level of off-axis color inaccuracy worsened with wide color gamut displays.

Resources

Why viewing angle is a key element in choosing an LCD screen refers to LCD monitors but is still very relevant (pdf)

Off-axis Color Performance “Surprisingly Poor” in Top LCD Displays