Definition of Bias angle & Viewing angle:

Since LCDs have limited viewing angles, they tend to lose contrast and become hard to read at some viewing angles. Generally the size of the viewing angle is determined by several factors primarily the type of Liquid Crystal fluid and the duty cycle. Because the viewing angle tends to be smaller than most people would like, a bias is designed into the module at the time of manufacturing.
This means the nominal viewing angle is offset from the perpendicular by some amount. Therefore; several LCD versions are offered with this bias set to different angles or positions to accommodate as many applications as possible. The termBias angle is often used erroneously with the term Viewing angle..
The Bias angle is the angle fromthe perpendicular fromwhich the display is best viewed.
This angle is determined when the display is designed and can be set at any angle or orientation.
The orientation of the Bias angle of a LCDdisplay is often stated with reference to a clock face.
If the offset is above the display, it is referred to as a 12:00 or Topview.

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The View angle isthe angle formed on either side of the Bias angle, where the contrast of the display is still considered acceptable. Generally, this contrast is specified as 1.4:1. A typical STN character display running at a duty cycle of 1/16 has a viewing angle of ±20 degrees and a bias angle of 25 degrees.
For example, assume the display is a 12:00 (top view) type. When the display is viewed from 25 degrees above the vertical, it will be at its maximumcontrast and best look. If the viewer moves their eyes further above the display by an additional 30 degrees, theywill see a contrast reduction, but the display will still be readable.

Moving the view position any further above the display will reduce the contrast to an unacceptable degree.

Source PDF =>http://www.newhavendisplay.com/app_notes/LCD_Viewing_Angles.pdf