Bias Angle
A “Bias Angle” is the angle from the perpendicular from which 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 LCD display 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 Top view.
The View Angle is the 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 maximum contrast and best look. If the viewer moves their eyes further above the display by an additional 30 degrees, they will 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.
Contrast Adjustment and its Effect on Viewing Angle
Adjusting the contrast voltage, VL, effects the Bias Angle to some extent, but not the View Angle. A top view 12:00 display can be optimized for a bottom view 6:00 viewing position by adjusting the contrast voltage. A 12:00 display set for a 6:00 viewing position will not have as great a contrast as a 6:00 display set for 6:00 viewing position and vice versa.
Generally, displays are optimized for straight-on viewing. Either a 6:00 or 12:00 module may be used and the contrast voltage can be adjusted slightly to optimize the display for that viewing position. In the above example, the viewing angles of both 6:00 and 12:00 modules actually overlap the perpendicular (or straight on) viewing position.
Contrast Adjusting
When the view position is established, the contrast setting can then be determined. This is done during product development on the prototyping units. Generally, a 10K ohm potentiometer is then connected between VDD and VSS in a single supply module, or from VDD to the negative rail in a dual supply module. The wiper of the pot is connected to the VL input of the module. (See Fig.2 ) The LCD is positioned at the nominal viewing position and the pot is adjusted to obtain the desired LCD appearance. The voltage on the VL pin is now measured and a pair of resistors are chosen to produce this voltage in the production units.
Choosing The Correct Module For Your Application
Most electronic products have a preferred view position. For a device that sits on a desk, such as a calculator, the display is usually viewed from below 6:00. This is also true for most handheld instruments. For a Liquid Crystal Display mounted in a dashboard of an automobile or airplane, a 12:00 module is suggested.