[Electrically Controlled Birefringence (ECB) mode]
The ECB mode uses the applied voltage to
change the tilt of the liquid crystal molecules, as a result, the
birefringence is changed as a function of the tilt angle
θ, which is
also called as the polar angle, the one between the substrate normal
and the molecule's long axis, shown in figure below, which also
shows the tilt change under an electric field, when a LC with Δε>0
is present.
Figure 1. A cell configuration of
ECB cell at field-off and field-on states.
and the birefringence Δn is defined in
the equation below:
In the TN mode, the transmission maxima will be reached when the
condition is met:
If we use applied voltage to
change the tilt angle to change Δn, we are able to shift the
wavelength of the maxima, i.e. we can shift the transmission color.
In general, ECB can be used to generate color to replace the color
filter, by applying different voltages on each pixel corresponding
the desired color. However the color obtained from ECB are not fully
saturated because the spectrum curve are not sharp (narrow) enough,
with some overlapping with neighbor colors. Therefore, it is not
used often in practical applications for color generations.
Further Readings
and References:
Yeh, P. and Gu, C.,
"Optics of Liquid Crystal Displays", John Wiley & Sons, Chichester
(1999).
