Brightness enhancement of transflective LCD's
Publication Date: 2002-Sep-24
The IP.com Prior Art Database
The workhorse for the LCD business is the transflective (F)STN LCD. The structure of this LCD is shown schematically in fig.1. Disadvantages of the FSTN LCD (reflective mode) are:
ii) relatively low brightness
The parallax results from the position of the transflector (location: externally, i.e. "behind" the rear polariser). The relatively low brightness in the reflective mode results from the absorption of light by the polarisers. Before the light reaches an observer it has to pass a polariser four times. Every polariser passage leads to some loss of brightness due to absorption. This is illustrated by fig.2 which shows the transmission of a stack of polarisers (mutually parallel transmission axes) as a function of the number of polarisers. To increase the brightness (in the reflective mode) CSTN LCD's were developed that make use of an internal transflector. Fig.3 shows the structure of these LCD's. The principle of operation of these displays is shown in fig.4 (reflective mode) and fig.5 (transmissive mode). In the reflective mode a dark state can only be realised if the light, which impinges onto the reflector, is circularly polarised. This implicates that the compensation film(s) plus the liquid crystal layer, should turn the light polarisation from linear to circular. The (metallic) reflector turns the circular polarisation from right to left or vice versa. The polarisation of the reflected light is changed from circular (at the reflector) to linear (in front of the polariser). However, due to the polarisation inversion by the reflector, also the linear polarisation has turned over 90o; the linear polarisation of the outgoing light in front of the polariser is now perpendicular to the polariser transmission axis; the display appears dark. In the on state the display appears bright. This can only be achieved if the light, which is incident onto the reflector, is linearly or almost linearly polarised. Upon reflection the linear polarisation is not changed and after passage of the LC layer and the compensation films the polarisation of the outgoing beam in front of the polariser is parallel to the polariser transmission axis; the display appears bright. So, the function of the compensation film and the liquid crystal layer is to turn the light polarisation from linear to circular (black state), from linear to linear (bright state) or from linear to elliptical (intermediate state.
The principle of operation of the transmissive mode has been shown in fig.5. The rear optical film stack consists of a polariser and a l/4 plate. The function of l/4 plate is to turn the light polarisation from linear (after passage of the rear polariser) to circular (after passage of the l/4 plate and the transflector). Often a wide wavelength band l/4 film is used that consists of two films, i.e. of a l/4 and a l/2 film.
The transmission of the LCD shown in fig.3 is rather low. This is illustrated by fig.6 which shows the transmission-volta...