**4.3 Trans-reflective LCD**

In order to overcome the drawbacks and to take advantage of both transmissive as well as reflective LCDs, trans-reflective LCDs have been developed, which use both ambient light and backlight to display images based on availability and necessity. It has a semi-reflective film in the back of LCD screen, the backlight can transmit through it so that it may work as a transmissive mode, but the front light cannot pass through it and get reflected, and it simultaneously works as a reflective mode. Broadly, trans-reflective LCDs are classified into four categories: (a) absorption, (b) scattering, (c) reflection and (d) phase retardation. As the name suggests, the first category absorbs light, and the corresponding device is referred to as guest-host (GH) display. The second one scatters light, and polymer-dispersed liquid crystal (PDLC), polymer-stabilized liquid crystal (PSLC) and LC gels are related technologies. The third category is based on reflection of light. The fourth one modulates the phase of an incident light. Here we will discuss operating principle of PSLC, PDLC and holographic PDLC (HPDLC).
