LCD classification

LCD Classification

This article makes an attempt to provide a classification to the numerous types of liquid crystal displays (LCDs) in order to remove overlaps and remaining gaps.

LCD operation in a nutshell

The operation of TN, VA and IPS-LCDs can be summarized as follows:
* a well aligned LC configuration is deformed by an applied electric field,
* this deformation changes the orientation of the local LC optical axis with respect to the direction of light propagation through the LC layer,
* this change of orientation changes the polarization state of the light propagating through the LC layer,
* this change of the polarization state is converted into a change of intensity by dichroic absorption, usually by external dichroic polarizers.

LCD Activation

Liquid crystals can be aligned by both magnetic and electric fields. The strength of the required magnetic field is too high to be feasible for diplay applications.

One electro-optical effect with LCs requires a current to flow through the LC-cell, all other practiced electro-optical effects only require an electric field (without current) for alignment of the LC.

Bistable LCDs

For some applications bistability of electro-optical effects is highly advantageous, since the optical response (visual information) is maintained even after removal of the electrical activation, thus saving battery charge. These effects are beneficial when the displayed visual information is changed only in extended intervals (e.g. electronic paper, electronic price tags, etc.).

Reduction of Variations with Viewing Direction in LCDs

With the direction of light propagation in the LC layer also the state of polarization of the light changes, and, as a consequence, the intensity and the spectral distribution of transmitted light changes too. In order to reduce such unwanted variations to a minimum, two approaches are used in actual LC displays: multi-domain approaches and application of external birefringent layers (retarder sheets).

References

Literature

* Pochi Yeh, Claire Gu, Optics of Liquid Crystal Displays, John Wiley & Sons, 1999
* D.K. Yang, S.T. Wu, Fundamentals of Liquid Crystal Devices, Wiley SID Series in Display Technology, 2006