How do segmented electrodes form numbers and letters on digital watches and calculators?
In nematic arrangements the crystals are randomly positioned with all their long axes pointing in essentially the same direction, given by a vector known as the director. In smectic arrangements the crystals are further constrained to parallel planes; in smectic A the director is perpendicular to these planes, while in smectic C the director is at some other angle to the planes.
In digital watches and calculators, numbers and letters are formed by patterning transparent electrode coatings into specific segments. These displays typically utilize twisted nematic (TN) cells, which consist of a liquid crystal layer sandwiched between two substrate plates and a pair of crossed polarizers.12
When no voltage is applied, the liquid crystal molecules adopt a twisted configuration that rotates the polarization of light, allowing it to pass through the display, which appears transparent. When an electric field is applied to a specific electrode segment, the liquid crystal molecules reorient perpendicular to the substrate plates. This change destroys the twisted state, preventing the rotation of light polarization and causing that specific segment to appear dark against the background.12
By selectively activating these individual segments, the display creates a patchwork of dark and light areas that form recognizable alphanumeric characters.12