A mathematical and numerical method for an analysis of a liquid crystal displays (LCD) has been presented. The assumptions of the calculations of a light propagation through the display are shown. The real conditions of a work of these displays have been taken into account. Basing of this mathematical model a computer program for a numerical modelling of the optical parameters of LC displays working in real conditions has been worked out. This program makes possible to obtain the most important parameters of a display such as: contrast ratio, luminance in on- and oft-state, spectral characteristics of light and its colour coordinates in these both states. The results obtained from this program have been verified experimentally. Very high conformity of these results has been obtained.