We present an experimental procedure for the determination of the physical properties of an arbitrary twisted-nematic liquid crystal cell. No assumptions are made about the physical properties of the cell, because the commercially available devices are generally produced under proprietary conditions. The techniques developed were then used to evaluate the Jones matrix of an Epson liquid crystal television, and reasonable agreement is observed between theory and experiment. Knowledge of the Jones matrix for these devices helps to evaluate their performance as spatial light modulators in optical processing systems, and such applications are discussed. It also provides an effective means for determining the spatial quality of the liquid crystal layer. We find that there is significant variation in the birefringence across this cell, and the influence that this has on the use of the cell in optical processing systems is discussed.