A method for reducing a size of data required for recording a physical characteristic of an optical device is proposed. By performing variable-length coding on sub-pixel data pieces that respectively represent physical characteristics of multiple sub-pixels of the optical device based on a similarity in spatial trend of the physical characteristics of the sub-pixels, a plurality of pixel-index data pieces are generated, and a part of the sub-pixel data pieces can thus be removed to obtain reduced sub-pixel data pieces. The pixel-index data pieces and the reduced sub-pixel data pieces cooperatively replace the sub-pixel data pieces to serve as an index for finding the physical characteristics of the sub-pixels.