1. A method of optical storage of a numerical image on a recording medium, said image being constituted by numbers each consisting of a plurality of digits and each digit being represented by a modification of an optical characteristic of the recording medium on a surface element of said medium, said optical characteristic being capable of assuming at least as many values as the number of different digits present in the number system employed for the image, wherein the numerical image is split up into at least as many separate sub-images as the digits which it is necessary to employ in order to represent the highest possible value for a number of the numerical image, each sub-image being conventionally associated with the rank of the digit, and wherein the surface element of the recording medium associated with a predetermined digit occupies within each sub-image a similar position to that occupied by the number which contains said digit in the numerical image to be recorded wherein a certain number of sub-images is reserved for additional information such as signs, a detection code or correction of errors.