are formed at the same location on the product accordingly the hidden image can be an animation image or a three dimensional image the lens can incorporate with printed color layers or photoelectric display device to detect the color hidden image wherein multiple hidden images animations three dimensional images or color images can be placed at one location of the printed product or mold product for the lens to read these applications can be used for u201c hint and answer u201d of teaching material or educational books to reduce the consumption of paper such that the present invention is an environmentally friendly device and can reduce the cost of printing and logistics furthermore it is easier for people to read and check the answer it also can apply for the paper product of the game product through the game development and lens technology and its various functions of the application the present invention inspires the memory of the participants and enhances the interest and attraction of the game the nano level optical lens and its manufacturing technology can be used for an automatic identification system that the hidden image can be obtained by the lens through the sensor instrument wherein the feature of the hidden image is extracted and analyzed for automatically recognition of the hidden image with special marks characters graphics or existing two dimensional codes and for connecting to the database to obtain more product information for testing record and management the hidden images are correspondingly applied on the lens surface of the lens on the product.