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We previously proposed a novel technology with which the images of real objects with no copyright protection could contain invisible digital watermarking, using spatially modulated illumination. In this “optical watermarking” technology we used orthogonal transforms such as a Discrete Cosine Transform (DCT) or a Walsh-Hadamard Transform (WHT) to produce watermarking images, where 1-bit binary information was embedded into each pixel block. Here, we propose a new robust technique of optical watermarking that varies the size of pixel blocks by a trade-off in the efficiency of embedded watermarking. We conducted experiments where 4×4, 8x8, and 16×16 pixels were used in one block. A detection accuracy of 100% was obtained by using a block with 16×16 pixels when embedded watermarking was extremely weak, although the accuracy did not reach 100% by using blocks with 4×4 or 8×8 pixels under the same embedding conditions. The results from experiments revealed the effectiveness of our proposed technique.