In this paper, we describe a new aspect to evaluating the robustness of the optical watermarking technique, which is a unique technology that can add watermarked information to object image data taken with digital cameras without any specific extra hardware architecture. However, since this technology uses light with embedded watermarked information, which is irradiated onto object images, the condition of taking a picture with digital cameras may affect the accuracy with which embedded watermarked data can be detected. Images taken with digital cameras are usually defocused, which occurs under non-optimal conditions. We evaluated the defocusing in images against the accuracy with which optical watermarking could be detected. Defocusing in images can be expressed with convolution with a line-spread function (LSF). We used the value of full-width at half-maximum (FWHM) of a Gaussian function as the degree to which images were defocused, which could approximate LSF. We carried out experiments where the accuracies of detection were evaluated as we varied the degree to which images were defocused. The results from the experiments revealed that optical watermarking technology was extremely robust against defocusing in images.
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