Watermark pattern, which is typified by Japanese paper, can reflect various patterns in gradation sometimes brightly and sometimes darkly unlike electronic watermark. Watermark paper is used for various interior items, gift wrapping paper and security paper. Fiber material used for watermark paper is also used for printing as banknote and passport, and contributes to prevention of forgery. In this article, we focused on the change of diffuse transmitted light to the angle of inclination, by irradiating light on watermark paper from different angles. We examined the new direction for image recognition from diffuse transmitted light, by receiving diffuse transmitted light through rod lens with narrow viewing angle. As a result, we revealed that image recognition of deeper gradation pattern can be gained compared to contact image sensor with reflected light source which reads the image printed on coated paper. We also confirmed that by knowing the light quantity of diffuse transmitted light that emits from black watermark part and white watermark part in watermark paper with pattern formed inside, we can achieve the optimization of irradiation angle and the downsizing of contact image sensor with transmitted light source. In other words, practical use of highly original thin type reading device can be achieved, which does not need the prevention of instantaneous malfunction of peripheral circuits by incidence of excessive direct transmitted light to receiving section, prevention of secondary radiation of primary incident light that re-enters light source, nor equipment of optical attenuation filter.
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