先前學者將個人化郵票結合網點位移藏密技術加入流水號，以肉眼無法直接察覺其藏密效果，需覆上柱狀透鏡方可解讀。惟其選定的藏密區域顏色範圍，是否為較優選擇，且藏、解密效果僅依靠人眼及覆上柱狀透鏡實際觀看，尚未建立評估模式，值得進一步研究。
本研究延伸探討上述個人化郵票結合網點位移藏密技術，在RGB色彩空間，以每隔51階間隔，共216顏色取樣，將隱藏訊息置換洋紅版的內容，組成加密圖像輸出郵票，以目測、光柵及數位程式進行評估驗證，得出網點位移藏密最佳使用色彩範圍，同時比較先前學者所使用顏色與本研究216色藏密及解密效果差異，以得出藏、解密最優的顏色範圍，而本實驗證實最優加、解密的顏色範圍為中間調。

關鍵字：網點位移、數位半色調、資料隱藏

Previous scholars have combined personalized stamps and dot phase modulation technique with serial numbers. The hiding information effect cannot be directly observed by naked eye, but only could be decrypted by covering a lenticular lens on it. However, whether the range of the hiding information on the selected color is a better choice or not. Meanwhile, the effect of hiding and decrypting only depends on the actual viewing of the human eye or covered with a lenticular lens has not yet been established with an evaluation model. This part is worthy of further study.
The study extends the discussion of the above-mentioned personalized stamps combined with the dot phase modulation technique. In the RGB color space, we separate colors in every 51 intervals, totally 216 colors for sampling. The content of Magenta printing plate has been replaced by the hiding information. It can be encrypted as a graphic image and printed as a stamp. Then, using visual inspection, raster and digital program to evaluate and verify furthermore. Finally, we obtain the optimal color range which could be selected when using dot phase modulation technique. At the same time, in order to know the best range of colors for hiding and encrypting, we compare the selected colors which adopted by previous scholars to the selected colors which adopted by our study the difference of hiding and encrypting effect between both. In conclusion, the middle tone is the optimal range of colors for hiding and encrypting.
Keywords：Dot phase modulation, Digital Halftone, Hiding Information

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