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To make us read the ruler on dark paper easily, we make the Black And White ruler design.After changing the black scales and figures to the white ones with black outlines, we can read them clearly on both dark and light color paper.On dark color paper, the white figures and scales appear clearly, while on light color paper ,the black ones can be identified easily.
A simple concept but really helpful.
Sun Chao, Hu Lingyue, Qin Xiang