| 摘要 |
The light dispersion portion is formed with the grooves having an identical depth D, and the spacing P between the light dispersions 15 is set 0.01 mm minimum and 0.2 mm maximum. In order to prevent the stripe pattern of brightness and darkness generated due to the light reflection at the light dispersion portion 15, it is preferable to make the spacing as small as possible. However, too small spacing makes it difficult to ensure workability. Accordingly, the minimum value of spacing is set 0.01 mm in view of workability. And, the maximum value of spacing is set 0.2 mm, which does not allow visual recognition of the stripe of brightness and darkness.
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