Reference for Niddah 122:3

תנא

and when worn out it should be examined in sunlight.<span class="x" onmousemove="('comment',' When holding up the garment to the light the place of the semen appears darker than the rest of it. A new garment, however, whose texture is close would not show up such a stain even in front of the light. ');">³</span>

Jastrow