White damask linen is woven in such a way that warpthread surfaces alternate with weft-yarn ones which results in a white-on-white pattern. An appropriate lighting and exposure angle may nevertheless reveal the original design of the damask. This article analyses the principles of this optical geometric phenomenon. One possible formula is a tissue with perfectly cylindrical and rectilineal threads. When the threads are completely brilliant, the damask tissue presents a pattern of brilliant lines on a surface that, in theory, is completely dark. When, on the contrary, the threads are mat and white, the design is visible all over the surface but with alternating contrast, that even may disappear with some lines. But flax threads (and fibers) are in reality half-mat and half brilliant, they are neither perfectly cylindrical nor rectilineal. Thus the pattern appears on a larger scale and with less contrast. These formulas make it possible to verify, a priori, how to dispose linen and source of light in order to obtain the best optical result.