Optimal achromatic wave retarders using two birefringent wave plates: Comment

In a recent publication, J. L. Vilas et al. [Appl. Opt. 52, 1892 (2013)] described a combined system comprising two wave plates and proposed an expression for the overall retardation. However, to the best of my knowledge, the proposed theoretical expression for the total retardation is incorrect. The authors improperly derived that expression by simply diagonalizating the Jones matrix of the cascaded system. The mistakes are pointed out and discussed in these comments.