I have just learned about Stokes vectors and Mueller matrices for description of polarized light. In the text I studied there is clear restriction for the Stokes vector $\vec S$ that $S^2_0=S^2_1+S^2_2+S^2_3$ but not characterization of Mueller matrices is given, they just say it is matrix $4\times 4$.
Obvious restriction is that Mueller matrix has to transform a Stokes vector to a Stokes vector. From the condition on Stokes vector $S^2_0=S^2_1+S^2_2+S^2_3$ we can see that Mueller matrix has to be a multiple of an element of group $O(3,1)$.
Can you direct me to some text where the connection between Mueller matrices and Lorentz group is made? I did a quick google search and got many articles about this topic. So I'm looking for concise exposition to the problem.