 If every \(Q^k\) is a projector, then implies that they are mutually orthogonal, and form a projective decomposition of the identity of \({{\mathcal H}}_s\). This means we are dealing with a projective measurement, though more general than the von Neumann model of Sec. [sbct7.1], as the \(Q^k\) may project on subspaces of dimension greater than 1. However, in general \(Q^k\) is not a projector or proportional to a projector, but since it is Hermitian it can be written in the form