  Here is considered a specific detection loophole, that is relevant not only
to testing of quantum nonlocality, but also to some other applications of
quantum computations and communications. It is described by a simple affine
relation between different quantum "data structures" like pure and mixed state,
separable and inseparable one. It is shown also, that due to such relations
imperfect device for a classical model may mimic measurements of quantum
correlations on ideal equipment.
