From my understanding decoherence occurs whenever a quantum object interacts with a macroscopic sized object. So for instance a measurement involving a photographic plate registering a particle will cause decoherence of the wavefunction, which appears to us as the wavefunction collapsing. However there are other instances where a quantum object interacts with a macroscopic object which doesn't cause decoherence. For instance a half silvered mirror that acts as a beam splitter in various experiments, and also the partition in the double slit experiment where the wavefunction encounters this but doesn't decohere. What is it physically that differentiates these objects from measurement devices such as photon detectors which does cause the wavefunction to appear to collapse?