  We study environmentally induced decoherence of an electromagnetic field in a
homogeneous, linear, dielectric medium. We derive an independent oscillator
model for such an environment, which is sufficiently realistic to encompass
essentially all of linear physical optics. Applying the ``predictability
sieve'' to the quantum field, and introducing the concept of a ``quantum
halo'', we recover the familiar dichotomy between background field
configurations and photon excitations around them. We are then able to explain
why a typical linear environment for the electromagnetic field will effectively
render the former classically distinct, but leave the latter fully quantum
mechanical. Finally, we suggest how and why quantum matter fields should suffer
a very different form of decoherence.
