  I study the response of a detector that is coupled non-linearly to a
quantized complex scalar field in different types of classical electromagnetic
backgrounds. Assuming that the quantum field is in the vacuum state, I show
that, when in {\it inertial} motion, the detector responds {\it only} when the
electromagnetic background produces particles. However, I find that the
response of the detector is {\it not} proportional to the number of particles
produced by the background.
