The electromagnetic field in a dielectric slab bounded by two dielectr
ic half spaces with arbitrary refractive indices is quantized by compu
ting the complete set of orthonormal electromagnetic modes. The zero-p
oint fluctuations of the electromagnetic field are determined as a fun
ction of position. The dependence of the rate of spontaneous emission
of thin dielectric films on the thicknesses of the films and the refra
ctive index of the substrate is studied and compared with experimental
results.