We propose to determine the IR spectra of reflection nebulae
 as a function of effective temperature of the central star.
 The goal is to study the energy budget of the dust and is
 complementary to an accompagning study of the energy
 budget of the gas (MSPAANS_COOLPDRS). In this way we can
 determine spectral variations in the mid IR emission features
 as a function of pumping photon energy. We can also test
 models for the photoelectric heating of of interstellar
 gas by PAHs and small grains. Finally, these observations
 will provide an independent measurement of the dust opacity.