Strong and broad emission bands commonly attributed to polycyclic hydrocarbons dominate the mid-infrared spectrum of interstellar matter. Early photo- and spectrometry by the ISO and IRTS missions has proved that these bands are widespread in the diffuse interstellar medium. We have sampled the galactic distribution of unidentified infrared band emission in the diffuse ISM component. The remarkable insensitivity of the shape of the UIR band spectrum is seen in this sample; little variation with line strength or galactic position is seen suggesting either a homogeneous history of the UIR band carriers within the Milky Way, convergent evolution of grains suppressing any differences seen in younger populations or an emission mechanism highly insensitive to particle type and physical conditions.