Lighting designers will be familiar with the illuminance of a planar surface, which is measured in lumens per square meter (or foot). The irradiance of a planar surface by a germicidal radiation source is conceptually the same, except that it is measured in watts per square meter (W/m2). Most designers, however, will not be familiar with the concept of the spherical irradiance (or equivalently, fluence rate) of a point in space. This is an essential metric for air disinfection by germicidal lamps, and so it needs to be understood.