System solutions for commercial applications such as autonomous driving, augmented reality, medical imaging, and security imaging, exploit active illumination. In these applications, the active source is used to provide photons but also to code and decode relevant information such as range or spectral response. The wavelengths of choice range from visible to millimeter waves depending on the application and associated requirements. Across these wavelengths, the targets range from Lambertian to specular. For single element and scanned systems, ranging is commonly modeled using conventions borrowed from the radar and antenna community. Staring and scanning systems that facilitate resolution in the cross-range are modeled using conventions borrowed from the synthetic aperture radar community or the passive imaging community. All the borrowed conventions, however, make assumptions about the size and nature of the target in relationship to the illumination and wavelength, unresolved versus resolved and Lambertian versus specular. These assumptions are relevant for the calculation of system signal to noise ratio and resolution; therefore, they should be carefully considered when adopting the conventions. Examples of systems where modeling falls between active radar and passive imaging include wide band Terahertz array imaging systems and solid state lidar systems. This paper generalizes and bridges the models used by the active radar community and the passive imaging community. We apply the model to a wide band terahertz array imaging system enabled by terahertz array technology recently developed at imec. The model is validated using simulated measurements from a two-dimensional terahertz array.