Application of surface BRDF to solar or other extended illumination requires generalization to spectrally- and spatially-broadband sources, which can be naturally accommodated by coherence theory. Here the previously-published Coherence Model [BG. Hoover and VL. Gamiz, J. Opt. Soc. Am. A 23, 314 (2006)] is generalized to broadband, extended-source illumination, with the predicted intensity reflected by a diffuse-gold calibration standard verified against surface parameters measured by an atomic-force microscope (AFM) and reflectance measured by a white-light scatterometer over the wavelength range 600-1800nm. Coherence theory reveals the reflected intensity as a non-geometrical convolution of the coherent BRDF with the source angular spectrum. The rendering routine for reflectance under a broadband source and the spectral invariance of Epner diffuse gold are demonstrated graphically. The results represent the first known BRDF solution for a diffuse metal reflector over this waveband verified against independent surface parameters.