1. A multispectral illuminator for providing EM radiation with a selectable frequency content, the multispectral illuminator comprising: a dispersive element which during operation provides an angular dispersion for incident EM radiation; a light source array comprising an array of light sources providing EM radiation at different wavelengths; and an optical system having an optical power, the optical system positioned relative to the source array and the dispersive element to image the dispersive element at infinity with respect to the light source array for at least one of the different wavelengths in a paraxial approximation, wherein the position of each light source along the array and the angular dispersion of the dispersive element are selected to cause at least a portion of the EM radiation from the source array incident on the dispersive element through the optical system to propagate along a common direction.