The invention relates to an incident illumination device for a microscope, for viewing a sample (1) in the microscope (10), having a planar light source (100) for incident illumination of the sample (1), wherein the planar light source (100) comprises a panel-shaped light guide having a lower boundary surface (111), an upper boundary surface, and at least one lateral surface, as well as at least one light-emitting means that is arranged so that it irradiates light, via at least one lateral surface serving as a light entry surface, into the light guide in such a way that said light propagates in the light guide due to total reflection; wherein the total reflection is disrupted in defined fashion by an element abutting at the lower boundary surface of the light guide against a contact surface so that an outcoupling of light occurs at the upper boundary surface of the light guide.