A dichroic mirror configured to separate a first type of radiation in a first wavelength range having an upper boundary from a second type of radiation in a second wavelength range having a lower boundary greater than the upper boundary of the first wavelength. The mirror includes a substrate, and a stack having a reflective surface facing away from the substrate and a width that increases stepwise in a direction towards the substrate. The stack is formed by alternating layers of first and second materials on the substrate. The reflective surface has steps with a width greater than the upper boundary of the first wavelength and less than the lower boundary of the second wavelength.