A microlithographic projection exposure apparatus includes a projection light source. The apparatus also includes a heating light source for generating heating light which is at least partly absorbed by an optical element. An illumination optical unit directs the heating light onto the optical element such that the heating light has a predefined intensity distribution on an optical surface of the optical element. The illumination optical unit includes a deflection element which is a diffractive optical element or a refractive freeform element. The deflection element simultaneously directs the heating light impinging thereon in different directions.