An illumination system of a microlithographic projection exposure apparatus comprises a pupil surface and an arrangement of individually drivable beam deviating elements. Each beam deviating element is configured to direct light impinging thereon onto different positions on the pupil surface in response to a control signal applied to the beam deviating element. According to the disclosure an attenuation unit is provided which is configured to reduce the intensity of light, which is directed by any arbitrary beam deviating element (onto the pupil surface, by more than 50%. This makes it possible to reduce the intensity of light in the pupil surface that has been reflected by defective beam deviating elements.