A first a confocal optical microscope in which a good image with high contrast can be observed with which a bright image can be observed over a long time interval. The confocal optical microscope has a quarter wave plate arranged in an optical path between an imaging lens and an objective such that it is inclined relative to a reference optics axis extending between the imaging lens and the objective. Furthermore, the confocal optical microscope has a shortwave radiation attenuator for attenuating light radiated from the light source with wavelengths of less than or equal to 340 nm. The confocal optical microscope can form part of a length measuring device which can take a length measurement of a sample to be measured with high measurement accuracy and high reproducibility. The length measuring device also has a movable carrier, a carrier device for a sample to be measured, a computing device, a display device and a control device.