Fluorescent screens are widely used for purposes of beam diagnostics in particle accelerators. When the screen intercepts the beam, it produces visible light, generating an image that can be visualized using CCD cameras. However, these materials are subject to saturation, when an increase in the beam charge is not followed by a proportional increment in the light emission. In this work we discuss how charge saturation influences the observation of charge distribution. Systematic errors are induced since the experimenter observes the beam wider than it actually is. Closer attention is given to gaussian distributions, but some other shapes are also discussed. An algorithm to correct beam diameter measurement is suggested and composite measurements are discussed. This study is relevant to improve the quality of the beam information provided to users.