This work makes a brief description of zenithal lighting used in museums, and other heritage sites. This type of lighting has interesting advantages: it produces a good level of illuminance with a correct color spectrum. In addition, it allows the observer to perceive the sensation of being outdoors, while protecting from wind and rain. However, skylights can also filter radiation that can be detrimental to the preservation of the assets below, as well as causing very marked gradients between the illuminance of the areas just below and those further away. Using l’Almoina Archaeological Museum (Valencia, Spain) as a case study, the aim of the present study consists of evaluating the transmittance of visible, ultraviolet and near infrared solar radiation through the large skylight that illuminates much of the interior space. This transmittance varies throughout the year and time of day. Empirical data were taken on summer and winter at three different times around noon using two spectrometers: one for the ultraviolet bands (A: 280–315 nm; B: 315–400 nm), and other for both visible (400–700 nm) and near infrared (700–1000 nm) bands. Several points under the skylight inside the museum were measured and quantified by the percentage of transmittance. A discussion in how this kind of illumination can affect the cultural heritage that houses, and the results of other zenithal lighting used in architecture is done.