The subject of the article is aspects of PV modules and cells measurement, with the use of natural sunlight. A light source is an important element during calibration and measurements of solar cells and modules. All designers of artificial light sources try to recreate natural light using so called measurement tables. The correctly performed measurement, i.e. meeting all the appropriate atmospheric conditions, guarantees obtaining the result with the use of a reference spectrum. The article has two main aims. The first aim of the article is to answer the question asked earlier - if the sunlight spectrum registered in appropriate conditions is so good that it serves as the reference spectrum - then, in practice, during measurements carried out with its use, certain problems occur regarding the correct measurement results or their interpretation. The second aim regards presenting detailed numeric procedures in order to enable readers to associate air mass with geographical coordinates and Local Solar Time of their study/laboratory location. Moreover, having the data from their local meteorological station, they will be able to estimate the occurrence of the measurement spectral error of the tested cell/module not only from the group referred to in the article but also for others, for which they have a dedicated characteristics of spectral response.