The Campbell-Stokes sunshine recorder (CSSR) has been one of the most commonly used instruments for measuring sunshine duration (SD) through the burn length of a given CSSR card. Many authors have used SD to obtain information about cloudiness and solar radiation (by using Ångström-Prescott type formulas), but the burn width has not been used systematically. In principle, the burn width increases for increasing direct beam irradiance. The aim of this research is to show the relationship between burn width and direct solar irradiance (DSI) and to prove whether this relationship depends on the type of CSSR and burning card. A method of analysis based on image processing of digital scanned images of burned cards is used. With this method, the temporal evolution of the burn width with 1 min resolution can be obtained. From this, SD is easily calculated and compared with the traditional (i.e., visual) determination. The method tends to slightly overestimate SD, but the thresholds that are used in the image processing could be adjusted to obtain an improved estimation. Regarding the burn width, experimental results show that there is a high correlation between two different models of CSSRs, as well as a strong relationship between burn widths and DSI at a high-temporal resolution. Thus, for example, hourly DSI may be estimated from the burn width with higher accuracy than based on burn length (for one of the CSSR, relative root mean squared error is 24 and 30%, respectively; mean bias error is -0.6 and -30.0 W m^-2, respectively). The method offers a practical way to exploit long-term sets of CSSR cards to create long time series of DSI. Since DSI is affected by atmospheric aerosol content, CSSR records may also become a proxy measurement for turbidity and atmospheric aerosol loading

This research was supported by the Spanish Ministry of Science and Innovation project NUCLIERSOL (CGL2010-18546). The first author enjoys a grant from the FPU program (FPU AP2010-0917) of the Spanish Ministry of Education. A. Sanchez-Lorenzo was supported by the "Secretaria per a Universitats i Recerca del Departament d'Economia i Coneixement, de la Generalitat de Catalunya i del programa Cofund de les Accions Marie Curie del 7e Programa marc d'R+D de la Unio Europea" (2011 BP-B 00078) and the postdoctoral fellowship JCI-2012-12508. The Negretti & Zambra Campbell-Stokes recorder was provided by the Pyrenean Institute of Ecology (CSIC). We thank Juan Jose Vizcaino Morillas (AEMET) for providing sunshine duration cards of the model Mod.98