A sun photometer measures aerosol optical depth (AOD) by pointing directly at the sun and recording light attenuation at multiple wavelengths (340–1,020 nm). The global AERONET network (NASA) operates hundreds of standardised photometers.
In Spain, the photometer network of the University of Valladolid and centres like AEMET-CIAI monitor Saharan dust episodes (AOD >0.5 indicates an intense event). The data calibrate aerosol transport models and improve solar radiation estimates for photovoltaic energy.