The measurement of the temperature of the glass that covers solar collectors or photovoltaic modules is very important for the characterization of the performance of these converters. Thermography is a non-contact thermometry technique that is capable to quickly scan and record surface temperature fields, but its accuracy depends on knowing the limitations and possible errors involved with the use of this technique. This paper identifies glass infrared reflection errors and their consequences when performing outdoor thermographs. The work also proposes and experimentally validates a correction method for correcting these errors. Finally is also presented a method for estimating a thermographic equivalent sky temperature that can be used in correction procedures for the own outdoor thermographic measurements. (c) 2006 Elsevier Ltd. All rights reserved.