Infrared Thermography (IRT) is a technique widely used to investigate thermal behavior of electrical components, and a representative number of papers is dedicated to applications of IRT in photovoltaics (PV). Despite this fact, few scientific publications discuss the challenges of measuring PV panels temperature outdoor, by means of IRT. From the back side, PV panels are opaque and diffuse surfaces, and IRT temperature measurement is straightforward, however, inspections are commonly made from the front side. From this latter perspective, PV panels are specular surfaces, highly directional-spectral dependent in the long wavelength infrared (LWIR) spectrum. Besides that, spectrum of incident LWIR thermal radiation, notably the sky and clouds, is discontinuous. Then, the conditions to assume PV surface gray are not sustained. To deal with this problem, we propose a post processing methodology. We use the full response of an IR camera, which is made considering that emissivity and transmissivity are equal to one, and consider the directional-spectral quantities to separate the target object signal from the other thermal radiation components. We applied the proposed methodology for outdoor PV panels in different sky conditions, resulting in a maximum absolute deviation of less then 3 degrees C, compared to a surface RTD.