The IR radiation of the a clear sky is referred to as "cold sky radiation". In a clear sky, cold sky radiation (~ -50 to -60 °C) and warm solar irradiation (~ 5500 °C) are reflected during the day. In terms of area, the sky outstrips the sun, which means that the reflected temperature in outdoor thermography is usually below 0 °C, even on a sunny day. Objects heat up in the sun as a result of absorbing sunlight. This considerably influences the surface temperature – some even hours after the solar irradiation.
Special aspects of outdoor thermography
- Ideally, measure in the early morning hours and/or under dense cloud cover. It should not be raining or snowing. Fog and strong winds are also unfavourable.
- During the measurement, change your position in order to recognize reflections. Reflections shift, thermal anomalies of the measurement object stay in the same place – even when the angle of view is changed.
- Avoid measurements close to very hot or very cold objects, or screen them off.
- Avoid direct solar iradiation, even several hours before the measurement. Take cloud cover into consideration several hours before the measurement.
- Do not measure if there is air humidity condensing on the thermal imager.
- Do not measure in extremely contaminated air (e.g. when dust has been freshly disturbed).