Factors affecting the accuracy of thermal imaging cameras in volcanology

Abstract

[1] Volcano observatories and researchers are recognizing the potential usefulness of thermal imaging cameras both before and during volcanic eruptions. Obvious applications include measurements of the surface temperatures of active lava domes and lava flows to determine the location of the most active parts of these potentially hazardous features. If appropriate precautions are taken, the new generation of thermal imaging cameras can be used to extract quantitative as well as qualitative information on volcanic activity. For example, they can be used to measure the temperature of lava on eruption and to reveal how the crust cools during flow emplacement. This is important for the validation of lava flow models. To ensure that meaningful temperatures are collected, thermal imaging data must be corrected for instrumental errors, emissivity of the surface being imaged, atmospheric attenuation, viewing angle and surface roughness. Controlled laboratory experiments have been undertaken to determine the emissivity of smooth and rough samples and the effects of viewing angle and to quantify the errors. Measured emissivities range from 0.973 ± 0.002 for smooth samples of basalt and 0.984 ± 0.004 for rough samples. Errors in emissivity-corrected temperatures are within ±15°C for lava at 1100°C. Variations from individual sensor receptors, which provide individual pixel temperature data, were found to be 0.6% and instrumental errors of the cameras used were 0.1%. Apparent temperatures were found to vary by less than the instrumental error for viewing angles up to 30 degrees from normal to lava, and thereafter increased by $1°C per degree. By increasing the apparent viewing distance of a small vent on Mount Etna from 1.5 to 30 m, the maximum temperature is shown to decrease by 53°C due to integrated averaging of radiance over increased pixel areas. At a viewing distance of 250 m the maximum temperature decreased by $200°C with a further 75°C decrease due to atmospheric attenuation for a relative humidity of 50%. However, errors in relative humidity measurements can lead to atmospheric attenuation correction inaccuracies up to 200°C at viewing distances of 1 km. We show how temperatures measured using thermal imaging cameras can be corrected to give improved estimates of temperature distributions on the surface of active lava flows.

Landslide Mapping and Characterization through Infrared Thermography (IRT): Suggestions for a Methodological Approach from Some Case Studies

“…While the last variables are easily obtainable in the field (by means of a thermo hygrometer and a distometer) emissivity can vary according to several factors: (i) the material surface roughness (the greater the roughness compared to the size of the incident wavelength, the greater the absorption and emission); (ii) the moisture content (the more moisture contained within an object, the greater its ability to absorb and emit); (iii) the viewing angle from which the object is surveyed [89][90][91][92][93]. In most applications, the emissivity of an object is based on values found in tables and literature (in many cases also the camera firmware has its own emissivity table) [67,68].…”

Topographic Maps of Mount Etna’s Summit Craters, updated to December 2015

“…This is equipped with an uncooled microbolometer, and is able to record 640 × 480 pixel images in the 8-14 μm waveband with sampling rates up to 30 Hz and temperature range between −40°C and 1500°C (Calvari et al, 2005;Harris, 2013;Spampinato et al, 2011). Ambient temperature and air humidity are measured each time at the start of the thermal survey, emissivity for lava flows is typically assumed to be between 0.95 and 0.98 (Buongiorno, Realmuto, & Fawzi, 2002), and these values are inserted in the thermal camera analysis software before recording starts (Ball & Pinkerton, 2006). Precision of the instrument is ±2%, and thermal sensitivity is <0.08°C at 30°C.…”

The ring-shaped thermal field of Stefanos crater, Nisyros Island: a conceptual model

“…This also leads to a lower sensitivity in the detection of apparent temperature contrasts. Further errors arise from assuming a single emissivity value, because it changes with the material and the viewing angle (Ball and Pinkerton, 2006). We use ε = 0.93 for the average site conditions following previous works (Lagios et al, 2007).…”

“…Otherwise T obj depends on parameters like the target-to-sensor distance (D), the emissivity of the target, and the transmittance of the atmosphere as a function of atmospheric temperature (T atm ) and relative humidity (RH). Finally, the accuracy of the measurements depends on the orientation of the field of view, which should be as parallel as possible to the target (Ball and Pinkerton, 2006).…”