A rigorous methodology for mapping thermal and RGB images on three-dimensional (3-D) models of building façades is presented. The developed method differs from most existing approaches because it relies on the use of thermal images coupled with 3-D models derived from terrestrial laser scanning surveying. The primary issue for an accurate texturing is the coregistration of the geometric model of the façade and the thermal images in the same reference system. This task is done by using a procedure standing out from other approaches adopted in current practice, which are mainly based on the independent registration of each image on the basis of homography or space resection techniques. A rigorous photogrammetric orientation of both thermal and RGB images is computed together in a combined bundle adjustment. This solution allows one to have a better control of the quality of the results, especially to reduce errors and artifacts in areas where more images are mosaicked onto the 3-D model. Several products can be obtained: 3-D triangulated textured models or raster products like orthophotos, having the temperature as radiometric value. The proposed approach is tested on different buildings of Politecnico di Milano University. Applications demonstrated the performance of the procedure and its technical applicability in routine thermal surveys.
Nowadays several thermal cameras capture images based on a pinhole camera model. This paper shows how multiple images of flat-like objects or 3D bodies can be mapped and mosaicked with a mathematical formulation between image and object spaces. This work demonstrates that both geometric and radiometric parts need proper mathematical models that allow the user to obtain a global product (orthophotos or 3D models) where accurate and detailed photogrammetric models and thermal images are registered in order to combine geometry and thermal information.
At present there is a lack of commercial software packages able to perform differential temperature gradient analysis. This is an innovative and fundamental tool to speed up the recognition of thermal anomalies revealing finishing damages like detachments.
This paper presents a photogrammetric methodology aimed at mapping IR thermal images on 3D models created with terrestrial laser scanning technology. The attention is focused on building, where a standard RGB texture of the 3D model will coupled to temperature values. Each facade will be then transformed into an orthophoto and processed in a GIS environment to support new thermal analysis. The developed image processing pipeline will be illustrated starting from data acquisition up to data visualization and management.
This paper presents two methodologies able to map a block of IR thermal and RGB images on 3D models derived from
terrestrial laser scanning surveying. Proposed methods stand out from other traditional approaches that are mainly based
on the projection of single images through approximate models. The first method is a rigorous photogrammetric
orientation through a bundle adjustment integrating both RGB and thermal data. Then, another complementary solution
based on the use of a calibrated 'bi-camera' system is illustrated. Both methods allows one to texture building facades
(reconstructed with 3D models) with their temperature values. Finally, several products can be extracted and managed in
different data processing environments: triangulated models to visualize 3D spatial information and to analyze 3D
heating diffusion on the surface; raster datasets (e.g. orthophotos or rectified images) with temperature as radiometric
value. Both approaches were tested on different buildings of Politecnico di Milano University, where a restoration
project of historical and modern facades is currently work in progress.
Many applications of IRT on buildings require active approach. The solicitation has to be properly calculated, and the application has to take in account the optical characteristics of the surface and its thermal properties. In fact, non-homogeneities of the surface definitively affect the absorbance and reflectance of materials, as shown in literature. Therefore, in case of different colors like artistic paintings, dark stains and salts deposition a convection heating results more effective for IRT inspection, because it does not stimulate different localized absorption due to the colors. Using fan coil heaters, major difficulty is to obtain an even heating on the wall under inspection. The laboratory tests permitted to verify that the strength of rising warm air is higher than the one due to the heater ventilation. As a consequence, the effects of heating on the wall start from the upper part and decrease in a non-proportional way to the bottom. On the other side, thermal flux from a heater changes direction according to the geometry of the room, ambient conditions (initial temperature of the air, openings, etc), technical characteristics of the heater (power, speed of the fan, shape, etc) and its location (orientation, elevation, distance from the surface under investigation, etc). In addition, the increase of air temperature does not directly correspond to the increase of the surface temperature. The paper shows the characterization of a convective heating source, by laboratory measurements; to map the distribution of heat in time, the 14.000-26.000 kcal/h heater flux was measured following a 3D grid, by anemometers, probes, and IR Thermography.
IRT technique applications for the detection of the plasters defects in historic buildings are widely documented by scientific literature. Previous studies demonstrated the advantages of tomographic techniques to obtain quantitative results by IRT. With a quantitative approach, the dynamic measures of IRT, versus time and maximum value of thermal contrast, allows to locate the delamination and calculate its volume inside the thickness of the plaster. Nevertheless, effects are not prominent if compared with the ones caused by the interaction between surface and irradiation. Moreover these effects are lower than the noise due to the approximation of the spectral coefficient value. Authors already showed that the multispectral evaluation of the reflectance coefficients, in the range of visible and near IR, contributes to a proper evaluation of the thermograms shot on surfaces affected by chromatic alterations. In the study case, the evolution of the surface temperature in time allows to quantify the effects of spectral absorption (absorbance) in the thermograms. By comparing the thermograms to the maps of damages and intervention, it has been possible to correlate the materials and its state of conservation to the evolution of the thermal profile corresponding to each analyzed area.
The object of this research is to develop a procedure using IRT to detect critical levels of moisture content in wood. Passive and active approaches are compared to define the most reliable procedure to map the moisture diffusion and to evaluate the moisture content of the surfaces. Laboratory research reported in the scientific literature has determined that the water content in porous materials is more related to the evaporative speed of the surfaces and the presence of soluble salts than to their absorption capability. Moreover, evaporative fluxes were studied at different environmental conditions and water content in order to determine a correlation between moisture content, evaporation and boundary conditions.
The thermal characteristics of timber are different those of porous materials such as brick and stone and mortar, particularly in that the thermal capacity of wood is lower. Nevertheless, because of the lower heat capacity of wood, the presence of water greatly affects the wood thermal capacity. Therefore, the active procedure guarantees the best results. Lab tests and a study case (Knight House, Kirtland OH) show the advantages and the limits of IRT techniques, and the results obtained demonstrate the sensitivity of the method in oak and pine wood species.
Knowledge of an historic monument can be substantially improved by locating hidden structures, openings and the wall bonding beneath the plaster. When restoring buildings, the physical connection between the walls must be known in order to predict the risk areas for structural weakness. IR Thermography produces remarkable results, especially by means of the quantitative approach. The temperature pattern detected by thermography and analyzed in space and time maps the hidden structure of the wall. Thick walls exposed to the weather represent a challenge in detecting hidden structures by means of thermography. Frequently output is very poor because testing conditions are not optimized. Hence, appropriate testing requires careful analysis of the wall system before and after taking the thermograms. Otherwise, false alarms render the images useless. This paper describes a general procedure applied to see the hidden wall structure. It works in three steps: a) a mathematical simulation of the real test by a dedicated software, implementing the 3D thermal problem; b) a transient thermographic test, delivering a suitable heating flux on the surface for the proper time; c) processing test data, including a thermogram sequence and air temperature analysis. Here, are reported tests achieved on a XV-XVIIth century Palace at Cremona (Italy) and in the Westcott House in Springfield (OH).
The presence of moisture in building materials causes damage second only to structural one. NDT are successfully applied to map moisture distribution, to localize the source of water and to determine microclimatic conditions. IR Thermography has the advantage of non-destructive testing while it allows to investigate large surfaces. The measures can be repeated in time to monitor the phenomenon of raising water. Nevertheless the investigation of moisture in walls is one of the less reliable application of Thermography IR applied to cultural heritage preservation. The temperature of the damp areas can be colder than dry ones, because of surface evaporation, or can be warmer, because of the higher thermal inertia of water content versus building materials. The apparent discrepancies between the two results are due to the different microclimatic conditions of the scanning. Aim of the paper is to describe optimal procedures to obtain reliable maps of moisture in building materials, at different environmental and microclimatic conditions. Another goal is the description of the related energetic phenomena, which cause temperature discontinuities, and that are detected by thermography. Active and passive procedures are presented and compared. Case studies show some examples of procedures application.
IR thermography allows to identify the thermal anomalies due to moisture in ancient walls. Wet zones can appear warmer or colder in IR images, according to the atmospheric conditions during the scanning; furthermore, thermal monitoring, even in qualitative thermography, allows to obtain a more effective diagnosis of the defects because it records thermal behaviors of the material in different environmental conditions. Thermographic system allows an accurate analysis of transpiration effects on buildings and precise measurements of water content starting from environmental temperature, relative balance and wind speed. These variables play a major role in the causes of damages in buildings. Particularly, the evaluation of transpiration is essential to determine the evaporative rate of water content within the wall. The research has been carried out on two ancient buildings during a period of several months. The main experimental tests were on the church of 'Guardia di Sotto', Corsico, a seventeenth century building on the bank of Pavese Canal. Five thermal scanning have been disposed in different seasons from March 14, 1996 to June 16, 1997. The causes of the wet zones were identified at the basis of the walls were rising damp and rain spread in the ground. The repeated thermographies and thermo-hygrometric test allowed to distinguish the size and the location of the areas damaged by the different causes. In other cases studied - Addolorate Church, Gessate the thermal scanning and the other supporting tests confirmed the list of optimal environmental condition required to detect humidity in walls by thermography.
Damage due to moisture and particularly to evaporation is one of the major causes of decay of wall surfaces in ancient buildings. The evaporative rate of water in building materials can be related to the alteration (chips, gallets) caused by salts crystallization when the water evaporates through the surface of the wall. Current and future usage of NDT heavily depends on the possibility to precisely measure physical variables which present large sensitivity to small variations of water content. A NDT thermography allows us to exactly determine the evaporation rate because of both the high value of water latent heat and the high sensibility of thermographic devices. The research has been carried out both in the laboratory and on the field measuring relative humidity and temperature in a frescoed wall of the castle of Malpaga (Northern Italy). In laboratory a climatic room has been set up using a thermovision system and a temperature & RH% probes, to analyze the evaporative phenomena. A mathematical model, although approximate, is proposed to describe the energy balance of the surface where evaporation is present. The model has been applied to the fresco to correlate the temperature to the evaporation rate. This method allows us to correlate the decay, due to the capillary raise of water in the masonry, to the transpiration phenomena.
This paper compares experimental results obtained on ancient buildings, following the usual qualitative inspection with a procedure based on time-domain analysis. The aim of the project was to study relationships between characteristics of defects and thermal behavior of the surface, using thermal scanning in various conditions. The other topic was to prove generally the usage of solar radiation as a heating source in thermal scanning. The 'Palazzo della Ragione' in Milan is more than 700 years old. We gave our attention to the upper part of the building. The conditions and damages of facades have been studied also using knocking tests. Defects we are looking for are delamination of plaster on the brick wall. The southward facade has been monitored 6 times during 1993 - 1995, using thermography. In addition, the actual weather conditions during scanning were measured. The results were qualitatively analyzed. A lot of knowledge is needed for the qualitative interpretation of thermograms, because different kinds of defects and structure variation may be confused. Because of limiting factors, any clear correlation between the temperature changes and the detected damages of coating cannot be found based on the first approach. A new series of tests has been made in October 1995. Experiments have been carried out also at a laboratory on a segment of wall, including known defects at various depth. A numerical simulation of the temperature pattern and its time evolution, during the heating and cooling of the wall, was also performed. The dynamic test has been planned, based on the experience of these preliminary studies. Experimental and simulated data have been compared for laboratory and in situ dynamic tests.