## 4 Stage II: Obtaining the True Temperature of an Object

From this point on, a surface of any structural element that has been recognised in the point cloud model of the scene will be denominated as an \"_object_\"; that is, the object classes are the following: the structural element (SE) (floor, ceiling and wall), the window frame or the door frame. It will, in turn, be assumed that a scene or zone will consist of a standard room (with doors and windows). We shall additionally assume that the thermal point cloud has been corrected for the repeatability effect (as shown in Section 3) and that it has been segmented into points corresponding to the floor, ceiling and each of the walls. We initially created a raw 3D point cloud in which each point has four associated values (RGB + Temperature). This point cloud is then segmented into smaller point clouds corresponding to the floor, ceiling and each of the walls. Orthoimages are subsequently obtained by projecting each segment from a specific point of view (for example, perpendicular to a certain wall) and creating the corresponding 2D images. The pixel of an image consequently maintains its associated RGB and temperature values. In the first case, we obtain a Colour Orthoimage (CO), and in the second, a thermal orthoimage (TO), which contains _apparent temperatures_. It is also assumed that the thermal camera has worked by default with emissivity one throughout this process. In this section, the temperatures of the objects contained in a TO are corrected by taking into account the emissivity and radiation effects. From here on, the corrected temperature will be denominated as the _real temperature_. Several targets comprising electrical tape and wrinkled pieces of aluminium foil are spread on the existing objects in the scene beforehand. Assuming that the target is a rectangle, the length of the sides of the electric tape and aluminium piece are in the ranges of 4 to 10 and 10 to 20 pixels, respectively, which in real dimensions are of 8 to 20 and 20 to 40 cm. As will be shown in the following subsections, these targets are necessary in order to make corrections derived from the reflected radiation and calculate the real emissivity of the objects. Let us suppose an _object_ O, detected and delimited in the colour and thermal orthoimages, which belongs to a certain facing on which an apparent average temperature of \(T_{object\_ap}\) has been detected. The apparent temperature correction is performed by employing the following steps. ### Step 1: Reflected Apparent Temperature

The reflected apparent temperature can be measured for one or several wrinkled pieces of aluminium foil placed on the _object_. This region can be delimited in CO when a high-frequency filter is applied to it. The aluminium target is usually placed in the geometric centre of each object (e.g., the centre of a wall). This piece is located automatically by means of a high-pass filter algorithm. This filter makes an image appear sharper and also emphasizes fine details in the image. After applying this filter, the wrinkled piece of aluminium foil will appear in the image as a region with rapid intensity changes within a non-texture background and will be easily detected. If there is more than one target on the surface of an object, the average temperature is calculated. When the emissivity is fixed to 1, it is assumed that radiation originating from outside or from other SEs is emitted by a diffuse reflector. The assumption of that distance from the camera to the object is ideally 0, thus making it possible to assume a null atmospheric effect.