# Influence of Material on the Density of a Point Cloud Created Using a Structured-Light 3D Scanner
## 2 Literature Review and Research Questions  
### Literature Review  
Structured-light 3D scanning technology is most often used in reverse engineering, quality control, control of the presence of parts on the equipment [18], and in general where it is necessary to achieve the capture of complex or otherwise difficult to measure objects(hot, soft, large, etc.) Although structured-light 3D scanning measurement does not achieve the most accurate results, the use of a coordinate measuring machine (CMM) for these objects would be inefficient [19]. Structured-light 3D scanners use so-called triangulation to capture the third dimension of an object. The principle of triangulation is based on the situation when the beam is reflected from the scanned object at a constant angle, so the distance of impact of the reflected beam on the sensor face is proportional to the distance of the scanned object from the sensor face. This means that neither the intensity of the incident beam nor the time of its flight is evaluated, but, rather, the place where the reflected beam hits is measured. For this reason, scanning is significantly more reliable and more resistant to interference, because the scannability condition is only the ability of the sensor's optical receiver to detect the impact of the reflected beam with an intensity that will be greater than the minimum detectable value [20]. The structured-light 3D scanner uses light to scan, so light scattering affects the resulting image. The light of the parallel rays incident on the plane interface will be parallel again. However, if the interface is not planar, the rays will bounce in all directions and, thus, scatter light. Therefore, if light is not reflected at either the planar or non-planar interface, the sensor would have nothing to capture and evaluate. Therefore, transparent or translucent materials cannot be scanned without surface treatment [1]. The color of the scanned object is closely related to light scattering and the scanning itself. Each color has its specific properties and, depending on the wavelength, is easier or harder to scan. Light is defined as a visible beam of an electromagnetic wave with a wavelength in the range of 380-780 nm. Visible light can be divided into seven spectral colors from the color with the shortest wavelength: purple, indigo blue, blue, green, yellow, orange, and red. When all of these colors are combined, colorless sunlight is produced. The human eye perceives the colors of an object as the reflected light or light passing through the object changes the wavelength and intensity of the light [21]. The coefficient of reflection or albedo indicates how much of the incident energy is reflected into the space, as follows [22]:  
\[\rho(\lambda)=(Er(\lambda))/(Ei(\lambda)), \tag{1}\]  
where \(Ei(\lambda)\) is the intensity of radiation incident on the surface of the object and \(Er(\lambda)\) is the intensity radiated back after reflection, corresponding to the wavelength of electromagnetic radiation. The coefficient of reflection depends on the wavelength of the incident radiation, the surface properties of the incident (ability to absorb radiation), and the three angles that describe the relationship between the light source \(\mathbf{L}\), the observer \(\mathbf{V}\), and the local orientation given by the normal n. The scalar product of vectors and, therefore, the reflectance function \(R\), is described by three scalar products of vectors, as follows [22]:  
\[R=R\;(\mathbf{n}\cdot\mathbf{L},\mathbf{n}\cdot\mathbf{V},\mathbf{V}\cdot\mathbf{L}). \tag{2}\]  
The surface reflectivity of a material is always somewhere between two extremes, namely the Lambert and ideal mirror surface [22]. A Lambert, or ideally matte, ideally diffuse surface reflects light energy evenly in all directions, and, therefore, the glow (brightness) from all directions is constant, i.e., it does not depend on the direction of view.