# Reflectance Transformation Imaging as a Tool for Computer-Aided Visual Inspection
## 3 Dynamic Perceptual Assessment for RTI-Based Visual Inspection: Methodology  
### Experimental Surface Samples and RTI Acquisition Set-Up  
As stated before, the adjustment of the RTI acquisition and modeling parameters have to be correlated with the type of inspected surface. For example, a homogeneous surface with Lambertian reflectance will not require a high density of lighting positions, with a low number of images distributed in the hemispherical space \((\phi,\theta)\) allowing one to correctly estimate the angular reflectance and the opposite for when the local reflectance is heterogeneous or more complex. We therefore chose three surfaces samples with different properties and reflectance behaviors, noted in the following as \(S_{1-3}\). Sample \(S_{1}\) was a Lambertian-type surface of paper material. Sample \(S_{2}\) was a sandblasted surface (Spectralon(r) SRS-05-020 Diffuse Reflectance Chemically Inert Standard) with a reflectance value of 20%. Sample \(S_{3}\) was a brush-finished metallic surface, inducing an anisotropic texture. These surface samples are illustrated in Figure 2.  
The RTI acquisitions were carried out using an in-house developed set-up (Figure 3). The system is based on a powerful white LED light source mounted on a motorized hoop, allowing one to position the light in the (\(\theta\),\(\phi\)) space. The imaging sensor is a monochromatic 2/3 active pixel-type CMOS sensor with a resolution of 12.4 Mp (4112 \(\times\) 3008). A precision micro-imaging modular and motorized magnification and focus has been implemented to be able to adapt the field of view and the focus. The lower lens and other optic components are fully modular, allowing one to cover a very wide range of the measurement scale, the main limitation being the difference between the maximum and minimum magnification  
Figure 2: Surface samples: (**a**) \(S_{1}\) = industrial paper, (**b**) \(S_{2}\) = Spectralon® Diffuse Reflectance Standard (SRS-05-020), and (**c**) \(S_{3}\) = metallic brushed or polished surface.  
(\([\times 1,\times 12]\)) and the space available to position the samples to be measured (maximum of approximately \(10\times 10\) cm). The device can be fully controlled with a user interface, allowing one to adjust all the RTI acquisition parameters [38]. In addition, the system is modular, and a specific light wavelength for RTI acquisition on particular surfaces, such as transparent or semi-transparent (varnished) surfaces, can easily be implemented.  
In particular, the acquisition parameters associated with the light energy emitted are essential in the RTI approach. Their adjustment requires special attention, especially since this adjustment also depends on the level of magnification. Indeed, the illumination parameters can strongly alter the quality of the acquisitions and therefore consequently the analysis performed from these data [52]. Generally, the exposure time is, in practice, arbitrarily chosen by the operator in charge of the acquisition, who tries to reach a compromise between the number of saturated and underexposed pixels for all acquisition angles. For this experiment, we therefore carried out a preliminary step to optimize the illumination parameter settings for each of the surface samples of the study according to the methodology presented in [52].