## 4 Conclusions  
This work aims to analyze the feasibility of non-contact measurement with the conoscopic holography (CH) technique in the measurement of ceramic parts manufactured by SLA. Originally, this study arose from the need to manufacture lightweight and customizable versions of ceramic reference spheres, widely used for the verification and calibration of non-contact metrology and reverse engineering equipment. The feasibility study of the CH measurement has been established in terms of dimensional and geometrical accuracy, thus by studying the dimensional (diameters) and geometrical deviations (form errors) made by the CH over the surface finish of ceramic spheres. This study involves measurements of the diameters and form errors of 18 spheres using a CH sensor. The spheres were fabricated through an additive manufacturing process (SLA) based on the ultraviolet laser photopolymerization of alumina (Al2O3) particles. The objective was to evaluate the optical performance of these components using conoscopic holography. From the presented data, it is observed that the dimensional quality, determined by the diameters of the spheres scanned from the point clouds, shows deviations of approximately 50 \(\upmu\)m relative to the reference values obtained by contact measurements. This value can be considered a threshold for determining the precision level suitable for potential industrial applications of different optical sensors [9]. However, it is evident that this dimensional quality is independent of the filtering level applied to the point cloud, as the mean deviations in diameters vary by only about 2 \(\upmu\)m.  
On the other hand, applying a filter to the standard deviation effectively improves the values obtained for form error, with this geometric parameter decreasing as the level of filtering increases. The form error reaches 22 \(\upmu\)m for a \(\sigma\) filter (the most extensive filtering performed in this study). However, it is worth noting a qualitative leap in the number of points removed when this filter is applied (25%), even though the most substantial improvement in the point cloud's quality is achieved with a 3\(\cdot\)\(\sigma\) filter (reducing the form error by 75%). A final conclusion is that, unfortunately, with the methodology followed and the rigorous experimentation carried out, this non-contact measurement technology does not offer sufficient quality for the parts manufactured with this process (SLA) and this \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(\sigma\) Filter & \(\sigma\) & 2\(\cdot\sigma\) & 3\(\cdot\sigma\) & **No Filter** \\ \hline Mean \(\mathcal{O}\) deviation [\(\upmu\)m] & 50 & 51 & 52 & 50 \\ \hline Mean Fe deviation [\(\upmu\)m] & 22 & 41 & 74 & 298 \\ \hline Improvement Fe deviation & 46\% & 45\% & 75\% & - \\ \hline Points number & 160,096 & 212,465 & 221,688 & 223,465 \\ \hline Variation points number & 25\% & 4\% & 0.8\% & - \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Mean diameter (\(\mathcal{O}\)) and form error (Fe) deviations with CH sensor. material (alumina) due to the impossibility of achieving a signal-to-noise ratio parameter higher than 50%. This leads precisely to the need to analyze the feasibility of other non-contact technologies. An additional conclusion, which is inferred directly from the most accurate measurement data (by contact, with the CMM), is that the SLA ceramic process is not suitable for the manufacturing of high-precision parts that can be used as reference elements, unless mid-range or high-range optical equipment is used. In any case, work remains to be completed for future research on the use of equipment with other optical metrology technology (laser triangulation, structured light, etc.) given that, even with the application of filters, stereolithography (SLA) is not valid in reference elements for low-range precision optics if the system chosen is conoscopic holography.