# Improved Autocollimator for Roll Angle Measurement with an Enlarged Measuring Range
## 4 Discussion and Conclusions  
This paper proposes an improved autocollimator for roll angle measurement, which can cover the whole range of 360\({}^{\circ}\) and maintains the optical structure of the classic autocollimator while incorporating a WP working in transmissive mode as a roll angle sensing element to dissociate the collimated beam into two beams. The roll angle of the WP can be solved through the moving paths of the two light spots on the CMOS. Furthermore, an experimental prototype was constructed, whose performance was estimated. The experimental results reveal that the improved autocollimator has a stability of 3.62 arcsec within 10 min, a repeatability error of 1.72 arcsec, an accuracy of \(\pm\)13.55 arcsec over a range of 360\({}^{\circ}\), and a resolution of better than 10 arcsec. It can be used for roll angle measurement where a large measuring range is required, such as measuring the roll angle of a simulated aircraft in a wind tunnel, calibrating the rotation accuracy of a rolling table, shaft, or mechanical arm, and guiding the assembly of structural parts including the alignment of the joints of two pipelines.  
Despite the positive advances of the improved autocollimator, further research is needed. Firstly, the measurement error owing to random and systematic errors needs to be reduced, which can be performed by digital filtering, error modeling, and compensation. Secondly, the roll angle resolution needs to be improved. According to the geometric relationship shown in Figure 2b, there are two remedies. One is to prolong the rotation radius of light spot 2, which can be achieved by increasing the focal distance of the OL and the wedge angle and refractive index of the WP. Another is to enhance the displacement resolution of light spot 2, which can be realized by optimizing the image-processing algorithm, as well as the shape and size of the AS. Thirdly, the experimental prototype needs to be industrialized, which can be accomplished by using smaller optical components and designing dedicated opto-mechanical structures. Error reduction, resolution improvement, and industrialization will be the focuses of our next steps.  
Figure 7: Actual test of linear stage: (**a**) experimental setup and (**b**) results.  
**Author Contributions:** Conceptualization, Y.G.; data curation, Y.Z. and J.J.; formal analysis, Y.G. and Y.Z.; funding acquisition, Y.G.; investigation, Y.Z. and J.J.; methodology, Y.G. and Y.Z.; project administration, Q.Y., H.D., L.W., and D.H.; resources, Q.Y., H.D., L.W., and D.H.; software, Y.Z.; supervision, D.H.; validation, Y.Z.; visualization, Y.Z.; writing--original draft, Y.Z.; writing--review and editing, Y.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Natural Science Basic Research Program of Shaanxi Province (2024)C-YBQN-0705) and the Shaanxi Province Postdoctoral Science Foundation (2023 BSHEDZZ251).  
Not applicable.  
The original contributions presented in this study are included in the article; further inquiries can be directed to the authors.  
We thank [PERSON] for his support in processing the wedge prism (WP).  
The authors declare no conflicts of interest.