# Comparison of the Observation Errors of Augmented and Spatial Reality Systems
## 4 Discussion  
In this experiment, two different AR device systems, HMD and SRD, were used to evaluate the visibility of 3D-CG models displayed stereoscopically in space. The Bland-Altman analysis suggested good agreements between the Model, HMD, and SRD measurements. The SRD and HMD measurements showed a minor variance. This experiment showed significant differences in the horizontal measurements in the SP and StP for the HMDs. The clinical accuracy of HMD for up to 5.0 mm has been previously reported [16; 18].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & \multicolumn{2}{c}{**Distinctness**} & \multicolumn{2}{c}{**Three-Dimensionality**} \\ \cline{3-6}  & & & **r** & **\(p\) Value** & **r** & **\(p\) Value** \\ \hline \multirow{4}{*}{HMD} & \multirow{2}{*}{SP} & Horizontal & 0.484 & 0.079 & 0.232 & 0.426 \\ \cline{3-6}  & & Depth & 0.198 & 0.498 & 0.026 & 0.931 \\ \cline{2-6}  & \multirow{2}{*}{StP} & Horizontal & 0.037 & 0.900 & 0.017 & 0.954 \\ \cline{3-6}  & & Depth & 0.185 & 0.527 & 0.122 & 0.678 \\ \hline \multirow{4}{*}{SRD} & \multirow{2}{*}{SP} & Horizontal & 0.128 & 0.663 & 0.098 & 0.738 \\ \cline{3-6}  & & Depth & 0.285 & 0.323 & 0.230 & 0.429 \\ \cline{1-1} \cline{2-6}  & \multirow{2}{*}{StP} & Horizontal & 0.137 & 0.642 & 0.119 & 0.685 \\ \cline{1-1} \cline{2-6}  & & Depth & 0.037 & 0.900 & 0.043 & 0.885 \\ \hline \multicolumn{2}{c}{HMD} & \multicolumn{2}{c}{Three-dimensionality 0.413} & 0.142 & & \\ \hline \multicolumn{2}{c}{SRD} & \multicolumn{2}{c}{Three-dimensionality 0.927} & 0.000 & & \\ \hline \hline \end{tabular}  
* [PERSON]’s correlation coefficients (\(\alpha\) = 0.01).
* SRD + distinctness vs. SRD + three-dimensionality (\(p\) = 0.000).  
\end{table}
Table 3: Measurement error and correlation coefficients of distinctness and three-dimensionality.  
Figure 15: Three-dimensionality and distinctness of HMD and SRD compared with those of the Model.  
The SRD showed no significant difference across all the measurements; the range of the differences from the Model measurement was \(-\)0.58-0.02 mm. The VAS scores showed significant differences in the distinctness of HMDs and SRDs. It has been reported that 3D objects displayed on HMDs can overlap with the actual surgical field and block the view [4]. It is thought that the overlap between the caliper tip and the object caused a variation in the measurements. However, the measured values did not show any correlations between the distinctness and the three dimensions. Problems with convergence distance and focal length have been reported when using HMDs [19], and HMDs at distances closer than 1.0 m can cause visual strain. The positional relationship between the eye, the AR monitor, and the object at a close range has also been reported to cause convergence and adjustment conflicts [20].  
In this experiment, the object-head position was set at approximately 30.0 cm for SP and 70.0 cm for StP. The suboptimal distance of the HMD may have influenced the large measurement values. The recommended working distance was used for the SRD.  
SRD may reduce the effects of congestion and adjustment conflicts that occur with traditional HMDs.