# Comparison of the Observation Errors of Augmented and Spatial Reality Systems
## 1 Introduction  
The use of 3D technologies such as virtual reality (VR) and augmented reality (AR) has improved in recent years [1; 2]. Technological advances have made it possible to capture the unseen [3; 4; 5]. These advances in technology allow visual aids through visualization to traditional blind tasks that rely on sensory approaches derived from clinical experience [6; 7]. In the dental field, its use for surgical guidance, navigation, and clinical education is being considered [2; 8; 9].  
AR devices allow users to see 3D-CG models displayed in real space in three dimensions and manipulate them freely. Evaluation of the accuracy of the displayed 3D-CG model is essential for clinical applications. For example, in dental care, high accuracy is required to take into account the sensitive sensation in the mouth [10]. Currently, mainstream AR devices are head-mounted display (HMD) types. A previous study evaluated the AR technology in HMDs [7].  
However, challenges with this technology have also been reported [11]. For example, several HMDs require the adjustment of the interpupillary distance (IPD) of the observer,which is processed for stereoscopic viewing. The IPD is different for each individual, and it needs to be readjusted for each person when several people use the same device, which is often time-consuming and labor-intensive. In addition, there seems to be an overlap of visual information. This occurs because the 3D-CG model overlaps a few centimeters away from the left and right eyeballs when observing the actual object.  
Several HMDs used in research are not manufactured for medical use. A typical example is the HoloLens2 (Microsoft, [PERSON], WA, USA), but the manufacturer recommends placing the 3D-CG model at least 1.0 m away [12]. In actual clinical practice, the distance between the surgeon and the surgical field is shorter than 1.0 m. The accuracy of 3D-CG models when the HMD is used within 1.0 m is valuable information.  
As a new AR device that differs from conventional HMDs, a 3D display that enables stereoscopic viewing with the naked eye has been developed [13]. A spatial reality display (SRD) (Sony, ELF-SRI, Tokyo, Japan, 2020) has an automatic IPD measurement functionality. The display recognizes the pupil of one person in front of it with the camera and provides stereopsis for the naked eye. Unlike HMDs, SRD Shows a 3D-CG model in front of the device. Figure 1 shows the positional relationships between the eyeball, the 3D-CG model, the AR device during the measurement, and objects (e.g., calipers).  
The manufacturer's recommended working distance for SRD is 30.0-75.0 cm. SRD has the potential to solve the challenges presented by HMD. In addition, the emergence of new system devices creates the possibility of using AR technology differently than before. By clarifying the impact of system differences on the accuracy of the displayed 3D-CG model, we can consider the criteria for selecting an AR device that meets our objectives. No studies have discussed the accuracy of 3D-CG models observed with AR devices within close range, which are required in clinical dentistry [14]. Although SRDs are beginning to be used in medical and dental education, their accuracies have not been compared with that of HMD [15]. Although SRD is beginning to be used in medical and dental education, its accuracy has not been verified [16]. In this experiment, four conditions were used for measurement: horizontal distance (horizontal) and depth and two postures, which were the sitting posture (SP) and the standing posture (StP).  
This study aimed to use two different AR device systems to evaluate the accuracy of the visibility of the 3D-CG models displayed stereoscopically in space.