# Image Quality Assessment of Augmented Reality Glasses as Medical Display Devices (HoloLens 2)
## 4 Discussion
### Limitations  
A key limitation of this study is the fact that some of the parameters affecting display image quality were not examined, such as pixel errors, image artifacts, and temporal responses. However, these factors are irrelevant to HoloLens 2 owing to its laser beam technology.  
HoloLens 2 produces images by continuously scanning a display with a laser, and the lack of a fixed pixel grid prevents pixel errors present in conventional displays. Additionally, static image artifacts (ghost images) are absent because the projection is continuously updated.  
To evaluate the temporal response of conventional display devices, their rise and fall time constants are measured to determine temporal resolution. In HoloLens 2, image content is generated through laser sweeps, with lasers activated and deactivated between pixels. Consequently, the rise and fall time constants are several orders of magnitude shorter than the laser sweep duration. Thus, temporal resolution depends on the laser scan frequency, making it critical for establishing the temporal resolution of HoloLens 2. The manufacturer specifies a scan frequency of 120 Hz, sufficient to display 60 contrasts per second. This exceeds the temporal resolution of the human visual system within the field of view of HoloLens 2 across all common scenarios, leading us to conclude that the temporal resolution is adequate [29].  
Additionally, the luminance of HoloLens 2 is highly angle-dependent owing to its laser beam projection technology. This study selected a measurement angle of 90\({}^{\circ}\), which ideally aligns with the user's eye position when wearing HoloLens 2. This angle is also used in conventional medical display systems; therefore, the corresponding requirements are tailored to this orientation. Existing methods address the angular dependence of see-through displays using light-emitting diode (LED) or organic light-emitting diode (OLED) technologies [30]. However, their application in the medical field remains unestablished and necessitates specialized measurement equipment. Moreover, it remains unclear whether these methods are applicable to see-through displays that utilize laser beam projection technology. Future research should, therefore, focus on evaluating the angle dependence of HoloLens 2.  
A further limitation concerns the approach used to assess color uniformity. In this study, measurements were performed by translating the measurement device across the display while maintaining a fixed orientation. Although this method enhances comparability with conventional display devices, it does not fully emulate the viewing conditions experienced when wearing AR glasses. A more realistic approach would involve positioning the measuring device at the typical eye level and tilting it to simulate various viewing angles. Future investigations should adopt this alternative method to more accurately capture user-perceived color shifts.  
Finally, this study did not analyze the spectral emission characteristics of HoloLens 2. The device utilizes an RGB laser system with discrete wavelengths, potentially including emissions in the eye-damaging blue light spectrum. Given that prolonged use may be necessary for medical applications, future studies should address these potential risks.