# Perceived Brightness and Resolution of Holographic Augmented Reality Retinal Scan Glasses
## 4 Discussion
Since we found an average perceived resolution threshold of 7.2 cyc/deg across all trials, we conclude that the perceived resolution was limited by the optical resolution of the RS glasses and not the visual perception of any of the participants. Our results indicate that the background conditions did not have a notable influence on the perceived resolution within the range of conditions we tested. This could, however, still be the case for other background parameter ranges. As expected, the RS luminance andthe ambient contrast ratio, which was above 0.5 in all trials, did not influence the perceived resolution thresholds, despite varying over several orders of magnitude. These findings agree with the previous literature in which it is argued that the neural response in the visual pathway saturates, and further contrast increases do not lead to improved perceived resolution thresholds [7]. We see our study as a first step toward more human-centered characterizations of augmented reality glasses and recognize the potential for follow-up studies in additional parameter ranges. In particular, the RS luminance and ambient contrast ratio could be decreased in a future study to values around and below 0.5 to also investigate the effect of contrast levels on the perceived resolution. Since calculating and setting the ambient contrast ratio of retinal scan glasses stimuli are only possible with the correct equation for the RS luminance, a follow-up experiment that varies stimulus contrast in addition to stimulus resolution would further illustrate the benefits of our theoretical derivation. We neither measured nor controlled for the polarization of light from the retinal scan glasses. The influence of the linear polarization from the laser source on the visual perception of retinal scan displays could be an interesting aspect for further investigation, especially when compared with conventional display architectures. We also measured the RS glasses' optical display resolution according to the IEC 61947-2 standard [20] and found a maximum value of 3.4 cyc/deg for monochromatic green images. In our psychophysical study, we found a perceived resolution threshold of 7.4 cyc/deg for monochromatic green images. The perceived resolution threshold adds additional interpretability to the IEC 61947-2 optical resolution threshold, which only uses a modulation depth of 30% to determine the spatial frequency cutoff, independent of visual perceptibility. Knowing the number of pixels per degree that humans can distinguish in the glasses helps in taking better-guided design decisions for future prototypes; for example, for finding the necessary field-of-view and holographic combiner area size to show image content with a certain desired size and resolution. Furthermore, in accordance with the ideas presented in Section 2.1.2, we expect the ratio between the perceived and optical resolution to be higher for retinal scan displays than for conventional Lambertian displays. A promising next step to test this claim would be to conduct a second, comparative study with a Lambertian augmented reality display that only differs in its optical architecture and is otherwise as identical to our retinal scan glasses prototype as possible. The BML500P prototype used in this study still had some known limitations. In Figure 4, one can see brightness inhomogeneities, particularly in the top left corner of the green and blue stimuli. Such inhomogeneities typically arise from local variations in the diffraction efficiency of the HOE. This is a manufacturing issue that will be resolved in the future. The second limitation was the small exit pupil of the single eyebox prototype. The RS display is only visible if the narrow light beam fully passes through the pupil. If this alignment is lost, the image is not visible anymore. Therefore, we added a 5 min fitting procedure for each participant at the beginning of our study. The nose bridge of the glasses could be quickly adjusted to fit to the pupil position of each participant (see Figure 1, left).