# A Stray Light Detection Model for VR Head-Mounted Display Based on Visual Perception
## 1 Introduction
It could be foreign material between the light source and the lens, such as fog, dirt on the windshield or lens surface, inter-reflection from the surface of optical components, non-uniformity, which can be reduced by an anti-reflection (AR) coating. Stray light might also be induced by the lens, aperture, shutter, and the parts of the exposure mechanism in the camera, spherical and coma aberration from the lens, and light leakage from the system. Previous studies have designated various types of stray light generated by different characteristics, such as lens flare, aperture ghosting, and veiling glare, with quantitate standards, such as ISO 9358 [37], ISO 18844 [38], and IEC 62676-5 [39]. The following articles revealed the adverse effects of stray light in different optical systems, and corresponding improvement methods were proposed. The stray light of the optical viewing system from a helmet-mounted display might distract visual attention. Therefore, the far-field model in optical simulation was adopted to search out the position where the light source's transmission path for the image and the glare were separated to further shield it [40]. For astronomical or surveillance camera systems, which are often placed outdoors, glare due to water droplets attached to the lens surface is likely to blur the image. The entrance position of the light attached by water droplets could be detected and closed by placing an LCD on the position of the camera's light barrier as a shutter array [41]. Using a light-field camera to capture 4D light field instead of general 2D image photography could effectively eliminate stray light signals of various light-emitting positions and apply image synthesis to remove the stray light of the original 2D image [42]. Compared with the stray light of a camera that could be blocked with physical shielding, absorption, light reduction, polarization control, and image signal processing to suppress stray light for post-processing of images, the research methods for VR/AR stray light are more limited. Some researchers designed the VR optical module based on the architecture of polarized catadioptric optics. In the simulation software, an ideal lens without aberrations was utilized instead of the human eye, and the image surface was used as a simplified retina. The analysis was carried out based on the three primary ghost paths caused by interface reflection and fitting problems of the degree of polarization. The light spot's size on the image surface of the ghost image was considered in the function for curved surface optimization so that the stray light energy was evenly distributed as low noise during the imaging of the human eye's retina and this reduced the chance of its being noticed [43]. At present, the primary sources of stray light in VR are all generated within the effective diameter from the lens of the optical system. Since the image light source and the stray light source have different light emission positions, there is still no way to optimize through image processing. Therefore, designing a fast and efficient stray light detection method, based on human perception, was the primary purpose of this research. The study combined psychophysical experiments and image-based measurements from three VR devices to establish a set of stray light detection methods based on visual perception. The stray light threshold can be clearly defined by introducing human factors and applying artificial intelligence (AI). It can also be used to design measurement equipment and provide optimized parameters for the VR viewing optical designer when setting the merit function with a goal. The study makes the quantitative evaluation of stray light a critical indicator of image quality and VR/AR content design.