# A Stray Light Detection Model for VR Head-Mounted Display Based on Visual Perception
## 1 Introduction
From the viewpoint of vision and optics, regardless of the difference in the visual field of view and light transmittance provided by VR and AR, VR/AR HMD still face lots of common problems, such as vergence-accommodation conflict (VAC), screen door effect (SDE) [18], image distortion [19], pupil swim, small eye-box [20], stray light interference, chromatic dispersion, non-uniform color and brightness [21], latency [22], cybersickness, motion sickness [23, 24], etc. Due to their larger field of view, VR users may more easily encounter these phenomena. Hence, image quality variance, influence, and acceptance of VR HMD on human visual perception are worthy of attention, research, and discussion and are expected to extend the experience of AR display applications. The VR viewing optical module is the most critical core in the VR HMD system, and the control of optical quality is essential to ensure visual quality. There have been many types of research and well-established standard documents in the consumer electronics industry to control optical quality in camera modules. Signal receivers are the most fundamental differential factor when comparing camera modules with VR. Digital camera modules have a charge-coupled device (CCD), or complementary metal oxide semiconductor (CMOS), and the captured signal can be further processed by the software and firmware, based on the requirements and image location, to perform noise suppression and make other adjustments. For the visual optics of VR, the signal receiver is human vision. The eyeball can passively receive all the light from the output of the VR viewing optical module. The human eye contains a series of physiological reflexes. The eyeball has the ability to accommodate the amount of incident light, to focus on different image surfaces of more than ten centimeters to infinity, tremble unconsciously, and can quickly scan an entire visual field of images [25, 26, 27, 28, 29]. These differences bring difficulties in detecting and suppressing stray light when producing VR display modules. Stray light generally refers to all rays generated along unexpected paths in an optical system or light noise that is not required by the system, usually caused by multiple interface reflections, refractions, or material scattering. Most of today's HMDs use a Fresnel lens with a convex lens function as the imaging lens, which increases refractive power, reduces weight, reduces volume, and saves a lot of material costs. However, in the mass production of the Fresnel sawtooth structure, in addition to the expected loss of optical efficiency caused by the ineffective surface, there is also the problem of connection surface inclination. It increases the light-receiving area of the ineffective area, thus resulting in more ineffective stray light generation. When the shape of the sharp corners of the Fresnel sawtooth is farther from the ideal sharp point, the more stray light is generated. The stray light produced by the Fresnel lens is very easy to detect, especially when the brightness and contrast of the showing pattern are relatively high. Therefore, it is dubbed, by VR developers and consumers, as [PERSON]'s rays or the ring effect. Observing the current development trends of many electronic products, it is not difficult to infer that there will inevitably be an increasing expectation for better optical system performance in the requirements of visual optical modules. According to the Fresnel equation [30], it can be expected that the qualitative and quantitative aspects of stray light will become too important to be ignored with increase in the complexity of lenses, films, and other components of visual optical modules and optical transmission interfaces in the future [31]. The performance of optical imaging systems will be reduced due to the intrusion of stray light from inside and outside of the system's field of view. Most research results have contributed to the camera [32; 33; 34; 35; 36]. ISO 12231 standard clearly describes the source of stray light in the field of electronic imaging.