# A Stray Light Detection Model for VR Head-Mounted Display Based on Visual Perception
## 2 Methods  
A total of 40 observers, corrected for hyperopia, or near-sightedness, participated in the study. Their binocular parallax was limited to not more than 200\({}^{\circ}\) to avoid the different image problems induced by a significant difference in spectacle lens magnification. The observers, 22 males and 18 females, that were recruited had experience in more than two VR HMDs. They were aged between 23 to 35 years of age and had normal color vision. To ensure the accuracy of the experiment, the day before the task, the observers were required to avoid alcohol abuse, excessive caffeine intake, staying up late, etc. Any glasses worn were not to have an anti-blue light function, and contact lenses were to be without dilated pupils or embedded with any style of pattern. In the psychophysical experiment, three kinds of VR HMDs were selected, as shown in Figure 1: Oculus Quest, Vive Focus Plus, and Vive Focus. The VR viewing optical modules of these three devices were all designed with Fresnel lenses. The monocular display module was an OLED display with a resolution of 1440 \(\times\) 1600 pixels. The advantage of the VR HMD with an OLED display as the light source was that the partial black background image would not have the problem of light leakage from the LCD backlight module, so the experiment could eliminate the stray light generated by the display element itself. The measurement was carried out by capturing the image of the VR viewing optical module. The FOV of the current mainstream VR HMD viewing optical module is about 110\({}^{\circ}\), so the FOV range to be obtained in the experiment had to be at least greater than or equal to that. Therefore, a wide-angle lens (Theia ML-183-M) with a horizontal field of view (HFOV) of up to 115\({}^{\circ}\) was used with a C-mount industrial camera (IDS UI-3590 CP-C-HQ). The power and the operation of the camera were controlled by the USB port of a personal computer. The experiment and measurements were conducted in a low-light dark environment with a temperature of 25 \({}^{\circ}\)C and a humidity of 60%. Three independent variables containing stimuli were investigated in the research to better know how they affect the threshold of stray light perceived by the observer: visual field, grayscale, and the VR HMD device. The stimulus of the visual field was the radius of the light source displayed on the image. The size of the stimulus was defined by adding the  
Figure 1: The VR HMDs for the experiment. (**a**) Oculus Quest; (**b**) Vive Focus Plus; (**c**) Vive Focus. inside and outside diameter pixels and dividing by 4. For example, the circle image number of the largest visual field stimulus was 750 ((1550 pixels of outside diameter + 1450 pixels of inside diameter)/4), as shown in Figure 2. Sixteen ring patterns, with sizes varying from 0 to 750, were acquired, and the difference between the inside and the outside diameter was 50 pixels. Figure 2 illustrates the images with different visual field stimuli. For grayscale stimuli, all the color depths of the VR HMDs were 8-bit RGB values, and, thus, 20 grayscales, ranging from 10 to 200, with ten intervals were designed and are shown in Figure 3. The test image background was set as 0 to exclude the stray light from the device itself. Both visual field and grayscale stimuli were controlled by the images presented on the VR HMD. In addition, the dependent variable was the threshold of the stray light perceived by the human eye. According to the observer's subjective evaluation, the visible stray light's grayscale value was recorded for each stimulus with different independent variables. In total 960 images (16 stimuli visual field \(\times\) 20 stimuli grayscale \(\times\) 3 VR HMD devices) with a resolution of 1600 \(\times\) 1600 pixels were tested in the experiment. The information on the parameters is listed in Table 1. The image VR content from three devices was identical and established by Oculus and Vive Wave software development kit (SDK) with Unity 3D engine.