Investigation of the Focal Plane of the Frontal-stray-light (FSL) Image of an Augmented-reality (AR) Device and Its Relation to the Observed Size of the FSL Image

The optical configuration of an augmented-reality (AR) device combines light rays from both the image source and the world side toward each eye of the person wearing the AR device, but some light rays emitted from the image source propagate toward the world side, and are referred to as frontal stray light (FSL). Consequently, people in front of the AR user can potentially identify the information in the AR image, which can cause privacy issues. To characterize the image caused by the FSL, two AR samples with different optical configurations are selected and the size of the FSL images observed on the world side are investigated. The observed size of FSL images for the two selected samples show different trends. As these different trends relate to the focal plane of the FSL image, methods to measure the focal plane are investigated. The measured values of the focal distance of the FSL image can explain these different trends, and can be used to estimate the observed size of the FSL image at an arbitrary position on the world side.