Visual Acuity

Reading is a highly visual task that requires rapid and accurate translation from orthography to phonology. If the reader is unable to perceive the letters clearly, then the reading experience will suffer. The reading performance depends on the visual acuity, clarity of vision which is dependent on optical and neural factors like the sharpness of focus within the eye, the sensitivity of nervous elements and the interpretative ability of the brain. Ophthalmologists use the ability to see and read the small letters at a distance, as a clinical method for measuring the visual function which is usually expressed in a fraction of 6/6 (or 20/20) which means the viewer can read at a distance of 6 meters (20 feet). (this video explains it well)

Visual acuity produced by AR headsets can be calculated based on pixels per degree. It is the number of pixels per degree it present to the eye, the number of pixels in the horizontal display line has to be divided by the horizontal field of view provided by the optics of the headset (lens). E.g. In a display with 1280 x 800 px, the pixels per eye are 640 x 800 px, and with a FOV of 90 degrees the PPD comes out to be 7.1 (640/90). This is way too low as compared to the retina resolution of the eye, the PPD of the human fovea is approximately 60 PPD. A lower pixel density can cause blurring of text, pixelation and screen door effect (the visible fine lines between pixels on a display when seen up close). Assuming the optics is perfect, the current headsets are in the range of 20/80 which is low in comparison to a human eye. Additionally, the magnification of the optics (in some cases) is never perfect, so there is a reduction in actual PPD which further reduces the acuity.