## 3 The Human Role in Teleoperation
### The Human Eye and the Real Scene Resolution  
Human visual acuity, that is the power to resolve fine details, relies on optics and neural factors. A person with \"normal\" visual acuity, i.e., 6/6 vision in meters (or 20/20 in feet), can discriminate the letter E in the Snellen chart at a distance of six meters. Given that the size of these optotypes is subtended in a visual angle of five arc minutes and the eye can resolve the gap between the five horizontal lines of the E letter, i.e., 1/5 of the arc, the human visual acuity is one arc minute.  
At a distance of six meters (or 20 feet), the eye can detect an interval lower than 1.75 mm between two contours. The discrimination of a line through optics suggests at least three photo-detectors (stimulated, not stimulated, and stimulated), so for each arc minute, there is a photo-detector. Thus, as one arc minute is equal to 1/60 degrees, there are at least 60 photo-receptors/degree in an angle subtended at the fovea [35] (an analogy with a camera could be 60 pixels/degree).  
The retina of the eye is composed of _cone_ cells that are sensitive to color and _rod_ cells that are sensitive to low light (about seven million cones and 120 million rod cells). It includes the fovea, which is a small pit region of the retina (1.5 mm wide) with a high density of cone cells (exclusively) and where the visual acuity is higher. Fixating on an object implies the movement of the eyes that makes the image fall into the fovea. The result is the sharp central vision essential for human tasks like reading or driving.  
Resolution acuity enables identifying two very close lines or contours, and there are people whose vision exceeds 6/6. Human grating resolution ranges from one to 0.3 arc minutes [36; 47]. Related also, _detection acuity_ refers to the ability to detect small elements in a well contrasted scenario, e.g., black points on a white background. Studies show that the human eye can detect a single thin dark line against a white background uniformly illuminated and subtended in just 0.5 arc seconds [48; 49] or 0.0083 arc minutes. It is approximately 2% of the diameter of the fovea's cones, which is a mechanism of the visual cortex and not exclusively based on the structure of the retina. Detection acuity seems to result from a spatial temporal averaging process involving the retina's peripheral region (even where rods and cones present a decreasing density) [50]. Considering that the eye's fovea has at least 60 photo-receptors/degree, an ideal display would have, at least, a resolution of 60 pixels/degree to stimulate the cells of the fovea. Of course, in the fovea's periphery, there is a decrease of photo-receptors; however, no one knows at what region of the display the user will look. Therefore, hypothetically, the ideal display would provide a resolution of 12,600 \(\times\) 8100 pixels to approximate human's vision (considering both the eye's FOV, 60 pixels/degree \(\times\) (210\({}^{\circ}\) (H) \(\times\) 135\({}^{\circ}\) (V)) [37]).  
Assuming that an immersive remote visualization system works like a _video see-through HMD_, whose cameras follow the exact user's head orientation, it is possible to analyze such a system without image transmission issues. Let us consider a setup composed of an HMD attached to cameras that stream video from the real world in front. _Video see-through HMD_ may acquire images of the real scene using miniature digital cameras and present them through the HMD's LCD or OLED displays. Then, the viewing optics of the HMD adapt these images to the human eye, using an eyepiece lens. Ideally, a _video see-through HMD_ should present images with resolutions similar to the real ones, but in practice, that may not be the case. Therefore, the perceived resolution of the real scenario is limited by the resolution specifications of either of these three system components: the video cameras, the HMD display, or the HMD's viewing optics.