Training and simulation A key application for HMDs is training and simulation, allowing to virtually place a trainee in a situation that is either too expensive or too dangerous to replicate in real-life. Training with HMDs covers a wide range of applications from driving, welding and spray painting, flight and vehicle simulators, dismounted soldier training, medical procedure training, and more. However, a number of unwanted symptoms have been caused by prolonged use of certain types of head-mounted displays, and these issues must be resolved before optimal training and simulation is feasible. Performance parameters Ability to show stereoscopic imagery. A binocular HMD has the potential to display a different image to each eye. This can be used to show stereoscopic images. It should be borne in mind that so-called 'Optical Infinity' is generally taken by flight surgeons and display experts as about 9 meters. This is the distance at which, given the average human eye rangefinder "baseline" (distance between the eyes or Interpupillary distance (IPD)) of between 2.5 and 3 inches (6 and 8 cm), the angle of an object at that distance becomes essentially the same from each eye. At smaller ranges the perspective from each eye is significantly different and the expense of generating two different visual channels through the computer-generated imagery (CGI) system becomes worthwhile. Interpupillary distance (IPD). This is the distance between the two eyes, measured at the pupils, and is important in designing head-mounted displays. Field of view (FOV) – Humans have an FOV of around 180°, but most HMDs offer far less than this. Typically, a greater field of view results in a greater sense of immersion and better situational awareness. Most people do not have a good feel for what a particular quoted FOV would look like (e.g., 25°) so often manufacturers will quote an apparent screen size. Most people sit about 60 cm away from their monitors and have quite a good feel about screen sizes at that distance. To convert the manufacturer's apparent screen size to a desktop monitor position, divide the screen size by the distance in feet, then multiply by 2. Consumer-level HMDs typically offer a FOV of about 110°. Resolution – HMDs usually mention either the total number of pixels or the number of pixels per degree. Listing the total number of pixels (e.g., 1600×1200 pixels per eye) is borrowed from how the specifications of computer monitors are presented.