So if you think about this, HMDs have various needs that need to be specified, for example you need to say what the image sources, which make HMDS different, you have to specify how the image, what the image is being displayed, for example in which direction you are looking through and if you know in which direction you are looking then the system can send the right image, appropriate image to the display and that monitoring is done by a tracking system then a system actually view the images that are being selected this upon what tracking system sees so you need a viewing system and then of course helmet which houses all this and sound system which we are not going to be talking about here. Actually nor are we going to be talking without helmet, we are only talking about two or three things, which are of interest here.

---