Banner or Ad impression script, how to track it or counter

A metric used in web site analytics, this is the number of times a banner ad is downloaded during a specific period of time. Note that because the ad has been downloaded does not necessarily mean the user has even seen it, never mind taken any notice of it. There is also the issue of whether or not the ad has actually downloaded onto the browser screen before the user clicks on a link and moves off of the page, particularly if it is not in the viewable area.