1080i video is a category of video formats. The number 1080 refers to the number of lines of vertical resolution in a single frame of video, and the i represents an interlace scan pattern. 1080i video is one of the available modes of HD (high definition) television.
The interlace scan pattern used by 1080i video was designed as an improvement over older scanning methods that displayed an image by projecting a single line of pixels, each line below the previous one, until the whole surface area of the screen was filled. Interlace scanning scans and displays only the odd-numbered lines of the frame, followed within 1/30 of a second by the even-numbered lines, which are projected in the gaps left from the previous scan. Because human vision works by retaining images for a brief period of time in the brain or retina, the interlace scan process makes two separate scans of an image appear as one frame. In the case of 1080i video, this means that there are two fields of 540 lines each that are perceived as a single video image. Scanning 60 fields per second instead of 30 entire frames per second has numerous advantages, most notably less flicker, improved detail, and reducing bandwidth used by half.
1080i video works well for older cathode ray tube (CRT) televisions. However, the equipment needed to encode interlaced images is expensive and complex. Newer progressive scan formats are cheaper and offer no flickering as well as increased vertical resolution. Progressive scan displays transmit images by drawing each line of the frame in sequence. The difference between this method and the older scanning methods that necessitated interlacing is that television technology has surpassed cathode ray tubes.
Newer LCD or plasma televisions are progressive in nature because unlike CRTs, they do not project an image onto a screen but rather contain lamps that illuminate to form an image. These television sets have to convert 1080i video to 1080p - the letter p standing for "progressive". Unless they are converting 1080i video, those sets already show images in the sharpest resolution, 1080p.
While the very newest televisions can display 1080p video, their source material has yet to reach that level of resolution. High definition broadcasts are still mostly in the 1080i video format. In addition, these 1080p sets are not equipped to receive an outside 1080p signal, so even though consumers may be paying top dollar for the latest television technology, they will not be able to make full use of it until broadcasters and DVD manufacturers begin to output 1080p. Therefore 1080i video will remain a viable format until that happens.