With progressive scan, an image is captured, transmitted and displayed in a path similar to text on a page: line by line, from top to bottom.
The interlaced scan pattern in a CRT (cathode ray tube) display completes such a scan too, but only for every second line. This is carried out from the top left corner to the bottom right corner of a CRT display. This process is repeated again, only this time starting at the second row, in order to fill in those particular gaps left behind while performing the first progressive scan on alternate rows only.
Such scan of every second line is called a field. The afterglow of the phosphor of CRT tubes, in combination with the persistence of vision results in two fields being perceived as a continuous image which allows the viewing of full horizontal detail with half the bandwidth which would be required for a full progressive scan while maintaining the necessary CRT refresh rate to prevent flicker.
This animation simulates a progressive and two interlaced images with the same line count - it is analogous to comparing NTSC with VGA. The interlaced images use half the bandwidth of the progressive one. The center image precisely duplicates the pixels of the progressive one, but interlace causes details to twitter. Real interlaced video blurs such details to prevent twitter, but as seen on the right, such softening (or anti-aliasing) comes at the cost of resolution. A line doubler could not restore the image on the right to the full resolution of the image on the left. Note - Some LCD monitors cause additional flicker in this image. The image on the right should have minimal flicker when viewed from a distance.
-- Only CRTs can display interlaced video directly — other display technologies require some form of deinterlacing.
Progressive or noninterlaced scanning is any method for displaying, storing or transmitting moving images in which the lines of each frame are drawn in sequence. This is in contrast to the interlacing used in traditional television systems.
The system was originally known as 'sequential scanning' when it was used in the Baird 240 line television transmissions from Alexandra Palace, England in 1936. It was also used in Baird's experimental transmissions using 30 lines in the 1920's.