Interlaced scanning
From The Glossary of Digital Photography
A technology used in video and television to display images electronically. The television image is made up of a series of lines (525 in the case of analog television in the United States). The picture is divided into two fields: one made up of even-numbered lines and one made up of odd-numbered lines. The fields are presented one after the other instead of both at the same time. Because of the persistence of human vision and the speed at which the fields are presented, the fields appear to blend together into one image. Each field is presented for 1/60th of a second, so it takes 1/30th of a second to present an entire image. When photographing images on a television or monitor, this phenomenon needs to be taken into consideration. Usually, selecting a shutter speed longer than 1/30th of a second will minimize the scan bar that will show up at faster shutter speeds.
