Experiments with facsimile image transmission systems that used radio broadcasts to transmit images date to the 19th century. It was not until the 20th century that advances in electronics and light detectors made what we know as television practical. A key problem was the need to convert a 2D image into a "1D" radio signal; some form of image scanning was needed to make this work. Early systems generally used a device known as a "Nipkow disk", which was a spinning disk with a series of holes punched in it that caused a spot to scan across and down the image. A single photodetector behind the disk captured the image brightness at any given spot, which was converted into a radio signal and broadcast. A similar disk was used at the receiver side, with a light source behind the disk instead of a detector.