Interlaced

Curvy Wurvy

Interlaced scanning is more or less an electronic equivalent of a much earlier trick used to make film smoother. Way back in the heydays of film, you would have a reel running the film at 30fps, with a flashing light timed to flash thorough each frame as it passed by the lens. This created an atrocious looking "stutter" to the moving image.

To overcome this deficiency, the frame rate needed to be increased. The problem was that at the time, the machinery wasn't capable of going any faster, so moving the film faster wasn't going to do it. The workaround that they came up with was to flash each frame on the screen twice, thus causing an effective doubling of the frame rate. This made for a smoother moving action on screen, but it introduced "flicker" into the image.

Interlacing

Interlaced scanning is pretty much the same idea, but implemented for a different reason. Television runs at 30Hz, or 30 frames per second. Early CRT were not able to energize the entire screen surface at an even rate, and the problem was compounded by the fact that the magnetic deflection which was used to cause the electron beam to sweep the screen also caused a weakening of the beam.

As a result, the top half of the screen would end up being brighter than the bottom half. Interlaced scanning was used to overcome this problem. By this method, the CRT would make two passes to draw the whole screen. The first pass would draw in the odd numbered lines (called the odd field), then return to the top and draw in the even numbered lines (the even field). Basically, the CRT would skip ever other line, then go back and fill them in with the next pass. This allowed for the entire screen to be lit at a much more even level, with the drawback being that the entire scene was a little duller.

This method served a dual purpose. Not only could you effectively light up the screen evenly from top to bottom, but you could also create a psuedo-60fps image with only a true 30fps field frequency. This worked out nicely because the components were under less stress, and could perform more reliably for a longer period of time without the risk of burning out.

Persistent little @#^^ Aren't you?

As discussed earlier, the CRT's screen is made up of phosphorescent materials, which glow when struck by the electron beam. The more "persistent" the phosphor, the longer, and more intensely it would glow after being energized by the electron gun. If the materials of the screen were too resistive, then the image would be dull. Over time, the screen phosphors begin to break down, and cannot hold as much of a charge, which is why older monitors tend to darken over time.

If the screen is over-energized, the phosphors would end up with an excess charge, causing them to glow too long. This excess energy would also have a tendency to leak into the adjacent pixels. The result here was that you ended up with ghosting along with a fuzzy image.

Persistence also partially accounts for the "flicker" present with interlaced scanning, since the first field would be weakened by the time the second (and fully charged) second field is drawn. Your eyes perceive that difference in luminance between the two fields that as flicker.