A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when a moving electron beam strikes a phosphorescent surface. There are three factors that limit the resolution on CRT display devices: screen dot pitch, electron beam size, and the bandwidth of the video amplifier. A typical CRT has a dot pitch around 0.8 - 0.9mm (much larger than a typical computer display). Lowering the dot pitch increases the display resolution, but increased dot pitch provides a brighter picture. Most CRT displays are configured to perform well with lots of ambient light, so dot pitch is typically higher. CRT televisions receive video signals at the rate of 30 frames a second. Each frame of video contains about 480 lines of information. A single frame is projected on the screen line-by-line in two passes (each pass is called a "field"). On the first pass, the beam projects all of the odd numbered lines from 1-479 from top to bottom. On the second pass, it projects all of the even numbered lines from 2-480. It takes 1/30 of a second to complete both passes. This process is called interlacing. CRT type TVs need time to reset the electronic beam to the top of the screen so it can get ready to paint the next sequence of lines. To accomplish, they build in an interframe gap that equals about 45 lines. There is no picture information here. So the total lines per frame are 525 (480 + 45). Thus standard definition TV (SDTV) is often referred to as 480i (interlaced).