Resolution: flat panel versus CRT
It is appropriate here to compare the resolution of a flat panel with that of a CRT display. CRTs are completely analog devices, and unlike FPDs, they do not have discretely addressable pixels that define a native resolution. A CRT resolution is defined as the maximum number of points or dots that can be displayed without overlap. A colour CRT has a matrix of red, green and blue phosphor dots. A phosphor dot is not a discrete pixel. The sweeping electron beam does not uniquely turn each individual dot on and off, as is the case with discrete pixel types of displays. Rather, as the beam scans across the face of the tube, the areas of phosphor that the beam strikes glow with an intensity proportional to the beam current’s instanta- neous amplitude. The size of the phosphor area illuminated by the beam depends on the focus of the electron beam and the distance between adjacent dots of the same colour. If the electron beam is sufficiently well- focused, the tube will be able to resolve an area smaller than a single dot which is in fact just a tiny area of continuous phosphor. If the electron beam’s focus is sufficiently diffuse, it may not just illuminate a single dot, but may also simultaneously illuminate a whole or part of an adjacent dot. The dot pitch plays a significant role in determining a colour CRT’s resolution capability, because it determines the distance between adjacent dots of the same colour. If we consider just blue dots, for example, one blue dot is not directly adjacent to another blue dot. There are green and red dots that are to some degree positioned between the two red dots. The result is a gap between adjacent blue dots, and the size of this gap influences the attainable resolution. The farther apart the blue dots, for example, the less blue resolution the screen is capable of displaying, and likewise for the red and green dots. The smallest dot pitch found in a typical top-quality monitor tube is about 0.22 mm, or about 4.45 dots per millimetre, which is about 115 dots per inch. Such a display with a width of 36 in. has about 4156 dots in a horizontal line. It is safe to say that it is possible for such a CRT monitor to display the resolution of HDTV, either 1280 X 720 or 1980 X 1020, if the proper conditions of dot pitch, electron beam focus and scanning speed are met.
To summarise, the resolution capability of a given colour CRT depends on a number of factors, the most important being the dot pitch, how tightly the electron beam is focused and the electron beam’s scanning speed. Most manufacturers do not list the dot pitch ratings anymore which are typically between 0.12 and 0.28 mm with an average of 0.25 mm. CRTs are, within limits, flexible in their resolution capabilities and can handle a video signal of any resolution. This is not the case with flat panel FPDs containing discrete pixels. If you want to display a resolu- tion other than the native pixel resolution on a display containing discrete pixels, the image must be re-scaled.