Image resolution describes the detail an image holds. The term applies equally to digital images, film images, and other types of images. Higher resolution means more image detail. Image resolution can be measured in various ways. Basically, resolution quantifies how close lines can be to each other and still be visibly resolved.
Resolution units can be tied to physical sizes (e.g. lines per mm, lines per inch) or to the overall size of a picture (lines per picture height, also known simply as lines, or TV lines). Furthermore, line pairs are often used instead of lines. A line pair is a pair of adjacent dark and light lines, while a line counts both dark lines and light lines. A resolution of 10 lines per mm means 5 dark lines alternating with 5 light lines, or 5 line pairs per mm. Photographic lens and film resolution are most often quoted in line pairs per mm.
Image resolution on raster displays
A television or raster image display with 525 scan lines makes a picture with somewhat less than 525 TV lines of resolution. The ratio of lines of resolution to the number of format lines in known as the Kell factor, after Raymond D. Kell, who worked out details of visual resolution in scanned systems at RCA in the 1930s.