Pixels per inch (PPI) or pixels per centimeter (PPCM) is a measurement of the pixel density (resolution) of devices in various contexts: typically computer displays, image scanners, and digital camera image sensors. It is defined as the horizontal or vertical density (for square pixels) as those are the same but the density on along the diagonal is lower. Square pixels are the norm (otherwise those densities would be different). PPCM can also describe the resolution, in pixels, of an image to be printed within a specified space. Note, the unit is not square centimeters. For instance, a 100×100 pixel image that is printed in a 1 cm square has a resolution of 100 pixels per centimeter (ppcm). Used in this way, the measurement is meaningful when printing an image. It has become commonplace to refer to PPI as DPI, which is incorrect because PPI always refers to input resolution. Good quality photographs usually require 300 pixels per inch, at 100% size, when printed onto coated paper stock, using a printing screen of 150 lines per inch (lpi). This delivers a quality factor of 2, which delivers optimum quality. The lowest acceptable quality factor is considered to be 1.5, which equates to printing a 225 ppi image using a 150 lpi screen onto coated paper. Screen frequency is determined by the type of paper that the image is to be printed on. An absorbent paper surface, uncoated recycled paper for instance, will allow the droplets of ink to spread (dot gain), and so requires a more open printing screen. Input resolution can therefore be reduced in order to minimise file size without any loss in quality, as long as the quality factor of 2 is maintained. This is easily determined by doubling the line frequency. For example, printing on an uncoated paper stock often limits the printing screen frequency to no more than 120 lpi, therefore, a quality factor of 2 is achieved with images of 240 ppi.