13 Replies Latest reply on Apr 20, 2018 2:37 PM by gener7

    Coming to terms with terms

    norman.sanders Adobe Community Professional & MVP

      To maintain constancy and eliminate the current confusion in definitions, I would like of offer a standard regarding terms for use in Forum posts. They refer to dpi, lpi and white balance. If I am lucky, in the future these terms will be as universally treasured as the flag, home and the loyalty of a fine dog. 


      PPI: Pixels – picture elements per inch viewed on a monitor. I think we are safe here.


      DPI: The number of inkjet printer ink droplets. It is measured in picoliters and may appear among a given machine’s specs. When printing, the number of droplets varies throughout the tonal scale with few ink dots deposited on a surface to create the lighter tones and more of them deposited to create the darker tones. We might refer to this variation in the number of dots as Frequency Modulation (FM) and it is the only time the term DPI should be used. Promise me.


      LPI:  Dots deposited on a surface by offset lithography or letterpress printing. When viewed from a distance, the dots seem to fuse and create a smooth gradation of tones. On the printed page smaller dots create the lighter tones and larger ones create darker tones, but the number of dots throughout does not vary. Since only dot size varies, we might refer to this as Amplitude Modulation (AM)


      Why LPI?

      Some halftone history: Before the introduction of computers in the graphic arts, the device used to create halftones consisted of two sheets of glass, each with fine parallel opaque black lines -- perhaps 150lpi – glued together (one vertical, the other horizontal) to create a grid of small clear windows. When this device, called a glass halftone screen, was positioned in front of high contrast film (not in contact with the film) in a camera and exposed to an image, it broke the light reflected from the subject being photographed into an array of various sized dots -- a halftone negative that was then used in plate-making.


      Today, glass screens and their successor -- plastic contact screens of a checkerboard-like design (which are placed in contact with the film) -- are gone but the term lines-per-inch (lpi) -- a vestige of the glass halftone screen era -- remains to designate the number of dots to the linear inch in the image on the printed page. That is why a 150 dots-to-the-linear-inch image on a printed sheet, for example, is referred to as a "150 line halftone." To repeat: a halftone is an example of Amplitude Modulation (AM), different from an inkjet’s FM.


      Now, on to White Balance (and don’t even try to talk me out of this) is defined as the neutral quality of only the highlight values in an image. The neutral quality of tone elsewhere in an image -- sidewalk, schnauzer, slate roof -- should rightly referred to as Gray Balance. These are not two arbitrary definitions: they are terms that have been used in the graphic arts for about a century. The camera manuals are wrong. There. I said it and I'm glad.


      If you concur, raise your hand.