Depending on the how a touch-screen monitor is going to be used, there are four major different technologies that allow the screen to record touches. The Apple iPhone uses a capacitive technology, which monitors changes in the electrical field caused by the human body. This is very accurate, but does not record touches if you are wearing gloves. There are acoustic technologies, which recognize touch by sound, by calculating the difference in time that each of four acoustic transducers "hear" the touch. Resistive touch-screens have been on the market the longest. They work by placing a flexible touch screen panel in front of the screen. The downside is that this panel can distort images or colors on the display. There are also infrared screens that place infrared emitters on the bottom and side of the display and receivers on the top and other side. When your finger breaks the beams, the display records the position. These displays can only record one touch at a time, and the touch points have to be fairly large; however, there is no layer that obscures the image.