Just like a light gun, a light pen works by sensing the sudden small change in brightness of a point on the screen when the electron gun refreshes that spot. By noting exactly where the scanning has reached at that moment, the X, Y position of the pen can be resolved. This is usually achieved by the light pen causing an interrupt ,at which point the scan position can be read from a special register, or computed from a counter or timer. The pen position is updated on every refresh of the screen. Since light pens operate by detecting light emitted by the screen phosphors, some nonzero intensity level must be present at the coordinate position to be selected, otherwise the pen wont be triggered.