of the light a second way of exploiting the aforementioned phenomenon is used in the case where the ink used in the pen 14 absorbs most of the light from the light source 28 39 in this case there is practically no signal from the light sensor 32 39 until the top surface of the ink gets to a point coincident to the line of sight between the light source 28 39 and the light sensor 32 39 at this point the turbulence and frothing of the ink will cause a portion of the light directed toward the receiver 32 39 to be reflected away or absorbed by the ink the amount of light so reflected or absorbed will vary with the changing shape of the surface of the ink and the positions of the bubbles therefore just as in the first case the signal from the light sensor will be erratic with distinct highs and lows however the excursion between the highest signal and the lowest signal from the light sensor 32 39 over a sample period will eventually reach the same threshold chosen for the case the light is being partially transmitted through the ink itself this result occurs because as before the turbulence of the surface tends to subside when the ink level gets low in addition this point will be reached before all the ink is gone so the pen 14 will not run completely dry before the plotting is interrupted so as can be seen the same optical.