example consistent with the present invention when an output value is smaller than the level it is determined that an input optical signal is provided by a stylus furthermore when an output value is greater than the level it is determined that an input optical signal is provided by a shadow in another example when an output value is greater than the level it is determined that an input optical signal is provided by a stylus furthermore when an output value is smaller than the level it is determined that an input optical signal is provided by a shadow in still another example the stable level is a gray scale value 128 given 8 bits per pixel a relatively white color optical input signal has a gray scale value ranging from 128 to 255 while a relatively black color optical input signal has a gray scale value ranging from 0 to 128 the compensation process therefore compensates for the variation in the background light and the differences of optoelectronic characteristics of the plurality of photosensitive transistors 14 1 as well consequently the output voltage of each of the plurality of photosensitive transistors 14 1 of the photo detector array 10 is maintained at a stable level in the absence of an input optical signal therefore a stylus may be used as an entry tool similarly the shadow of finger chopstick or ordinary pen may also serve as an entry tool in one example an input optical signal having a diameter of.