when the sensor has 3 kinds of lights (red, green , blue), it could select the best light according to the difference of envelope papers. The setting method is :
1) Adjust the height of sensor supporter until the distance between lens of sensor envelope paper is 10+-1mm.
2) Press "MODE key" , the sensor will be "TEACH" mode, "TEACH" indicator flashes, the screen will show "1-st"(see figure below)