Epson ColorWorks C7500 Technical Reference Guide - Page 206

Paper detection method Sensor Judging method Label sensor (Transmissive photo interrupter) Black mark sensor (Reflective photo interrupter) The label sensor and black mark sensor detect the position of the paper.  When using die-cut label without black marks, the light transmission rates must be: 40 % or higher for the backing paper, and 23 % or lower for the labels.  When using paper with black marks, the black mark reflectance rate must be 10 % or lower, and the white reflectance rate is 70 % or higher.  When preprinting on the back of the continuous paper, do not preprint on the area where preprinting is prohibited that is shown in the figure below to avoid affecting paper detection. 15 mm {0.59"} 15 mm {0.59"} Paper feed direction Back Paper feed direction Front 13 mm {0.51"} 13 mm {0.51"} Paper Area where preprinting is prohibited 206