A photoelectric device has been developed with which it is possible to measure automatically the relative times of exposure of a photo‐cell placed in the path of a moving beam of light. When the photo‐cell is illuminated, a relay is energized which closes contacts for an electrical stop watch. Accordingly, this clock runs only when the beam of light is projected upon the photoelectric cell. By adjusting the position of this cell upon a graduated upright or within a predetermined area, it is possible to determine by the stop watch reading the time of the photo‐cell exposure to the illumination at a given location. Frequency distribution curves obtained in this manner were analyzed and have been found valuable in the analyses of filamentous materials such as textile yarns.