The response speed of the photointerrupter must be taken into account during high-speed switching operations. Please ensure that sufficient margin exists when using ON/OFF control elements operating in the usec order. Also, reduce the resistance at the phototransistor side during high-speed operation, as well as the resistance in the infrared block, for greater current flow. Another consideration is the amount of light blocked by the light shield. Set the light shield/aperture to 3-10%.