As shown in the figure, it is a schematic diagram of the testing principle using a safety light curtain to detect the entry of objects (such as hands). In the figure, multiple infrared emission tubes are installed at equal intervals on one side of the light curtain, and there are corresponding infrared receiving tubes of the same number and arrangement on the other side. Each infrared emission tube corresponds to a corresponding infrared receiving tube, which is installed in the same straight line. When there are no obstacles between the infrared transmitting tube and the infrared receiving tube on the same straight line, the modulated signal (optical signal) emitted by the infrared transmitting tube can smoothly reach the infrared receiving tube. After the infrared receiving tube receives the modulation signal, the corresponding internal circuit outputs a low level. However, in the case of obstacles, the modulation signal (optical signal) emitted by the infrared transmitting tube cannot reach the infrared receiving tube smoothly. At this time, the infrared receiving tube cannot receive the modulation signal, and the corresponding internal circuit outputs a high level. When there are no objects passing through the light curtain, all modulated signals (optical signals) emitted by the infrared emission tubes can smoothly reach the corresponding infrared receiving tube on the other side, thereby causing all internal circuits to output low levels. In this way, information on the existence of objects can be detected by analyzing the internal circuit status.

The theoretical maximum number of infrared emission and reception channels for a safety light curtain can be countless. Considering the actual height of the light curtain and the distance between the upper and lower channels, it is generally not more than 48.

The safety light curtain is divided into linear scanning mode and cross scanning mode. In the linear scanning mode, the microcontroller sends the same path selection signal to the sending and receiving ends each time, namely, the first path sends the first receive signal, the second path sends the second receive signal,... the fifteenth path sends the fifteenth receive signal, and the sixteenth path sends the sixteenth receive signal. In the cross scan mode, the microcontroller sends different path selection signals to the sender and receiver each time. That is, the first route sends the second route for collection, the second route sends the first route for collection,... the 15th route sends the 16th route for collection, and the 16th route sends the 15th route for collection. In contrast, the cross scanning mode is more accurate in measuring the height of objects, and the detection accuracy is highest at the center of the detection area at one-third. The minimum detection height can be reduced to 2/3 of that in linear scanning mode. However, considering practical needs, the commonly used safety light curtains are linear scanning methods.