Linear light sensor is also called phototransistor. The integrated chip is very sensitive to light changes. Therefore, it is usually used to detect the current ambient light level and convert the light energy into electrical signal output. It can be used as follows:
Black, white and grayscale can be determined by sending a beam of light to the object to be measured and accepting the reflected light with a linear light sensor. The color of an object can also be detected by using different emitting wavelengths of luminophore (light emitting diodes in usual) and linear light sensors sensitive to different wavelengths. In this way, the optical signal is converted into electrical signal, and then a series of processing is carried out. The common "automatic tracking car" uses linear light sensor to detect the black line on the ground.
Optocoupler is an electronic switch which is made of light-emitting diodes and linear light sensor. Its function is similar to relay (therefore also known as photorelay), the input and output are isolated, and the signal is transmitted in one direction.
The short distance communication such as construction site or mountain climbing can be converted and modulated into optical signal at the sending end, received by linear optical sensor at the other end, and restored to electrical signal. Therefore, you don't have to worry about eavesdropping or using a transceiver.