LED Sensors are found in many common devices: remote controls, traffic lights, on/off indicator lights on many electronic goods, etc. In a way, LED Sensors are similar to electric motors. Connect an electric motor to a battery, and the motor turns. Spin the motor's axle, and electricity is generated. The faster the axle is rotated, the greater the amount of electricity produced. Similarly, connect an LED to a battery, and it emits light of a narrow range of wavelengths. Shine light of similar color (actually slightly shorter wavelengths than what is emitted) onto the LED, and electricity is produced. The greater the intensity of light, the greater the voltage created. By connecting the LED to a voltmeter (a device that measures voltage), the LED becomes a simple light detector. This equipment does not produce calibrated measurements, but it is very inexpensive and LEDs are extremely rugged.
Also, use LED Sensors to control the light illuminating objects. Filters that transmit (pass through) a narrow bandwidth of light are very expensive; however, in a darkened space, LED Sensors are an inexpensive way to essentially filter light. The ALTA II reflectance spectrometer is an example of combining LED light sources with a broad spectrum light detector.