There is also a distinction made between passive sensors that do not produce signals of their own and active sensors that emit signals that are subsequently sensed, perhaps in an attenuated, distorted, or delayed form, to extract information about the robot's environment. By emitting a signal that is different from the background signals we expect to encounter we can make it much easier to detect a particular signal. Active sensor strategies include using frequency or amplitude modulation or projecting patterns that are easy to distinguish from other signals. The Lego light sensors are active sensors. IR devices are used in TV remotes and to flush toilets or open doors.
All sensors require some interpretation.
Here is the general form for a sensor of the sort we'll encounter in programming Lego robots. Advanced Sensing Polaroid ultrasonic ranging sensors are used to estimate distances to nearby objects by calculating the time it takes an ultrasonic pulse to travel from the Polaroid sensor to the object and then back to the sensor. The sensor is called a transceiver because it includes both a transducer for emitting a high-pitched pulse of sound and a receiver for detecting the energy of the reflected pulse. For a transmitted pulse to leave the sensor, strike a target two feet away, and then return the sensor requires an elapsed time of approximately 3.5 milliseconds. Polaroid sensors are typically packaged with circuitry to generate the ultrasonic pulse and then calculate the time of flight which is then converted into distance using the speed of sound.