Mostly a set of light source and detector is used. The light source will emit the light which will be projected on the strip and reflected back, intensity of reflected light will be dependent on the surface from which light is reflected. So we get the data whether it is just above the strip or not.
Now guess what will happen when there is enough light in the stage, then our system may not work !!
To avoid this some engineers suggested to use Infrared Sources (IR LEDs). So this will make us free from interference by surrounding light.
But there may be IR sources (as IR sources is nothing but heated materials) in the surroundings. So the next improvement was to add some modulation to the light or simply send a pulse train from the source and get back the pulse train.
