Popular line sensor use IR LED as transmitter and IR photodiode as receiver to distinguish reflective color such as white and less reflective color like black. Consider that infrared LED is hard to see by naked eye therefore hard to know if the LED still working or not and infrared ambient noise in not less than visible light why it is so popular?

  • \$\begingroup\$ Start by asking yourself: "What other alternatives are there?" and tell me your answer. \$\endgroup\$ Jan 14 '18 at 14:45
  • \$\begingroup\$ @Harry. Visible light, the reason is mentioned in my question above. \$\endgroup\$ Jan 14 '18 at 14:51
  • \$\begingroup\$ So your eye sensitivity peaks around 500 nm wavelength. A silicon photodetector peaks at 900 nm wavelength. What fits your robot's needs: a human sensor or a silicon sensor? \$\endgroup\$
    – glen_geek
    Jan 14 '18 at 15:15
  • \$\begingroup\$ Another method is squarewave modulation of the light source; the receiver signal is similarly correlated with the squarewave, and averaged over some long period to remove most of the noise. Analog Muxes may provide the correlation behavior. \$\endgroup\$ Jan 16 '18 at 3:43

Most likely because there are commonly available devices that have an infrared emitter and detector pair in a single unit.

Easy to install, easy to use.

There is, of course, a lot of ambient infrared. But, the detectors have a filter in front of them to only allow a certain (narrow) range of infrared wavelengths. The emitters only transmit in that particular range. This reduces interference from ambient infrared.

So, an IR emitter detector pair is actually less sensitive to ambient IR than an unfiltered visible light LED emitter/detector pair would be to ambient visible light.

You could do the same with an LED of a particular color as an emitter and a bare phototransistor or photodiode as a detector - but you would have to have a color filter to make it only receive from your LED.

There aren't many easily available filters that match the typical LED colors so you would have to find some piece of transparent plastic or glass of the correct color. Good luck with that.

There may well be commercially available colored visible light emitter/detector pairs.

IR is common, available, cheap, and gives good results so that's what gets used.


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