Why can't IR sensor detect black and white on a paper?!


I drew some squares on a white paper and coloured them completely black. However, why cannot an IR sensor differentiate the black and white colours on the paper and continuously detect the whole white paper as an obstacle, even with decreasing the detection distance?


  1. Some IR sensors recognize only light that is modulated, for example at 38 kHz. Post a link to your sensor.

  2. Black, in visible light, may not be black to an IR sensor.

Thank you for your reply. Here is the link to the sensor I purchased from Amazon: www.amazon.com/gp/product/B01I57HIJ0/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

If it doesn't detect it as black, then how can I know that? and how can I have my sensor detect the greyscale when it produces a digital signal?

If it doesn't detect it as black, then how can I know that?

The sensor reports what it detects, if it is working properly. Use it.

If you bought 10, try another. Have you tried changing the pot setting?

What your eyes report with visible light is NOT the same as what the IR detector sees.

how can I have my sensor detect the greyscale when it produces a digital signal?

You can't. You need a different detector for greyscale, such as this one or this one (for visible light) or this one for Near IR.

Maybe your squares are too small to detect - assuming black is truly black and does not reflect IR, while the white is truly white and does reflect it. Indeed what your eyes see is not necessarily what the IR sensor sees - after all it's looking at a different colour of light.

This sensor has an IR LED that basically floods the surrounding (mostly in front of it but it's got a significant cone) and an IR photodiode which records whatever light comes back. It will detect the white next to a small black square.

It is also highly sensitive to ambient IR, thus will not work outdoors during the day (lots of IR also when it's cloudy), or close to heat sources such as incandescent lights.

... and not to forget: what is black in visual light is not necessarily black in IR - same goes for white.

This sensor detect black and white at a distance of about 1cm, maybe one inch, it is used by line follower robots.
It use a IR led to illuminate the target and read the reflected light with the phototransistor.
Then there is a comparator so if light is under a certain level the output pin is LOW, HIGH in the other case.
You can variate the level turning the trimmer.

The ir sensor works at a specific wavelength ; you won’t get grey scales from it . You get light reflected , at the particular IR wavelength at varing intensity.
White is mixture of wavelengths , black tends to be absorbing all wavelengths , but maybe or maybe not IR as mentioned by others.

This may sound silly, but, find a friend with a trail camera that can take pictures at night using IR. Set the camera up in a room that can be completely darkened. Walk in front of the camera holding your prepared paper so the camera can see it. Your heat and motion will trigger the camera to take an IR photo. Do this several times and then look at the recorded image on a PC.