I've always seen that all the old model remotes that is used in DVD players or TVs, it emits electromagnetic waves, having wavelength somewhere in the Infrared region. But when my phone's average quality camera (Moto E) is held between the emitter and my eyes, it converts the infrared into visible ones and shows a whitish purple colour. I don't know why always the purple is shown. I'll be very happy if you explain this to me. Thank you.

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    $\begingroup$ I don't really think this has any meaning behind it. The camera detects the radiation emitted by the remote. This information get's processed into a signal which than is displayed. There obviously is not a 1:1 transfer from what is detected to what is displayed (since than you couldn't see the infrared signal from your remote anymore). $\endgroup$ – rtime Jan 25 '16 at 16:28
  • $\begingroup$ @t.rathjen most run of the mill digital cameras use rgb photodiodes and then some interpolation. Isn't it strange, that the camera appears to behave, as if the blue diode is being excited? Unless of course the interpolation is cyclic, which would sort of make sense. $\endgroup$ – LLlAMnYP Jan 25 '16 at 17:31
  • $\begingroup$ Interesting observation. I've noticed that infrared from tv remotes appear as purplish in color but never really thought about it. Apparently, both 'red' and 'blue' pixels are responding to the infrared. The 'red' and 'blue' pixel sensors are essentially the same, except that they are covered by red and blue filters, respectively. I would guess that whatever material is being used for the blue filters has high optical transmittance not just in the blue light range but also some transmittance in the near infrared. $\endgroup$ – Samuel Weir Jan 25 '16 at 18:07
  • $\begingroup$ @LLlAMnYP, there's no such thing as an rgb photodiode. Cameras use red, green, and blue filters to discriminate colors. As Carl Witthoft's answer (below) explains, the color filters actually allow a small portion of IR to reach the sensor, and the light from a remote is sufficiently bright for that small portion to show up in the image. $\endgroup$ – Solomon Slow Apr 10 '18 at 15:31
  • $\begingroup$ @jameslarge I wish I could even remotely remember what I meant by "rgb photodiode". Probably three separate diodes, much like human photoreceptors. $\endgroup$ – LLlAMnYP Apr 14 '18 at 16:37

The comments are on the right track. First thing: the R,G,B filters in a standard digital camera each pass some IR light, i.e. they're not low-frequency blocking. Usually cameras add an IR-blocking filter on top of the color grid because of this. An example RGB filter from FujiFilm is shown in this picture:

FujiFilm which is from somwhere near here You can see the Blue sensitivity in the Near-IR.

EDIT: Just to clarify: the three curves are for different thicknesses of the color filters, not (0.7, 0.9, 1.1 $\mu m$) wavelengths Camera manufacturers are usually unwilling to release their color filter specifications, unfortunately. You might get lucky if you Google around for your particular model.


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