Using a LDR or Photoresistor, are they more sensitive to one color or wavelength compared to another? Or are they consistent with any light? Why?

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    \$\begingroup\$ How is this not directly answered in the datasheet of whatever photoresistor you are looking at? \$\endgroup\$ – Olin Lathrop Jan 17 '16 at 19:18
  • \$\begingroup\$ @olin apparently not all datasheet have them, and you'd have to know what you are looking for. \$\endgroup\$ – Passerby Jan 18 '16 at 0:15
  • \$\begingroup\$ If you hadn't been someone who generally writes good posts and has earned some respect here, I'd have voted to close this. As it is, you're on thin ice. Obviously sensitivity will vary across the electromagnetic spectrum. I still can't tell what specific device you are asking about. Do you have a CdS cell? How do you know if you don't have a datasheet? This question is basically "guess what I've got, then read me its datasheet". \$\endgroup\$ – Olin Lathrop Jan 18 '16 at 11:47

Based on a random LDR's data sheet, yes. A photoresistor's sensitivity is a byproduct of the chemical used to create it.

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This one, a CdS cell, peaks around 545 nm wavelength, which means it is much more sensitive with Green, Green-Yellow light than it is UV/Violet/Blue or Red/IR. It's also fairs better along the higher wavelength colors, Yellow/Orange/Red, than it does with lower wavelength colors, Cyan/Blue, based on it's sharp decline to the lower wavelength.

It also depends on the type of LDR you are using. Typical visual light LDRs use Cadmium-Sulfide, CdS. Other LDRs are IR range, based on Lead sulphide, PbS, or variants.

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There are also variants of the CdS with different peak wavelength response, based on this data sheet:

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Cadmium Selenide, CdSe, peaks just outside of Red into InfraRed. Hybrid cells using both, Cadmium Sulfide Selenides, Cd(S-Se) range in the Orange and Red visible wavelengths.

Notably, natural sunlight, while covering the entire visible spectrum, slightly peaks in the blue-green and green ranges, matching the peak response range of a CdS Photoresistor nicely.

  • \$\begingroup\$ Which means that CdS is as good as useless for common IR LED wavelengths (so is CdS.Se, but not CdSe). Of course most applications with an IR LED would use a photodiode anyway. \$\endgroup\$ – Chris H Jan 18 '16 at 9:15

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