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I am doing experiments using a red (623nm) LED. However, when I measured the power output I noticed that it is higher at smaller wavelenghts. I did the measurements using two different power-meters and both gave me that result.

First, I am confused since I though LEDs should only emit light in a small wavelenght range. Second, I don't understand how I can see the LED as red when it emits light in all the visible light spectrum and emits even more in the blue/violet range that in the red range..

The LED is #9008097 in the following datasheet: https://asset.conrad.com/media10/add/160267/c1/-/gl/000180735DS01/datenblatt-180735-ledxon-9008097-highpower-led-modul-rot-3-w-865-lm-10-2-v.pdf

Thanks in advance!

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  • \$\begingroup\$ Do you have part number or datasheet for the LED? Could you edit your question to add that information? Maybe it is not a normal red LED, but a UV LED coated with phosphors. Quite a few LED datasheets have spectra. \$\endgroup\$ – mkeith Feb 16 at 9:31
  • \$\begingroup\$ @mkeith good idea, I added it already. It doesn't have the spectrum though \$\endgroup\$ – Sofia Feb 16 at 9:45
  • \$\begingroup\$ It definitely seems odd. White LED's typically have strong emissions in the UV region because they are really UV LED's with phosphors. But red LED's that I have dealt with in the past were actually emitting red light directly from the semiconductor junction. The forward voltage of the red LED is much lower than the others, which makes me think it is a normal red LED junction. But the presence of UV makes me think it is a phosphor-based LED. Maybe you should inquire from the manufacturer. I think your confusion or curiosity is very warranted. \$\endgroup\$ – mkeith Feb 16 at 10:03
  • \$\begingroup\$ This sounds like a power measurement error, and you haven't given us the faintest clue how you are measuring. \$\endgroup\$ – user_1818839 Feb 16 at 14:55
  • \$\begingroup\$ @BrianDrummond I used an optical power meter from ThorLabs - PM100D. I held the sensor towards the LED at a distance of 10/20cm \$\endgroup\$ – Sofia Feb 16 at 15:37
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It sounds like you might be using the power meter incorrectly. The power meter has a wavelength setting, but this only affects the calibration. It does not change the operation of the sensor. The wavelength setting is used to compensate for differences in the response of the sensor so you can get an accurate reading. For instance, most sensors have a peak somewhere, and the response falls off on both sides of that peak, so the meter has to report a higher power for the same sensor output the further the wavelength is from the peak. Here is the responsivity curve for the S120C power sensor from ThorLabs, which uses a silicon photodiode and has a wavelength range of 400-1100 nm (https://www.thorlabs.com/images/tabimages/S120C_Responsivity_780.gif):

S120C Responsivity

In your case, the LED is likely producing most of its output power around 623 nm. Shining that light on the sensor will result in the sensor emitting some amount of photocurrent. The meter then converts that photocurrent to optical power by looking at the responsitivity curve. If you select 400 nm instead of 623 nm, the responsivity of the sensor at that wavelength is lower, so the power meter will compensate for that and report a higher power. If you set the wavelength to 950 nm, you would get a lower power. If you set it to 1100 nm, you would get an even higher power.

So in your case, you aren't measuring the LED spectrum, you're measuring the power meter calibration curve. If you want to measure at different wavelengths, then you have to use an optical filter of some sort to select the wavelength that you're interested in.

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  • \$\begingroup\$ Just for the record, I agree. Now that I see OP is using a wavelength-non-selective measurement technique, I agree that it is not valid for characterizing the spectral content of the LED light. \$\endgroup\$ – mkeith Feb 16 at 22:23
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However, when I measured the power output I noticed that it is higher at smaller wavelengths.

That's what the theory says as well.

and emits even more in the blue/violet range that in the red range..

Laser beam's power, among other factors, highly depends on the energy levels of the material's atoms' electrons: The shorter the wavelength, the higher the beam (photon) energy. That's why you see a blue (~450-480nm) or violet (~400-450nm) LED has a higher power level than a red one (~600-650nm).

First, I am confused since I though LEDs should only emit light in a small wavelenght range. Second, I don't understand how I can see the LED as red when it emits light in all the visible light spectrum

This depends on the LED's materials and construction. An example: A white LED is actually a blue LED, but the construction makes the beam look white.

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