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Can anybody explain to me the output waveform generated on pins of an ordinary LED when it is exposed to very short light pulses?

I can clearly see that there is a precise difference in the length of the response by 55ns when the led is exposed to 110ns pulse instead of 55ns pulse. However the overall length of the waveform is much longer and there is a sharp pulse followed by a dip about 400ns later.

55ns light pulse response

110ns light pulse response

100us light pulse response

It seems that there is a rapid negative response first and then the expected voltage spike hundreds of nanoseconds later.

What are the mechanics of this?

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    \$\begingroup\$ hint for the future: most modern digital oscilloscopes have a screenshot function that can even save to a file. Also, when really having to take a photo of an oscilloscope screen (don't.), you should make sure to take it straight from the front, using e.g. a cardboard box as fixture for your camera or phone, so that pictures are directly comparable. \$\endgroup\$
    – Marcus Müller
    Commented Aug 9, 2019 at 11:22
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    \$\begingroup\$ What are you plotting here, exactly? what kind of signal is that? \$\endgroup\$
    – Marcus Müller
    Commented Aug 9, 2019 at 11:22
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    \$\begingroup\$ The vertical scale factor on the third shot is very different. The first two simply look like electrical noise that's getting coupled into your probe. \$\endgroup\$
    – Dave Tweed
    Commented Aug 9, 2019 at 11:26
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    \$\begingroup\$ What does the LED data sheet tell you about it working as a photo detector? \$\endgroup\$
    – Andy aka
    Commented Aug 9, 2019 at 12:39
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    \$\begingroup\$ @ChrisH I think the OP is using the LED as a photodetector, not as a photoemitter. The 6000 lumens is coming from another source. \$\endgroup\$
    – Elliot Alderson
    Commented Aug 9, 2019 at 13:05

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