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Do all LEDs have the same optical power vs current curve?

(NOTE: Thanks to some guidance and patience from some patons of EESE, not to mention my naieve/unfortunate/inaccurate word choice in the original version of this question, I have been able to condense it down to just a single sentence. As such, it takes away the context under which the two answers were given. For example, the answers mention "an indicator LED" and a situation where I "replace an old (indicator) LED with a modern one". Rather than rewrite this question from square one so as to maintain context and express a coherent question (assuming I could even do this), I have decided to take the advice given to me and ask the question that best solves my problem. But since I didn't want to ignore the help I had getting what I came here for, I thought I should write this note. As far as context (for the theoretical reader who might be wondering), I realized that this now-viable question doesn't supress the context , it has found it a nice home: in the edit history.)

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  • \$\begingroup\$ Flicker indicates either variability in the current supply, or imminent failure such as a faulty connection which makes or breaks with die temperature. \$\endgroup\$ Sep 30 '20 at 10:43
  • \$\begingroup\$ If these are indicator LEDs, I wouldn't assume modern are more efficient. High efficiency LEDs are usually larger, SMD parts made for high power applications. Lots of old factories churning out cheap LEDs for low power applications that don't need high efficiency. \$\endgroup\$ Sep 30 '20 at 13:53
  • \$\begingroup\$ I didn't understand what are you asking. If you are asking about the different response curves of different LEDs, there's possibly no ready-made answer, because even in the same batch of LEDs you can find significant differences in their response curves. The only way to compare their response curves is connecting two LEDs side by side, drive them with equal signals, and objectively measuring their brightnessess (even a simple circuit made with a phototransistor). By the way, do you expect to be able to visually perceive brightness differences about 10%? \$\endgroup\$
    – mguima
    Sep 30 '20 at 14:38
  • \$\begingroup\$ @ Brian Drummond I mentioned in my edit that "flicker" was a poor word choice on my part. \$\endgroup\$
    – user19267
    Oct 1 '20 at 1:44
  • \$\begingroup\$ @ user1850479 my use of the word "efficiency" was eliminated in the first edit \$\endgroup\$
    – user19267
    Oct 1 '20 at 1:54
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Most LEDs have a pretty linear luminous intensity vs current curve at low currents, but lose efficiency at higher currents.

enter image description here

This matters a lot for lighting where LEDs are run close to the max power they can take without burning before the end of the warranty. However in the case of an indicator LED, current should be much lower, so you can expect a pretty linear relationship with intensity.

However, two different LEDs of different brands/models/series will have different efficacies thus the slope of the curve will not be the same.

Now if you are interested in LED current, why not measure it directly in the circuit?

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LEDs are unlike lightbulbs where you apply a voltage and then a current is drawn (depending on the bulb's rating). LEDs have to be provided with a certain current (indicator LEDs usually need 20 mA or less) otherwise they'd draw too much current and destroy themselves. So a series resistor needs to be used. That resistor then sets the current.

If you replace an old (indicator) LED with a modern one, indeed it might be much brighter. The safest and simplest way to reduce that brightness is to increase the value of the series resistor. Since a higher value is needed, you can simply leave the existing resistor in place and add a 2nd series resistor. You can mount that close to the LED if convenient. Start with a 1 kohm resistor and see what that does.

The first part of your question can only be answered if we have datasheets for all LEDs. But I think investigating curves etc. is overkill as you simply want to replace some LEDs and make them burn less bright. And that is easy to do.

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