Nichia cs500's getting dimmer, Huh?

I have 3 of them in my computer case with a resistor in series off of a 12 volt lead from my power supply.
It has taken less than 20 hours of use for all 3 of them to be NOTICEABLY dimmer than when they were 1st installed.
This is part of the reason why leds have not come far enough to replace other white light sources.

Sounds like they are being driven too hard. The Nichias are good but they are not invincible.

How is it configured (series/parallel) and what value resistor did you use? For a 5mm LED on all/most of the time, you will not want to drive it much over spec.

To give you a rough example for a single LED off 12V, your resistor should be around 300 ohms. If you are driving the three in series, the resistor should be around 50 ohms. Of course, since Vf changes between LEDs, you will have to verify with a DMM and adjust as required.

Wow. That's pretty dramatic.

True, defintely sounds like you are grossly overdriving them way beyond their specifications.

You can get away with blasting the daylights out of them in a flashlight, as typically for most people, it would take a long time to get 24 hours on them.

Even Bulks LC/LH series, some of them actually blast the LED at 1.9A, confirmed by several folks. And I know of one person that noticed the Bulk light Luxeon dimming (according to a light meter) just in one discharge. There are other threads here on cpf, where folks noticed their Bulk lights dimming in just a few discharges.

Go try and shove 2X-3X the current into your incandescent bulb for 24 hours, no need to back to me on the results, they'll be dead...