I thought the community might benefit from some of what I have been finding in examining old XR-Es that have some service life on them. It probably is no shock to many of you that I love the XR-E package as it is what I have used for years to make the longest throwing flashlights in the world. For my application they are simply optically superior as well as better thermally than the newer XP-E package design. Many people over the years have noticed that the hard driven XR-Es however can burn up and reduce the output significantly making some appear to be amber LEDs.
Here is an example of a really bad one.
Conventional wisdom has always said that the phosphor was burnt. In dissecting these I have found that it is not the phosphor but the silicone gel that is burning. Look at that picture again and you can see that the burnt part extends out away from the chip.
Removing the dome this is what we see.
And now with the burnt gel removed.
And yes these pictures were made using my iPhone along with an assisting lens.
This is looking back through the dome with the gel still attached.
I have done extreme tests on these newer style LEDs that have a solid silicone construction(XP-E/XP-C) and they seem to last much longer as the silicone can take higher heat.