All well and good.. but I'm not seeing where they were attached to the heatsink (probably were) when you measured them. not seeing the drive current or not seeing that you used copper heat sinks anywhere and I assume they were all aluminum..
copper is much better than Al in thermal conductivity.
It is very important point. I'll try to explain. Since the crystal is quite small (a power LED's crystal usually has the surface of only 1-2mm2 or 0.0015-0.003in2), its generated heat density is quite large. In fact, it is about 30 times as large as the heat density through the soleplate of a household iron! So removing heat from LED heatspreader is critically important. We should use best possible technology for do this, at least for dense LED assemblies.
After removing the heat from LED heatspreader we should to distribute it evenly across whole heatsink. You right, material with best thermal conductivity will better for this. But we have technology, that provide the MORE best thermal conductivity than all material, include copper. It is heatpipes. It's thermal conductivity is more than 100 times more than copper. We have used for testing this heatsink
http://reefll.com/index.php?route=product/product&path=65_69&product_id=69
In short word, combination of SinkPAD MCPCB and heatsink with direct contact of heatpipe to MCPCB will provide the best possible thermal mode for LEDs.
That said ... sinkpadded LEDs heat sinked to copper would be even better..
Though expensive..
Agreed, it is not cheap. But good thermal mode for LED will provide not only long lifetime, but also best efficacy. Depend on type of LEDs the difference may be 10-50%. Also, with using the best way, described above, we can design fixture with passive, i.e. fanless, cooling. By the way, temperature of LED crystal on SinkPAD MCPCB, installed on that heatsink with heatpipe, on 100% current is around 60C. I think, it is awesome
All commercial design is almost always a tradeoff ....
Exactly right. But if our target not a cheapest prime cost, but best solution, we should use best technology. To be honest, it is not very pricey