Alan:
Thanks for that. I've seen LED bulbs with large heat sinks, but I personally have no experience with them. Where I was/am going with this 'heat' thing is to compare the radiated heat of a PAR/HAL and/or HR lamp to the equivilent lumen output LED.
The reason for the heatsink is all physics. You have a fixed unit quantity of heat... say 10 watts. Apply that heat to a small 10 gram mass... it'll be HOT. Apply those 10 watts to a 10kg mass... you might or might not feel a difference.
The heat sinks are large mostly because there is no accompanying fan, so they have a very high surface area to encourage convection currents over the fins.
