I thought others considering LEDs for their enlarger heads might find this story useful:

The halogen bulb burned out in the color head of my Durst M605, so I replaced it with a new 100w "EPJ" lamp, intended for projectors. It worked, but it was too bright for my taste and much brighter than the old, unmarked bulb. I got exposure times of a few seconds at f8 with a 35mm neg enlarged to 5x7 on Kentmere VC (a fairly fast paper). And that was with the diffusor set to the dimmer 6x6 setting.

So I thought I might experiment with one of the new LED bulbs. I knew it would be less bright, which I wanted, and I also figured that the accuracy at short exposure times would improve, because the halogen bulbs take time to warm up and also glow for a short while after power is cut, whereas LEDs turn on an off instantly.

I bought a 2W warm white MR16 LED bulb one, said to be equivalent to a 20W halogen. It fit into the socket in the enlarger head with a little wiggling. It didn't burn out the transformer, and the light looked bright to me. But by the time it came through the diffusion box and the optics, it was barely visible on the baseboard - a real lesson in how inefficent the diffusor head is.

I gave LEDs one more chance. I bought a 4W cool-white bulb, figuring the added blue and overall higher efficiency as compared to warm white would help. It was significantly brighter, but it was nowhere close to bright enough. In fact, it was 9 stops dimmer than the 100W bulb! Filtering the light through a blue filter brought the difference down to about 8.5 stops, but that would still mean unacceptably long exposure times.

So it's back to halogens for me - I've ordered a 50W bulb. Now, what I really want is to build a head with banks of green and blue LEDs that can be switched independently for split printing ...