Quote Originally Posted by glbeas
I use a little desk lamp with a dimmer wired in plugged into an enlarger timer and hundg on the wall for some distance. It's a very simple procedure to lay a sheet on the table under the lamp and give it a preflash bump before loading it into the easel.

For what its worth this is the very same technique used in graphic arts predigital while making halftones. They required a preflash to bring up the shadow detail and correct for ink bleed in the printing process. The time was standardised for all shots as it was just to bump the dot formation over the threshold exposure.
FWIW, I use a method very similar to what Gary describes. I use a 7 watt bulb that screws into the base of a lamp socket mounted in the ceiling of my darkroom positioned directly above the counter next to my enlarger. The bulb is almost completely masked by black photo tape and the exposure is controlled by a timer. I usually need a few attempts to get the pre-flash exposure correct. In short it's an easy technique to use if you need it.