Hi, I have been doing a number of tests with various ES bulbs on a condenser enlarger. Spot light bulbs seem OK. The ones I have tested have on writing onn the end and you can use a lower wattage as they concentrate the light. My tests also included 'low energy' in particular Phillips bulbs - the ones that look like normal light bulbs. Causes real trouble with multigrade paper; a grade 0 filter comes out as grade ~3-3.5 with a compresed contrastr range.
Stick to the real photographic ones, or try the spot light bulbs (the glass was 'milkly' over the filament area).