[QUOTE=Sean Tang,

How do you convert it to work with tungsten-halogen bulbs? Would anyone care to tell?[/QUOTE]

I have a 138S as well. I haven't had to deal with the conversion yet. I still have a 300 watt and a 500 watt Thorn lamp. Eventually, obviously the time will come. What I would do is use a minimum of 1000 watt halogen lamp and use the same principal that Jensen Optical did with their $800 upgrade. I would build a reflector that would be placed to the rear of the halogen lamp. The material that I would use for the reflector is Mirro which has above 90% reflective qualities. The reflector would need to be curved and emulate the radius of the halogen lamp.

The reason for the reflector is that it utilizes the light radiating off the rear of the lamp envelope and additionally it creates a larger effective light source to the condensor path. My thoughts are that the reflector diameter would be approximately that of the Thorn lamp.