With the introduction of Halogen headlights, night driving is a little safer due to more illumination. However, this improvement comes with a negative for those still using a generator for their electrical charging system.
To get the extra lighting from Halogen bulbs, the available amperage should be about 60. This will come from an alternator systems which has a charging ability of at least 75. If you are still using your original 6 or 12 volt generator, as was on most pre 1963 vehicles, the available amperage is approximately 45 at normal driving speed.
Therefore, with a generator charging system, there is not the amperage created to get the proper Halogen lighting. When at engine idle speed the lights dim much like the generator lighting systems. When at faster RPM, the advantage of Halogens is not reached.
Suggestion: Keep your original headlights when you have a 6 or 12 volt generator.