Hi,
First : Sorry for my bad english :)
I invite you to consult this page if it is not already done :
I have not (not yet) Radiance but in your visualsetting.dat, catch this lines (~500) :
car.defaultlight.day.emissive.on
car.defaultlight.night.emissive.on
car.defaultlight.day.emissive.off
car.defaultlight.night.emissive.off
I noticed that by increasing day.emissive.off the brightness will be increase (my setting below) :
car.defaultlight.day.emissive.on 900.50
car.defaultlight.night.emissive.on 900.50
car.defaultlight.day.emissive.off 20.00
car.defaultlight.night.emissive.off 20.000