Dam strange time to be taking travel photos.
I wonder if they were using infared film?
It was a man and a woman. They were probably on a porno shoot for the blind. Yeah,it's a small market,but there's not much competition.
Not to be defending these 'gentlemen', but I find it not strange in the least. Most dams that I have seen are brightly lighted, and by that light look very from their daytime appearance. I have photographed numerous structures using only their nighttime illumination.
That said, I seriously doubt that these characters were interested in the artistic effect of the sodium vapour lamps.