I Know this is a bit old but answers may be useful for some one else. Rosco day/night backdrops are as you describe, they are not the only people who do it but their softdrops have a certain look that many favour. As a translight photographer for a day to night backdrop you would ideally capture both day and night image. The night image may well be enhanced if the are not enough windows etc lit up, the key is that you are using specific equipment to produce very large files which then must be matched so as rgistration can be achieved between the day and night shot prior to printing. You can achieve both a dawn and dusk effect by having a combination of both front and rear lighting. windows that are illuminated in the night shot can be blocked out to create some change in the window views. Real strength of the day to nights is that with just a change of lighting you can jump between day and night pretty quickley.
Allan Staley translight photographer
Transligtht Images
