My agency just started a poster campaign with "city light posters" and while city light primarily refers to the operator of these backlit billboards, in marketing it became customary to refer to the specific format of these posters as "citly light" as opposed to DIN A 0 for ex. cf:
http://www.citylightcontact.de/index.php?article_id=2 so I reckon the actual translation would need to go either with the american equivalent of the format or as suggested the backlit billboard when it refers to the thing itself. Best, Chris