Red Light District, Amsterdam, Netherlands

One of the many terms used for a red-light district in Japanese is akasen (赤線), literally meaning "red-line." (This has independent origins from the same term in English). Japanese police drew a red line on maps to indicate the boundaries of legal red-light districts. In Japanese, the term aosen (青線), literally meaning "blue-line," indicates a non-legal district, also exists.
Still others believe that the term originated from an early 20th century piece of autobiographical writing entitled "Street Walker Unveiled", written by then-reputable New York City high-class prostitute, Nicole Sullivan, who at the time worked for the notorious "mobster" Most Jokes. In the work she mentions how she would "[walk] about the dim streets in the early morning, red light bathed across [her] face."