Why fluorescent lamp instead of a signboard or advertisement? That’s because, by law, soliciting and advertising for sex is illegal in Hong Kong but prostitution in private is not. But how it is going to happen without soliciting? That’s how the fluorescent lamp came in.
In this over-crowded city, you can imagine the conflict between resident and sex worker. Indeed when I take this picture, a woman (I believe she is the shop keeper nearby) warns me not to do so because she thinks that is not deserved to be photographed and recorded, I seriously doubt that.
Further information, you may refer to https://en.wikipedia.org/wiki/Prostitution_in_Hong_Kong