Last month a TV station in Los Angeles went off the air for equipment maintenance. Yet the overnight Nielsen ratings suggested that more people were watching its blank screen than the programming on a number of other local stations. That lead to renewed cries that the Nielsens are screwy. Well, in the television business, ratings are a lot like the weather: Everybody squawks about them, but no one does anything about them. This week’s episode of Nova, “Can You Believe TV Ratings?” on PBS (Tues., Feb. 18, 8 P.M. ET), suggests that may be changing. The networks, Nielsen’s loudest critics, are interested in a photosensitive device, now being developed by several audience-research companies, that will be mounted on top of the TV and will sweep the room periodically to monitor who is really sitting in front of the set. In other words, very soon, when you’re watching TV, the TV may be watching you.