Are television crime-show writers running out of characters? That might explain the proliferation of Kevin Federline storylines of late. When not described dismissively as the husband of trampy chanteuse Britney Spears, Federline is most often characterized as “an aspiring rapper.” He guest stars as an obnoxious street punk on tonight’s “CSI” (8 p.m., CBS).

Readers may recall a recent episode of “Law & Order” featuring fictional characters clearly based on Spears and Federline. In that case, the aspiring rapper turned out to be a backstabbing, two-timing cad who inadvertently killed a cop while trying to retrieve incriminating photographs of his dimwit wife. Good thing that was only fiction!

To see how badly Federline’s character behaves (or how badly Federline acts), you’ll just have to tune in to “CSI.”

¢ “CSI” and its proliferation of spin-offs depend on the ability of forensic scientists to come up with slam-dunk evidence in the space of 60 minutes. But just how dependable are forensic labs? “Naked Science” (9 p.m., National Geographic) examines two of the mainstays of forensics science – fire investigation and firearms analysis – to determine whether the methods that make for such good television can also make for good police work and result in good justice.

Apparently, there is plenty of room for doubt.

Investigators, crime analysts, attorneys and a federal district court judge express deep concerns about unsound forensics. Some describe common forensic methods as “rules of thumb” masquerading as science and demonstrate how slam-dunk evidence of arson could be interpreted in four different ways.

That’s not good enough for Barry Scheck, founder of the Innocence Project. He argues that forensics still needs to prove that it is “a disciplined, objective, independent investigation of evidence” before “CSI”-style findings are considered final.

Tonight’s other highlights

¢ A feline wrong is set right on “My Name Is Earl” (7 p.m., NBC).

¢ The New York Mets host the St. Louis Cardinals in Game 1 of the National League Championship Series (7 p.m., Fox).

¢ Betty woos a local talent for the magazine on “Ugly Betty” (7 p.m., ABC).

¢ Groucho Marx appears on a vintage helping of “The Dick Cavett Show” (7 p.m., TCM).

¢ Believing that the staff is in mourning, Michael offers consolation on “The Office” (7:30 p.m., NBC).

¢ Addison loses confidence on “Grey’s Anatomy” (8 p.m., ABC).

¢ A former pal becomes a chief suspect on “Shark” (9 p.m., CBS).

¢ Abby and Kovac may need outside help for the baby on “ER” (9 p.m., NBC).

¢ Twenty cast members engage in 16 competitions on a new season of “The Real World/Road Rules Challenge: The Duel” (9 p.m., MTV).