Every engineering hire goes through a very structured interview process. If you travel to Coopersburg for the interview, you'll be met at the airport, taken to your hotel, picked up in the morning, taken for a tour of Lutron's local facilities--it seems kinda low-key.
It is not. Every single person you meet is part of the interview team. One legendary applicant was "dinged" for how he behaved at *breakfast.* (Here's a tip--don't offer to finish your host's bacon.)
When you get to campus, you'll have an overview conversation with someone from HR, followed by technical interviews with different pairs of interviewers. The interviewers are asking standard questions from scripts--if you do not understand the question, ask for clarification. An applicant who (a) is not intimidated, (b) asks questions, and (c) seeks to clarify what is being asked will do well--even if you don't have the faintest idea what the answer might be.
The biggie in the on-campus interview is the role play. You will be asked to play the role of a tech support engineer (regardless of your experience, every engineering hire spends time on the tech support hotline--it's a Lutron thing). You will be presented with a problem--a really, really strange problem. This will almost certainly be based on a real tech support question that Lutron has received.
The purpose of the role play is to see how you perform under pressure, and whether you can ask enough questions to find a solution to the problem. Keep asking questions! The problem can be really tough--but if you don't pass the role play, your chances for an offer are exceeding slim.
If you have done well, you'll end the day with an interview with a senior executive, who'll tell you about working for Lutron. It's a great company, filled with insanely smart people.