Dear Brent, Thank you for your reply, I think Your method would suit my purpose. The only thing I don t understand is the setup: how do you manage to put laser and detector outside the chamber and get an image? Are these two components just kept outside the plasma range? Is that what you are trying to do? Why do you use HeNe laser? Is it because it is not disrupted by the frequency in the chamber? Also one more question: what type of photodetector do ypu use? is it a CCD? Many thanks again. Regards. Ghazal ________________________________________ From: Brent Garber [garber@engr.uconn.edu] Sent: Friday, March 06, 2009 8:23 PM To: General MEMS discussion Subject: Re: [mems-talk] CCD in plasma chamber Ghazal, I bounce a HeNe laser off my sample and it reflects on to a photo detector which is connected to a strip chart recorder. The laser and detector are outside the chamber. Brent