>Anyone have a URL where I can learn why the IR needs to be "chopped"? I >have been searching unsuccessfully this morning. thx, jf Because the best IR sensors that are affordable only work in AC (sense change not absolute) so if you have a constant or slowly changing target you need to supply your own 'variation' in the form of chopping. This exposes the sensor alternately to the target radiation and to some reference radiation (like the temperature of the chopper blades which is known). IR transmissive and dispersive sensors are often used in process control in the chemical industry. Locate a chemical process control and instrumentation textbook in a university library and you will find several (dozen) examples of chopper based IR instruments. Peter -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu