by Swaraj » Mon Jul 12, 2010 1:15 am
Arbitrary unit means that you cannot calibrate your data or compare to other data. "Arbitrary" means you produced it.
In other words, if you walk in the mountains, and you measure your distance with a stick, then, as a scientist, you'd write "the distance was 5000 au", without any information about the length of your stick.
Or a more serious example: imagine you're looking at a cell through a microscope, and would like to say how big the cell is, but you don't have a scale in milimeters. Then you could put a small object, let's call it a "red ball" next to the cell, and when you say "my cell was two red balls wide", then you're using your arbitrary unit of "red ball".
Since i am working in the field of non-linear optics, i have found the use of arb. units for 'Intensity', 'Conversion Efficiency' in a no. of journals.