Technical Note: Goniometric Radiometer Alignment

Misleading results can be obtained when performing single axis or perpendicular axes scans if the device under test is either not centered in the entrance aperture or if it is not pointed reasonably well into the instrument. Some users assume the measurement is made through the device optical centroid axis and it may not be so. Only when the instrument optical axis and the device optical centroid axes are coincident will this be the case.

What is "reasonably well pointed" depends on the device under test. A smooth LD with divergences of 12-20 X 30-45 degrees seems to give comparable angular widths when mis-pointed by a degree or two. We have tried to devise methods to detect mis-pointing without doing a full 3D centroid axis analysis. Software could notify the user of a possible problem if we can devise a fail-proof method; no method is obvious. Device differences prevent a single solution.