Demonstration of confocal laser induced fluorescence at long focal lengths.

We add a collection path obscuration to a confocal telescope and confirm theoretical predictions of significant improvement in the longitudinal spatial localization. The improvements of spatial localization permitted an extension of the confocal telescope's focal length from 150 mm to 500 mm. At this longer focal length, millimeter-scale spatial localization is confirmed by comparing radial profiles of metastable state density obtained via confocal and conventional optical arrangements in a helicon source. The long focal length arrangement enables the measurement of argon neutral velocity distribution functions in the conventionally inaccessible region under a helicon source antenna.