There are various measures of the impact of a researcher, but they tend to be highly correlated. The most commonly mentioned one is the h-index.  One can compute the h-index (and all of the other impact indexes) for any faculty member by using a tool such as the one from Harzing's Publish or Perish.  While it is relatively easy to do this in most cases, it can be a little more difficult when a faculty member's name is the same as other authors. Misspelled names and misspelled article titles also complicate the computation.
The h-index was developed by Hirsch for computing the research impact (productivity) of physicists. For other fields, one should scale the h-index to the value for physicists. This is called the H-index.  After scaling, the traditional thresholds for academic ranks are as follows:
|12||Tenure at a major research university|
|18||Full professorship at a major research university|
|20||Fellow of a major professional organization in the field|
|45||Membership in a National Academy|
 Wikipedia entry for the h-index, http://en.wikipedia.org/wiki/H-index.
 Harzing's Publish or Perish, http://www.harzing.com/pop.htm.
 JE Iglesias and C Pecharroman, "Scaling the h-index for different scientific ISI fields," http://arxiv.org/pdf/physics/0607224.