This study evaluated (1) the effect of known reductions in the output of curing lights on the depth of cure of various resins as determined by hardness measurements, and (2) the ability of the clinician to detect reduced light output by use of an explorer to compare the hardness of the top and bottom surfaces of resin specimens. Curing light output was reduced stepwise from 10 to 70% with neutral density filters. Hardness values indicated polymerization of the top surface to be generally unaffected by light blockage. Bottom surfaces were greatly affected: more with a 30-second than a 60-second cure time. Three clinicians utilized an explorer to compare the tops and bottoms of specimens of known hardness. Evaluators were unable to routinely detect differences of less than 20 to 30 Barcol numbers. The data indicate that a light meter is a more efficacious means of monitoring curing light performance than is a tactile test of resin surface hardness.