Objectives To test the hypothesis that a blue light emitting diode (LED) light curing unit (LCU) can produce an equal dental composite depth of cure to a halogen LCU adjusted to give an irradiance of 300 mWcm–2 and to characterise the LCU's light outputs. Materials and methods Depth of cure for three popular composites was determined using a penetrometer. The Student's t test was used to analyse the depth of cure results. A power meter and a spectrometer measured the light output. Results The spectral distribution of the LCUs differed strongly. The irradiance for the LED and halogen LCUs were 290 mWcm–2 and 455 mWcm–2, when calculated from the scientific power meter measurements. The LED LCU cured all three dental composites to a significantly greater (P < 0.05) depth than the halogen LCU. Conclusions An LED LCU with an irradiance 64% of a halogen LCU achieved a significantly greater depth of cure. The LCU's spectral distribution of emitted light should be considered in addition to irradiance as a performance indicator. LED LCUs may have a potential for use in dental practice because their performance does not significantly reduce with time as do conventional halogen LCUs.