A dental instrument, or a coupling connectable to the dental instrument, comprises an illuminator having a plurality of light emitting diodes that are each capable of emitting light at a selected wavelength in a range 260 to 880 nm. White or near white light emitting diodes may also be included together with a switch. Fluorescence-based diagnosis can be assisted by the use of a single dye or mixture of dyes. Light of about 400-540 nm wavelength may be used to distinguish composite, porcelain or other tooth coloured filling materials from normal tooth structures. Light of about 260-450 nm wavelength may be used to identify dental caries, calculus and/or dental plaque. Light of about 350-500 nm wavelength may be used to cure dental composite. Typically, the dental instrument is, or comprises, a drill, de-scaler, or other instruments such as for cleaning, examination or diagnosis of dental conditions.