The design and evaluation of a novel instrumentation system for the advanced monitoring, characterisation and evaluation of fossil-fuel-fired flames in utility boilers are presented. The system comprises an optical probe with a wide viewing-field, a CCD camera, a frame grabber and associated image-processing software. Experimental trials of the system were conducted on a 10-MWth natural-gas-fired boiler in Guy's Hospital, London. The flame in the furnace was visualised continuously using the imaging system and then characterised in terms of geometrical and luminous parameters. Results obtained over a range of combustion conditions demonstrate that the system is capable of identifying the flames both qualitatively and quantitatively. Correlations between the measured flame parameters and corresponding fuel/air flow rates and emissions are identified.