The present investigation is aimed to develop and test a NO, diagnostic system based on a spectral ultraviolet/visible imaging device. The study was performed in a small-scale laboratory furnace fired by a propane or ethylene swirl burner. The data reported include flue-gas concentrations of O-2, CO2, CO, unburnt hydrocarbons and NO,, obtained with conventional techniques, and photometric data of spontaneous emission of OH and CH radicals, obtained with the imaging device, from various regions of the flame. All these measurements have been obtained for twenty three furnace operating conditions which quantify the effects of the gaseous fuel (propane and ethylene), flue-gas recirculation, heat losses through the furnace walls to the surroundings and excess air. Against this background, it was possible to obtain a wide range of NO, emissions. In addition, the analysis of detailed near burner in-flame data of local mean major gas-phase species and gas temperatures collected for four furnace operating conditions has indicated that NO, formation was strongly dominated by the prompt (or Fenimore) mechanism. The results reveal that there is an excellent correlation between the NO, emissions from propane or ethylene flames and the OH + CH photometric data, which can be mathematically expressed as a logarithmic function, provided that the radicals images are collected from a flame zone close to the burner exit.