Characteristics of a flame within a turbine or burner are determined based upon ultraviolet, visible, and infrared measurements of the flame. The measurements include a measurement of the amplitute of frequency bands that are indicative of an efficient combustion process, such as those that increase when the flame temperature increases. The measurements also include of the amplitude of frequency bands that are indicative of an inefficient combustion process, such as those that do not vary, increase a relatively small amount, or decrease when the flame temperature increases. The temperature of the flame may therefore be determined accurately, to facilitate efficient operation of the turbine or burner while minimizing polluting emissions. A fiber structure, suitable for remote location of sensors and processing equipment passes energy for several spectra by providing a hollow core that passes infrared energy, in combination with a core of visible-transmissive material that passes visible or ultraviolet energy. Contaminants in the turbine or burner are detected, and a degree of contamination measured, by detection of energy levels for particular wavelengths associated with a respective contaminant.