42 of light pipe 40 is optically coupled to infrared source 44 infrared light 46 from source 44 is adapted to strike the surface 48 of light pipe 40 at an angle such that infrared light 44 is totally internally reflected back into light pipe 40 and by repeated total internal reflections propogated along light pipe 40 to output face 50 infrared light 46 is then optically coupled to detector array 52 array 52 can have the same structure as detector means 22 as is known for total internal reflection the index of refraction of light pipe 40 must be greater than the index of refraction of fuel 12 also although light 46 is totally internally reflected the evanescent field of light 46 penetrates into the rare medium i e fuel 12 with the magnitude of the electric field decaying exponentially outside of light pipe 40 the energy content of fuel 12 is determined by first employing beer 39 s law in which the transmitted intensity at wavelength u03bb i u03bb is given by if it is assumed that u03c3 at u03bb is proportional to the number of carbon carbon and carbon hydrogen bonds in a material which absorb at u03bb and the absorption cross section.