A system and method for optical interrogation and measurement of a hydrocarbon fuel gas includes a light source generating light at near-visible wavelengths. A cell containing the gas is optically coupled to the light source which is in turn partially transmitted by the sample. A spectrometer disperses the transmitted light and captures an image thereof. The image is captured by a low-cost silicon-based two-dimensional CCD array. The captured spectral image is processed by electronics for determining energy or BTU content and composition of the gas. The innovative optical approach provides a relatively inexpensive, durable, maintenance-free sensor and method which is reliable in the field and relatively simple to calibrate. In view of the above, accurate monitoring is possible at a plurality of locations along the distribution chain leading to more efficient distribution.