1. An apparatus for providing an indication of the presence of a gas of interest having a background concentration within a scene having a surface comprising: a backscatter absorption gas measurement system to generate light to illuminate the surface and determine an indication of absorption by the gas of interest within the scene; a range measurement system to determine an indication of the distance from said backscatter absorption gas measurement system to the surface illuminated by said backscatter absorption gas measurement system; and a processor to combine the determined indication of absorption and the determined indication of the distance to provide an indication of the presence of the gas of interest in excess of the background concentration.