Abstract: Systems and methods are provided for measuring the thickness of a film on an interior surface of an exhaust pipe system of a vehicle. The film is typically a layer of a nitrogen-oxides reductant, such as urea, deposited during operation of a selective catalytic reduction system for eliminating pollutants in an exhaust stream. The methods and systems are directed to optical measurements that directly probe the interior surfaces of the exhaust system to determine the thickness of a urea film built up during operation of the engine and the selective catalytic reduction system.