As always when measurements of legal importance are concerned, PTB must examine the respective measuring instruments very thoroughly. In addition to electronics, ease of handling and stability, the measuring technique used is tested. As for many measurements in the field of physics, an optical method seems to be an obvious choice. A spectrogram allows the individual constituents of the exhaust gas and their proportions to be determined. In a still simpler way – by examination of the opacity of the light beam – the soot content in the exhaust gas of diesel cars is determined. The official exhaust gas analysis was introduced in 1993. Since the relevant Act entered into force, the car industry has reduced the exhaust gas values for its most recent models by 95%. Keep at it!