The emission of light during combustion is investigated on standard production engines. For this purpose, a light pipe is integrated into the center electrode of the spark plug. The emerging radiation is measured and converted using simple silicon photocells.The emission of light during combustion has a similar time curve to that of pressure. The start of light emission correlates with the start of mass burn. The position of maximum radiation coincides with the position of maximum combustion pressure.From the curve of light emission received, one can draw conclusions concerning combustion. From the position of the light emission over the crank angle, one can read whether the mixture has been ignited at the correct time or not. Cyclic fluctuations give information on the smooth combustion of the engine. Any knocking which occurs can be detected with an appropriate filter.The measuring spark plugs which have been developed allow light to pass through for a considerable period of time when used with unleaded fuel.