Two types of in-cylinder optical probes were applied to a single cylinder CFR engine to detect knocking combustion. The first probe was integrated directly into the engine spark plug to monitor the radiation from burned gas in the combustion process. The second was built into a steel body and installed near the end gas region of the combustion chamber. It measured the radiant emission from the end gas in which knock originates. The measurements were centered in the near infrared region because thermal radiation from the combustion products was believed to be the main source of radiation from a spark ignition engine. As a result, ordinary photo detectors can be applied to the system to reduce its cost and complexity.It was found that the measured luminous intensity was strongly dependent upon the location of the optical sensor. However, good correlations existed between the luminosities measured from two probes and cylinder pressure quantities in terms of crankangle and magnitude of peak pressure of normal combustion cycles. Knock can be detected by the spark plug optical probe as a high frequency ripple similar to that observed on the pressure waveform. The knock intensity was quantified by the peak-to-peak band-pass filtered luminosity with the center frequency set at the resonant frequency of the combustion chamber. This luminous knock intensity was found to correlate well with the integral of the root-mean-square (rms) of the band-passed cylinder pressure signal.Filtered peak-to-peak oscillations in luminosity measured by an inexpensive infrared detector sighting across the combustion chamber offer an alternative to pressure or acceleration measurements in the sensing and control of engine knock.