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A specialized optical probe together with a spectrometer setup has been used to make spectrally resolved measurements from the spontaneous diesel flame emission in a Wärtsilä 6L46 production-line-type, six cylinder, medium speed diesel engine. The test engine has 460 mm cylinder bore and it was operated at 500 rpm and full power (5.8-6.8 MW). The fuels used in the experiments were standard light fuel oil, standard heavy fuel oil and low quality, high ash content heavy fuel oil. This paper reports the in-cylinder emission spectra as a function of crank angle for the different fuels in realistic engine operating conditions. The measurements were performed in the 400-900 nm wavelength range using 1 nm spectral resolution. The recorded spectra show, as expected, that the diesel flame radiation is dominated by the continuous, black body soot radiation.