This paper describes the development and a demanding field application of a double charge-coupled device (CCD) camera system, which allows simultaneous imaging at two different wavelength bands through common front-end optics. The imaging system has been assembled using two 1280 × 1024-pixel, 12-bit digital cameras together with standard, commercially available optical components for image splitting and filtering. The pixel resolution and wavelength separation of the system can be changed according to the requirements of the given application and the basic optical set-up can be used together with a variety of different types of front-end optics. The construction and performance of the double-camera system are described, together with preliminary experimental results from applying the system to in-cylinder combustion diagnostics of heavy-duty size-class, direct-injection diesel engines. To provide optical access into the combustion chamber of series-production-type engines, endoscope-based optics were used together with the camera system. Images taken at the two separate wavelength bands were used to calculate pyrometric diesel flame temperatures based on the blackbody radiation of the soot contained in the flame. The measurement system has been applied to engines with cylinder bores ranging from 20 cm up to 64 cm, delivering 0.15-1.6 MW power per cylinder during the measurements. In this paper specific results from measurements with Wärtsilä 4L20 and 6L32 engines are reported. These engines have 20 and 32 cm cylinder bores, respectively. In addition to the two-colour flame-temperature measurements, spectrally resolved measurements were performed to investigate a cloud-like, self-luminous phenomenon that was observed above the piston top even prior to the injection of fuel into these engines. This phenomenon was associated with the evaporating and chemically reacting fuel that originates from the residues which have been carried onto the piston top from the previous cycles. For the in-cylinder measurements reported here, the production-type test engines were only slightly modified to accept the endoscopic optics. During the measurements the engines were operated with realistic diesel fuels and other operation parameters. To the best of our knowledge, there have been no previous corresponding measurements reported in the literature for diesel engines of this size.