Reactivity Controlled Compression Ignition (RCCI) is an approach to increase engine efficiency and lower engine-out emissions by using in-cylinder stratification of fuels of differing reactivity (i.e., autoignition characteristics) to control combustion phasing. RCCI is defined by an early, high-pressure, direct injection of a high-reactivity fuel into a premixture of low-reactivity fuel and air that yields a significant dwell before start of combustion. The degree of in-cylinder stratification of the two fuels can be altered by varying the injection timing of the high-reactivity fuel, causing transitions between various regimes of combustion. These progress as injection timing is retarded from highly-premixed autoignition to sequential autoignition driven by reactivity stratification (i.e., RCCI) to more diffusion-controlled, diesel-like combustion. Control authority, performance, and emissions are most favorable in the RCCI regime, but the effects of charge preparation and fuel properties on the transitions between regimes and the extent of each regime are not well understood. To provide insight into these transitions, three different optical diagnostics were applied in a single-cylinder optical engine, and conventional engine diagnostics were applied in a multi-cylinder, all-metal engine. Both engines were operated with iso-octane and n-heptane as the low- and high-reactivity fuels, respectively, and operating conditions spanned a range of iso-octane fuel fractions between 70% and 90% of the total fuel energy at a global equivalence ratio of 0.35 with no exhaust gas recirculation. In the optical engine, high-speed imaging of natural luminosity was used to identify ignition sites and type of combustion progression in each regime, while visible-wavelength Mie scattering and single-shot, band-pass infrared (IR) imaging emission near 3.4 m provided qualitative information about liquid spray penetration and vaporized fuel distribution. Conventional metrics employed on the all-metal engine included efficiency, emissions, and heat-release rate analysis. Similar combustion mode regimes were observed for both experimental engine platforms, allowing an opportunity for optical engine data to provide insight into fundamental phenomena affecting regime extents and transitions.