Local measurements from optical fiber micro probes are presented on vertical subcooled flow boiling of refrigerant Novec 649. Experiments were conducted in a rectangular flow channel with a cross section of 40 mm by 40 mm at a mass flux density of 1000 kg m(-2)s(-1) and a subcooling of 13 K. Boiling regimes ranged from single-phase convection to fully developed film boiling. Detailed description of the measurement methods and in particular of the fiber optical micro probes is given. Average boiling curves are provided for three measurement positions along the direction of flow. Void profiles above the copper heater surface are presented for void fraction, void ligament velocity, void ligament size and detection frequency. Time-resolved data of the void phase at critical heat flux was obtained at a distance of 100 mu m away from the heater surface. The dataset is discussed with respect to a previously developed conceptualization for the critical heat flux process in flow boiling and the recent consensus on dry spot formation and re-wettability being the main cause of critical heat flux in pool boiling. A phenomenological extension to flow boiling is presented. (C) 2019 Elsevier Ltd. All rights reserved.