An optical coherence imager comprising: a laser source for illuminating an area of tissue surface; a detector system comprising an array of detector elements capable of operation at video frame rates, the detector system being arranged to detect laser light scattered from the tissue surface and to generate laser output signals indicative of scattered light intensities; signal processing apparatus arranged to read the laser output signals generated by the detector system and, from these signals, to generate an optical coherence image and to further process selected one or more optical coherence images to form a representative image of blood flow within the tissue; and a display screen (18a, 18b) adapted to display the representative blood flow image; wherein the signal processing apparatus is further configured to: (i) obtain the selected one or more optical coherence images from output signals that are generated by the detector system only during a time period that is designated a stable time segment; (ii) select the stable time segment, during imager operation, from a plurality of tested time segments, each tested time segment corresponding to different time-domain subsets of a period of imager operation; and (iii) assess each tested time segment by extraction of stability indicators from a Fourier Transform analysis of time-varying values extracted from a region of interest within the illuminated tissue surface area that extends spatially over a number of image pixels. a laser source for illuminating an area of tissue surface; a detector system comprising an array of detector elements capable of operation at video frame rates, the detector system being arranged to detect laser light scattered from the tissue surface and to generate laser output signals indicative of scattered light intensities; signal processing apparatus arranged to read the laser output signals generated by the detector system and, from these signals, to generate an optical coherence image and to further process selected one or more optical coherence images to form a representative image of blood flow within the tissue; and a display screen (18a, 18b) adapted to display the representative blood flow image; wherein the signal processing apparatus is further configured to: (i) obtain the selected one or more optical coherence images from output signals that are generated by the detector system only during a time period that is designated a stable time segment; (ii) select the stable time segment, during imager operation, from a plurality of tested time segments, each tested time segment corresponding to different time-domain subsets of a period of imager operation; and (iii) assess each tested time segment by extraction of stability indicators from a Fourier Transform analysis of time-varying values extracted from a region of interest within the illuminated tissue surface area that extends spatially over a number of image pixels. (i) obtain the selected one or more optical coherence images from output signals that are generated by the detector system only during a time period that is designated a stable time segment; (ii) select the stable time segment, during imager operation, from a plurality of tested time segments, each tested time segment corresponding to different time-domain subsets of a period of imager operation; and (iii) assess each tested time segment by extraction of stability indicators from a Fourier Transform analysis of time-varying values extracted from a region of interest within the illuminated tissue surface area that extends spatially over a number of image pixels.