Abstract: The present invention relates to a method and device for ascertaining characteristics relating to porosity and grain size in rock sample. Four illuminators A, B, C and D direct collimated illuminating beams to a point O at the centre of a sample 40 and have projections onto the plane of the surface of the sample 40 at points A1, B1, C1, and D1. Angles <AOA1, <BOB1, <COC1, and <DOD1 have the same value of 45°. A camera (41) located directly above the point (O) acquires four images as the illuminators are individually illuminated. A processing section receives the acquired image data on line (42) and, after filtering, a common point distribution is evaluated based on the common points in the light intensity distribution data of two pairs of images with opposite illumination directions is found (A & C, B & D). At the same time, an edge enhancing distribution is evaluated based on the opposing value points in the light intensity distribution data for the opposing directions.