An optical measuring device (10) for detecting the coloring of test fields of a test strip (12), which is to be wetted with a liquid for detecting substances in said liquid, whereupon the reflectivity of the test fields changes depending on the concentrations of the substances to be detected, comprising: a measuring plane (14) wherein said test strip (12) is to be placed; an illumination device (16) for illuminating the measuring plane (14); a planar image sensor (36); an optical system for imaging said measuring plane (14) onto the image sensor (36); and an electronic evaluation unit (52) for evaluation of the signals detected by said image sensor (36), said illumination device (16) comprises light sources (18, 20, 22) or other means of different colors which serve to alternatively illuminate said measuring plane (14) in different colors, wherein said electronic evaluation unit (52) detects the coloring of the test fields from the images obtained under different color illuminations.