A portable instrument for measuring a physiological quantity arranged to contact with the surface of organic tissue includes, essentially arranged in the same plane: an illumination device including at least one light source with an illumination surface for subjecting a portion of organic tissue to light emission in at least one wavelength range; and a detection device distant from the illumination device for detecting intensity of light emission produced by the illumination device after propagation in the organic tissue. The illumination device includes an optical element forming a guide coupled to the at least one light source for guiding light emission from the source by total internal reflection in a substantially parallel direction to the surface of the organic tissue and for distributing light emission into several illumination zones on the surface of the organic tissue over a substantially broader area than the illumination surface of the light source.