The method enables a concentration of a species in a culture medium (12) to be estimated using an estimation system (10) comprising a light source (16), a transparent substrate (14) and a matrix photodetector (18), the substrate being located between the source and the photodetector, the medium comprising biological particles (32) and changing color when said concentration varies. Said method comprises the following steps:—placing the medium on the substrate,—illuminating the medium via the light source,—acquisition of an image of the medium via the photodetector, each image being formed by a ray transmitted by the illuminated medium and comprising at least one diffraction pattern, each diffraction pattern corresponding to the waves diffracted by a biological particle when the medium is illuminated,—and calculating an estimate of said concentration as a function of a pixel intensity of the acquired image.