Method of using an immersible probe (70; 80; 92), the immersible probe comprising: - a light source (71; 81), arranged to illuminate a sample of a liquid solution containing particles; - a matrix photodetector (76; 86) disposed facing the light source, arranged to acquire a global diffraction figure of the sample, the global diffraction figure comprising a plurality of elementary diffraction figures (20; 30) each associated with a particle; - a first tight housing (711; 811), holding the light source and having a first transparent window (712; 812) between the light source and the matrix photodetector; - a second tight housing (761; 861), holding the matrix photodetector, and having a second transparent window (762; 862) between the first transparent window and the matrix photodetector, the distance between the first transparent window and the second transparent window being variable in space or variable in time; in which the method comprises the following steps: - the immersible probe (70; 80; 92) is immersed in a liquid solution comprising particles such as cells, said liquid solution occupying the space between the first transparent window (712; 812) and the second transparent window (762; 862); - a distance between the first transparent window (712; 812) and the second transparent window (762; 862) is defined as a function of the particle concentration in the liquid solution and such that at least 30% of the elementary diffraction figures (20; 30) in a global diffraction figure are not superimposed with another elementary diffraction figure, each elementary diffraction figure being associated with a particle such as a cell; and with the use of the matrix photodetector (76; 86), and for said distance thus defined, a global diffraction figure of the sample is acquired, wherein the sample is formed by a portion of said liquid solution disposed between the first transparent window (712; 812) and the second transparent window (762; 862). - a light source (71; 81), arranged to illuminate a sample of a liquid solution containing particles; - a matrix photodetector (76; 86) disposed facing the light source, arranged to acquire a global diffraction figure of the sample, the global diffraction figure comprising a plurality of elementary diffraction figures (20; 30) each associated with a particle; - a first tight housing (711; 811), holding the light source and having a first transparent window (712; 812) between the light source and the matrix photodetector; - a second tight housing (761; 861), holding the matrix photodetector, and having a second transparent window (762; 862) between the first transparent window and the matrix photodetector, the distance between the first transparent window and the second transparent window being variable in space or variable in time; - the immersible probe (70; 80; 92) is immersed in a liquid solution comprising particles such as cells, said liquid solution occupying the space between the first transparent window (712; 812) and the second transparent window (762; 862); - a distance between the first transparent window (712; 812) and the second transparent window (762; 862) is defined as a function of the particle concentration in the liquid solution and such that at least 30% of the elementary diffraction figures (20; 30) in a global diffraction figure are not superimposed with another elementary diffraction figure, each elementary diffraction figure being associated with a particle such as a cell; and