The present invention provides a method for identifying biological objects on a substrate; the method comprises: (i) illuminating the substrate carrying the biological objects with a light beam; (ii) acquiring at least one optical image of the illuminated substrate; the light beam comprises a wavelength band, the wavelength band is selected to induce at least one optical aberration in the optical image of the illuminated substrate; the optical aberration is predetermined as characterizing the biological objects; and (iii) processing said at least one optical image to provide a value or a combination of values indicative of presence or absence of said at least one optical aberration; said value or combination of values permitting detection or identification of the biological objects on said substrate. The invention also provides a processing unit, a system and a database for use in connection with the method of the invention.