A particle characterisation instrument (200), comprising a light source (201), a sample cell (202), an optical element (204) between the light source (201) and sample cell (202) and a detector (203). The optical element (204) is configured to modify light from the light source (201) to create a modified beam (207), the modified beam (207): a) interfering with itself to create an effective beam (208) in the sample cell (202) along an illumination axis (206) and b) diverging in the far field to produce a dark region (209) along the illumination axis (206) that is substantially not illuminated at a distance from the sample cell (202). The detector (203) is at the distance from the sample cell (202), and is configured to detect light scattered from the effective beam (208) by a sample in the sample cell (202), the detector (203) positioned to detect forward or back scattered light along a scattering axis (306) that is at an angle of 0° to 10° from the illumination axis (206).
