A particle characterisation instrument configured to perform dynamic light scattering measurements and/or static light scattering measurements of particle size or particle size distribution, comprising: a light source ; a sample carrier defining a sample position; an optical element (204, 704, 804, 904a-c, 1304) between the light source and sample carrier , the optical element (204, 704, 804, 904a-c, 1304) configured to modify light from the light source to create a modified beam , the modified beam : a) interfering with itself to create an effective beam at the sample position propagating along an illumination axis ; and b) diverging to produce a dark region (209, 210, 509, 609, 610, 709, 710, 809, 909a-c, 910a-c, 1310) along the illumination axis that is substantially not illuminated at a distance from the sample carrier ; a detector (203, 403, 603, 653, 703, 903a-c, 953a-c, 1353), at the distance from the sample carrier , configured to detect light scattered from the effective beam by a sample in the sample carrier , the detector (203, 403, 603, 653, 703, 903a-c, 953a-c, 1353) positioned to detect forward or back scattered light scattered from the sample along a scattering axis that is at an angle of 0° to 10° from the illumination axis ; a processor configured to determine a particle size or particle size distribution from the scattering measurements by performing a dynamic light scattering analysis or a static light scattering analysis based on a scattering angle defined by the angle between the scattering axis and the illumination axis of the effective beam. a light source ; a sample carrier defining a sample position; an optical element (204, 704, 804, 904a-c, 1304) between the light source and sample carrier , the optical element (204, 704, 804, 904a-c, 1304) configured to modify light from the light source to create a modified beam , the modified beam : a) interfering with itself to create an effective beam at the sample position propagating along an illumination axis ; and b) diverging to produce a dark region (209, 210, 509, 609, 610, 709, 710, 809, 909a-c, 910a-c, 1310) along the illumination axis that is substantially not illuminated at a distance from the sample carrier ; a) interfering with itself to create an effective beam at the sample position propagating along an illumination axis ; and b) diverging to produce a dark region (209, 210, 509, 609, 610, 709, 710, 809, 909a-c, 910a-c, 1310) along the illumination axis that is substantially not illuminated at a distance from the sample carrier ; a detector (203, 403, 603, 653, 703, 903a-c, 953a-c, 1353), at the distance from the sample carrier , configured to detect light scattered from the effective beam by a sample in the sample carrier , the detector (203, 403, 603, 653, 703, 903a-c, 953a-c, 1353) positioned to detect forward or back scattered light scattered from the sample along a scattering axis that is at an angle of 0° to 10° from the illumination axis ; a processor configured to determine a particle size or particle size distribution from the scattering measurements by performing a dynamic light scattering analysis or a static light scattering analysis based on a scattering angle defined by the angle between the scattering axis and the illumination axis of the effective beam. <EOS>