A particle characterisation instrument (200, 500, 600, 700, 800, 900, 1300) configured to perform dynamic light scattering measurements and/or static light scattering measurements of particle size or particle size distribution, comprising: a light source (201, 501, 601); a sample carrier (202, 502, 602, 702, 802, 1002) defining a sample position; an optical element (204, 704, 804, 904a-c, 1304) between the light source (201, 501, 601) and sample carrier (202, 502, 602, 702, 802, 1002), the optical element (204, 704, 804, 904a-c, 1304) configured to modify light from the light source (201, 501, 601) to create a modified beam (207, 507, 607, 707, 807, 1307), the modified beam (207, 507, 607, 707, 807, 1307): a) interfering with itself to create an effective beam (208, 508, 608, 708, 1308) at the sample position propagating along an illumination axis (206, 506, 606, 706, 806); 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 (206, 506, 606, 706, 806) that is substantially not illuminated at a distance from the sample carrier (202, 502, 602, 702, 802, 1002); a detector (203, 403, 603, 653, 703, 903a-c, 953a-c, 1353), at the distance from the sample carrier (202, 502, 602, 702, 802, 1002), configured to detect light scattered from the effective beam (208, 508, 608, 708, 1308) by a sample in the sample carrier (202, 502, 602, 702, 802, 1002), 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 (206, 506, 606, 706, 806); 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 (201, 501, 601); a sample carrier (202, 502, 602, 702, 802, 1002) defining a sample position; an optical element (204, 704, 804, 904a-c, 1304) between the light source (201, 501, 601) and sample carrier (202, 502, 602, 702, 802, 1002), the optical element (204, 704, 804, 904a-c, 1304) configured to modify light from the light source (201, 501, 601) to create a modified beam (207, 507, 607, 707, 807, 1307), the modified beam (207, 507, 607, 707, 807, 1307): a) interfering with itself to create an effective beam (208, 508, 608, 708, 1308) at the sample position propagating along an illumination axis (206, 506, 606, 706, 806); 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 (206, 506, 606, 706, 806) that is substantially not illuminated at a distance from the sample carrier (202, 502, 602, 702, 802, 1002); a) interfering with itself to create an effective beam (208, 508, 608, 708, 1308) at the sample position propagating along an illumination axis (206, 506, 606, 706, 806); 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 (206, 506, 606, 706, 806) that is substantially not illuminated at a distance from the sample carrier (202, 502, 602, 702, 802, 1002); a detector (203, 403, 603, 653, 703, 903a-c, 953a-c, 1353), at the distance from the sample carrier (202, 502, 602, 702, 802, 1002), configured to detect light scattered from the effective beam (208, 508, 608, 708, 1308) by a sample in the sample carrier (202, 502, 602, 702, 802, 1002), 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 (206, 506, 606, 706, 806); 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.