Particle characterisation instrument

A particle characterisation instrument configured to perform dynamic light scattering measurements and/or static light scattering measurements, comprising:

• a light source configured to create a light beam;
• a sample carrier defining a sample position;
• an optical element between the light source and sample carrier, the optical element configured to receive the light beam from the light source and to modify the light beam to create a modified beam, the modified beam:
• a) interfering with itself to create an effective beam at the sample position along an illumination axis;
• and
• b) diverging in the far field to produce a dark region along the illumination axis that is substantially not illuminated at a distance from the sample cell;
• a detector, at the distance from the sample cell, configured to detect light scattered from the effective beam by a sample in the sample cell, the detector positioned to detect forward or back scattered light along a defined scattering axis that is at an angle of 0° to 10° from the illumination axis.