received by the International Bureau on 04 May 2018 (04.05.2018)

CLAIMS

1. A particle characterisation instrument configured to perform dynamic light scattering measurements and/or static light scattering measurements, comprising: a light source;

a sample carrier defining a sample position;

an optical element between the light source and sample carrier , the optical element 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 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; and

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. 2. The particle characterisation instrument of claim 1 , wherein the sample carrier is a sample cell, and the modified beam interferes with itself to create an effective beam in the sample cell along the illumination axis.

3. The instrument of claim 1 or claim 2, wherein the detector is a first detector positioned to detect forward scattered light, and further comprising a second detector positioned to detect back scattered light.

4. The instrument of claims 1 or 2 or 3, wherein the detector comprises an active detector area, and wherein the active detector area is located entirely within the dark region.

5. The instrument of any preceding claim, wherein the effective beam approximates a Bessel beam.

6. The instrument of any of claims 1 to 5, wherein the optical element is an axicon lens.

7. The instrument of any of claims 1 to 5, wherein the optical element is an obscured lens, the lens comprising a light -blocking obscuration at its centre.

8. The instrument of claim 6, wherein the detector or second detector positioned to detect back scattered light forms the light-blocking obscuration. 9. The instrument of claims 7 or 8, wherein the lens comprises an aperture through its centre; the detector or second detector positioned to detect back scattered light is positioned within the aperture.

10. The instrument of any of claims 1 to 5, wherein the optical element comprises: a beam splitter configured to split light from the light source into a first portion and a second portion; and

at least one lens positioned to receive the first portion and the second portion, and wherein the at least one lens is configured to direct the first portion and the second portion onto the sample cell.

1 1. The instrument of claim 10, wherein a lens is positioned to receive both portions of the light from the light source, and the lens is configured to direct both portions of light onto the sample cell, wherein the lens is optionally positioned such that the first portion and second portion are incident at opposing positions on the lens relative to the centre of the lens.

12. The instrument of any preceding claim, wherein the sample carrier comprises a droplet surface for holding a droplet of liquid sample, or wherein the sample carrier comprises a cuvette.

13. The instrument of any preceding claim, further comprising a light deflecting element configured to deflect light scattered along the scattering axis to the detector.

14. The instrument of claim 13, wherein the optical element, sample carrier and light deflecting element are supported on a body, the body optionally configured to be received in an optical instrument comprising the light source and the detector. 15. The instrument of any preceding claim, further comprising an additional detector configured to detect light scattered from the effective beam by a sample at the sample position, the additional detector positioned to detect scattered light along a defined scattering axis that is at an angle of 85° to 95° from the illumination axis. 16. A method of characterising particles dispersed in a sample using dynamic light scattering, the method comprising:

placing the sample on or in a sample carrier;

providing a light source to illuminate the sample;

modifying light from the light source using an optical element placed between the light source and the sample, the optical element configured to modify light from the light source to create a modified beam, the modified beam interfering with itself to create an effective beam in the sample along an illumination axis and 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;

detecting light that is forward or back scattered from the sample along a defined scattering axis that is at an angle of 0° to 10° from the illumination axis with a detector placed at the distance from the sample.

17. An optical assembly comprising:

a body, receivable in a cuvette holder, configured to support or house a liquid sample;

an optical element attached to the body, configured to modify light from a light source to create a modified beam, the modified beam: a) interfering with itself to create an effective beam in the body propagating 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.

18. The optical assembly of claim 17, wherein the body is configured to house a liquid sample and allow illuminating light from outside the body to communicate with the sample and scattered light to communicate from the sample to outside the body.

19. The assembly of claim 17 or 18, further comprising a light deflecting element attached to the body and positioned in the dark region, the light deflecting element configured to divert light scattered from the liquid sample in the direction of the illumination axis along an axis that is at a non-zero angle to the illumination axis.

20. A non-volatile machine readable medium comprising a computer program configured to operate an optical instrument, the instrument comprising a light source for illuminating a sample position along an illumination axis and a detector configured to detect light scattered away from the illumination axis by a sample at the sample position;

wherein the computer program is configured to determine if an optical assembly according to any of claims 17 or 19 is present in the instrument, and wherein:

if the program determines that the optical assembly is not present, to operate in a first mode for performing an off-axis scattering measurement; and

if the program determines that the optical assembly is present, to operate in a second mode for performing a zero-angle scattering measurement. 21. The machine readable medium of claim 20, wherein the computer program is configured calculate a physical property of the sample .

22. The machine readable medium of claim 21 , wherein the physical property comprises a particle size .