Charged particle assessment system and method

A method of operating a charged particle assessment system, comprising:

• holding a sample in a sample holder;
• projecting a charged particle beam towards the sample using a charged particle-optical device, the charged particle beam having a field of view corresponding to a portion of a surface of the sample, the charged particle-optical device having a facing surface facing the sample;
• and
• directing a light beam along a light path using a projection assembly such that the light beam reflects:
• off the surface of the sample up-beam, with respect to the light path, of reflecting off the facing surface;
• and
• at least twice off the facing surface up-beam, with respect to the light path, of being incident on the portion of the surface of the sample.

A charged particle assessment system, comprising:

• a sample holder configured to hold a sample having a surface;
• a charged particle-optical device configured to project a charged particle beam towards the sample, the charged particle beam having a field of view corresponding to a portion of the surface of the sample, the charged particle-optical device having a facing surface facing the sample holder;
• and
• a projection assembly arranged to direct a light beam along a light path between an optical system and the sample holder to be incident on the portion of the sample surface and such that along the light path, the light beam reflects:
• off the surface of the sample up-beam, with respect to the light path, of reflecting off the facing surface;
• and
• off the facing surface at least twice up-beam, with respect to the light path, of being incident on the portion of the surface of the sample.