Abstract: Method including providing a substrate having a non-metallic surface with particles on the surface; depositing a layer of metal on the surface and the particles, so that each particle and an area of the surface without particles has a layer of metal, wherein, each particle has a gap between metal on that particle and metal on the surface; illuminating metal with electromagnetic rays, so that rays are scattered by metal on particles to produce scattered rays; or rays are absorbed by metal on particles and another portion of rays are reflected by metal on the surface to produce reflected rays; receiving scattered or reflected rays at an array of photodiodes; forming an image as pixels corresponding to photodiodes, wherein the colour of each pixel corresponds to the intensity and/or frequency of the rays received at that pixel; processing the image to determine the size of the particles.