1. A method of forming a desired optical image having a give minimum feature size on a selected surface of a layer of material sensitive to optical radiation, the method comprising the steps of: providing a source of optical radiation having at least one predetermined wavelength &lgr;; providing a layer of material, sensitive to optical radiation having the wavelength &lgr;, with a selected surface to receive an optical image thereon; positioning a mask, comprising image information and being at least partly transparent to the source radiation, between the radiation source and the layer of sensitive material, so that source radiation is transmitted through the mask towards the sensitive layer, and illuminating the selected surface with source radiation received by transmission through the mask to produce the desired optical image, characterized in that use is made of a mask in the form of a diffractive element wherein the image information is encoded in a two-dimensional array of image cells having dimensions which are smaller than the minimum feature size, each image cell having one out of at least two specific transmission levels and one out of at least three phase levels.