COMPLEX-VALUED SPATIAL FILTER USING PHASE MODULATION ONLY

1319941 Phase spatial filter INTERNATIONAL BUSINESS MACHINES CORP 4 Jan 1971 [19 Jan 1970] 255/71 Heading G2J An optical phase-type spatial filter, for producing an image in an incident beam of radiation by selective phase and amplitude modulations of different parts of the beam and the consequent operation of interference patterns, comprises a phase modulating medium having phase modulation characteristics according to first and second components, the first component representing the desired phase modulation on the incident beam, said second component having an efficiency which varies as a function of the desired amplitude modulation on the incident beam and having a frequency which is at least the minimum value required to place undesired interference products outside a selected order interference pattern, whereby the image can be produced in said selected order interference pattern. If desired the second component may have a fixed frequency. It is preferred that the depth of the second component of phase modulation is used to control the efficiency. The interference products of this second component, e.g. orders other than zeroth order, lie on an axis not parallel to the primary axis of the image in the zeroth order. The second component is represented by a pattern of dots, the dots having such size to represent spatial frequency to place the undesired interference products outside the selected image and a depth proportional to the desired amplitude modulation. The filter may be fabricated on a computer driven plotter. Alternatively to such a "single plot" a photosensitive film may be exposed to achieve the desired modulation as if the process were to be carried to completion. Over the film is then laid a mask having a given transmission such that exposure through the mask gives the desired phase modulation that would have been achieved had the mask not been there. Once the filter has been made copies can be pressed from a metal master.