Design of a variable attenuator and fabrication of its shutter element

A variable attenuator employs a shutter element that is inserted into the path of a beam of radiation. The shutter element comprises a transparent substrate with an opaque layer formed on top of the substrate. The opaque layer forms a pattern in the shape of dots on one part of the substrate and in the shape of a solid layer with holes therein on another part of the substrate. The pattern of holes and dots are such that the radiation transmission function of the element varies as a smooth linear function with the length of the shutter element. Therefore, by inserting the shutter element in a direction along its length into the path of the radiation beam, the amount of attenuation of the beam can be accurately controlled.