Optical apparatus for controlling the distribution of illumination

Abstract

In order to provide a laser beam having a relatively constant width (i.e. transverse spacing between points receiving a theshold irradiance), a converging lens 15 having substantial negative spherical aberration is used. In the far field, the beam comprises paraxial rays 18 which have diverged to provide the desired beam-width. In the near-field, the smaller, higher-irradiance beam formed by these rays is augmented by a `sheath` of marginal rays 19 which are not refracted towards the optical axis as strongly as the paraxial rays. The shape of the beam in the far field is determined primarily by the shape of the (laser) light source 10, while that of the near-field beam is controlled by a mask 14. In another arrangement, a lens 102 having substantial positive spherical aberration is used to create a `light source` having an accurately-controlled brightness distribution and very small size. Paraxial rays 109 are brought to a focus at a plane 108 defining the `position` of the `light source`, while the more strongly refracted marginal rays 110 cross the optical axis before reaching this plane, thus creating a halo round the bright central beam produced by the paraxial rays. The brightness distribution at the plane is controlled by adjusting the spherical aberration of the lens, and by masking.