Illumination apparatus includes a light source which directs a polarized source beam in a downstream direction along an optical axis and desirably focuses the beam. A polarization-altering element downstream from the source such as a birefringent plate alters the polarization in a portion or portions of the source beam, so that the altered beam includes portions having different polarization directions. The altered beam passes downstream through a polarization-selective filter. The output beam from the polarization-selective filter can include either portion depending on the orientation of the filter. The arrangement can be used to provide a spot of a given size and shape at the focal location in a scanner such as a bar code scanner. The size and depth of field of the spot can be varied dynamically during operation.