An optical system, for example in an optical pen, is arranged to illuminate an
object plane and to transmit an image of the object plane to an image plane. The
system has an optical component that reflects first radiation, which is emitted
by a radiation source, towards the object plane while also transmitting second
radiation from the object plane towards the image plane. The optical component
comprises a reflective surface portion arranged to reflect the first radiation,
and a spatially separate transparent surface portion arranged to transmit the second radiation.
