Determination of optical properties from reflection of polarised light

An optical apparatus, which may be used an ellipsometer, comprises a unit 10 for emitting a beam B1 of polarised light onto a sample 12, the unit 10 being rotatable to rotate the plane of polarisation of the incident beam B1, and a photodetector 14 for receiving the beam B2 reflected from the sample. The unit may comprise a laser, a pin-hole aperture and a polariser (fig. 2) rotatable as a whole by means of a motor. No other optical elements are required between the unit and the detector. The sample 12 may also be rotated by means of a motor.