Abstract: Light measuring instruments have a sensitivity to polarization states of the light being measured. To minimize such sensitivity a depolarizer is used. The depolarizer includes a half wave polarization plate between the source of light being measured and the light measuring instrument or within the instrument, but ahead of the polarizing elements. The half wave plate is rotated through an angular sector of at least 90.degree. (.pi./2 radians) for each measurement period to ensure that all polarization states are measured. If the measurement period is sufficiently long, its half wave plate may be rotated asynchronously with respect to the measurement period through at least two and one-half revolutions.