Light from a light source 10 is linearly polarized by a polarizer 11. Then
it propagates via a half-mirror 12 almost parallel to the normal to a
reflective liquid-crystal panel 13 and falls on the reflective
liquid-crystal panel 13. The reflected light reflected by the reflective
liquid-crystal panel 13 is received by a detector 15 via the half-mirror
12 and an analyzer 14. In this state, the reflective liquid-crystal panel
13 is rotated about an axis almost parallel to the normal to the
reflective liquid-crystal panel 13 and an angle (extinction angle) at
which the output signal of detector 15 reaches minimum is measured. Then,
the gap of the reflective liquid-crystal panel 13 is detected based on
the measured extinction angle. It is also possible to measure the output
signals of detector 15 by arranging the analyzer 14 in a state in which
the transmission axis thereof is almost parallel to the polarization
direction of the incident light and a state in which it is almost
perpendicular thereto and to detect the gap of the reflective
liquid-crystal panel 13 based on the measured signals.