Drive shaft coupling having in-series bellows segments of different flexural resistance. Each lock includes a body portion, a handle at one end of the body portion for manually pivoting the lock, a latch at an opposite end of the body portion for latchably engaging with a complementary connector and a spring portion between the handle and the latch. The first and second deflecting members are provided in the light guide plate. Calibration and feedback control can be provided to stabilize the discriminated wavelength peak, which remains at the same center wavelength over the span of the internal field of view notwithstanding the difference in retarder thicknesses traversed along paths that are normal to the retarders versus oblique to the optical axis.