When observing biological materials and other transparent media with a microscope, stray light from outside the
plane of focus of the objective blurs the image. This is the significance of the confocal microscope.
In the confocal microscope a pinhole is placed such that only light that passes through a confocal point is viewed, thus blocking the unwanted scattered light and allowing all of the light that passed through the subject to be imaged.
The polarization longitudinal slit (PLS) achieves the function of the pinhole, transmitting only light from the focus. The PLS employs two photonic crystal wave-plate arrays each integrating two 1/4 wave-plates with orthogonal axes.
Linear polarized light incident on this structure is rotated by it.
Rays of light passing through the focus, shown in red, pass through wave-plates of the same orientation, thus the
effect of the wave-plates is cumulative and the azimuth of the polarization of the incident light is rotated
through 90 degrees.
Because light outside the focus, shown in blue, passes through wave-plates of opposite orientations the effect of
the wave-plates is cancelled and the azimuth of the polarization of the incident light is preserved.
Thus, light passing through the focus has a different polarization from that either side of the focus, and defocused light can be removed with a polarizer.