Patent ID: 8284488

Claim:
A low pass filter which separates incident light into four light beams of two light beams composed of a component in an A polarization direction, and two light beams composed of a component in a B polarization direction orthogonal to the A polarization direction, comprising: a first optical path separation birefringent plate for separating the components in the two polarization direction in a first separation direction; a wave plate; and a second optical path separation birefringent plate for performing separation in a second separation direction intersecting the first separation direction for each of the components in the two polarization directions which are disposed therein in an order of incidence of the light, wherein the wave plate includes at least one phase plate for changing a polarization state of the incident light, when the wave plate is constituted of one phase plate, an optic axis of the one phase plate is so disposed as to intersect both of the A polarization direction and the B polarization direction, when the wave plate is constituted of two or more phase plates, the respective optic axes of at least two of the phase plates are so disposed as to intersect each other and intersect both of the A polarization direction and the B polarization direction, and when a light quantity of light having passed through the wave plate in the A polarization direction with respect to a total light quantity of light of a given wavelength out of light having passed through the wave plate is assumed to be I x %, a diremption value |50−I x | defined by using a value of the I x which is maximally deviated from 50% with respect to light in a wavelength range of 410 to 600 nm out of the light having passed through the wave plate is not more than 20%, wherein the light having passed through the wave plate is a linearly polarized light or an elliptically polarized light.