Patent ID: 7597446

Claim:
An illumination optical system comprising: a light source; a polarization conversion element which converts a luminous flux from the light source into a luminous flux having a certain polarization direction; a polarization beam splitter which splits the luminous flux from the polarization conversion element on the basis of the polarization state of the luminous flux; and a phase plate which gives a phase difference to the luminous flux from the polarization beam splitter, wherein the illumination optical system illuminates a surface to be illuminated with the luminous flux from the phase plate and satisfies the following conditions: θx<θy 0°<φy≦θy where a direction which is orthogonal to one of fast and slow axes of the phase plate and in which the luminous flux is reflected and split by the polarization beam splitter is defined as the x direction, a direction which is orthogonal to the other of the fast and slow axes and to the x direction is defined as the y direction, θx and θy represent the maximum incident angles of the luminous flux onto the surface to be illuminated in the x and y directions, respectively, and φy represents an incident angle of the luminous flux onto the phase plate at which the phase difference at a principal wavelength of the luminous flux that passes through the phase plate is 90 degrees.