Illumination optical system and image projection apparatus

An image projection apparatus comprising:

• an image display element;
• an illumination optical system which illuminates the image display element;
• and
• a projection optical system which projects a luminous flux from the image display element onto a projection surface,
• wherein the illumination optical system comprises:
• 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 the image display element 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 image display element 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.