Projection optical systems and methods having variable diaphragm based on average luminance and grayscale

Abstract

An illumination device according to the present disclosure includes: an illumination optical system that includes a first variable diaphragm and a second variable diaphragm, and that generates illumination light used for generation of an image by a pulse width modulation method on the basis of light from the light source; a luminance calculating section that calculates average luminance at least for each frame in an image signal; a gray-scale evaluation computing section that evaluates a gray-scale value in a screen for each display period of at least a unit gray-scale bit; a first aperture diameter determining section that determines an aperture diameter of the first variable diaphragm on the basis of the average luminance calculated by the luminance calculating section; and a second aperture diameter determining section that determines an aperture diameter of the second variable diaphragm on the basis of evaluation by the gray-scale evaluation computing section.