Optical imaging system and method, and aperture stop assembly and aperture element

A method for providing multiple focal lengths for inspecting a target having variable positions with respect to an optical imaging system employed in an inspection device, the method comprising:

• decomposing un-polarized light reflected or diffused from the target into first linear polarized light and second linear polarized light under different refractive indexes to respectively form a first focal length and a second focal length in the optical imaging system;
• modulating a state of polarization of the first and second linear polarized light in response to control signals;
• and
• filtering out one of the modulated first and second linear polarized light for creating a single image.

An aperture stop assembly, comprising:

• an aperture element comprising a polarizer part and defining an aperture part in the center of the polarizer part, the polarizer part configured for filtering out linear polarized light in a first fixed state of polarization;
• a light modulating element configured for modulating a state of polarization of linear polarized light in response to control signals;
• a polarizer element configured for filtering out linear polarized light in a second fixed state of polarization;
• and
• wherein the light modulating element is located between the aperture element and the polarizer element.

An aperture element, comprising:

• a linear polarizer comprising a polarizer part, the polarizer part configured for filtering out linear polarized light in a fixed state of polarization;
• and
• a first aperture part comprising an aperture through the linear polarizer in the center of the polarizer part, the aperture configured for not changing light propagated within a circumference of the aperture,
• wherein the linear polarizer and the first aperture part comprise a second aperture part.