Adjustable range finder and the method thereof

A method for adjustable range finder, comprising the steps of:

• projecting a first beam containing information relating to an object on a refractive optical element, which is comprised:
• a liquid-crystal layer, electrically connected to a voltage device, and a transmission blazed grating, so as to generate a second beam containing information relating to the object;
• enabling the voltage device to provide a first voltage to the liquid-crystal layer for forming an energy-concentrated Mth-order diffraction image by the projection of the second beam upon an optical imaging device;
• adjusting the voltage device for enabling the same to provide a second voltage to the liquid-crystal layer for forming an energy-concentrated Nth-order diffraction image by the projection of the second beam upon the optical imaging device;
• forming a series of images by the use of the Mth-order diffraction image and the Nth-order diffraction image;
• and
• comparing the disparity between corresponding points in the series of images so as to obtain the distance between the object and the refractive optical element.