<P> Anatomically, there are 3 levels of binocular vision required to view stereo images: </P> <Ol> <Li> Simultaneous perception </Li> <Li> Fusion (binocular' single' vision) </Li> <Li> Stereopsis </Li> </Ol> <Li> Fusion (binocular' single' vision) </Li> <P> These functions develop in early childhood . Some people who have strabismus disrupt the development of stereopsis, however orthoptics treatment can be used to improve binocular vision . A person's stereoacuity determines the minimum image disparity they can perceive as depth . It is believed that approximately 12% of people are unable to properly see 3D images, due to a variety of medical conditions . According to another experiment up to 30% of people have very weak stereoscopic vision preventing them from depth perception based on stereo disparity . This nullifies or greatly decreases immersion effects of stereo to them . </P>

A steroscope provides the illusion of depth by