Monocular vs Binocular Testing
The autostereoscopic screen on the Nintendo 3DS presents an entirely different set of pixels to the right eye and the left eye. As a result, stereo vision can be tested without the aid of 3D Polaroid or red-green glasses. It also allows the ability to test only one eye. PDI Check has future versions that present monocular acuity screening using a concept called "rivalry" where one eye sees one Landholt C image as "C" while the other eye views a rotated optotype as "U."
Being careful to not produce eye strain and headache- with hours of game use- Nintendo has certain rules for games one of which is to avoid "rivalry." As a result, the Quick Screening version of PDI Check screens visual acuity with both eyes. Monocular acuity can be simply determined by closing or covering the non-tested eye.