Visual Acuity is a very sensitive test for blurring of the image and therefore this test will indicate whether or not the user is seeing the screen clearly. 

click the screen for a larger image

Visual acuity is a measure of the resolution of the eye i.e. the smallest detail the eye can resolve  For simplicity, the Vision Screener uses a Square with one side broken, which is presented in one of four orientations (left, right, up or down).  To begin with, the square is displayed in the smallest size possible on the screen (1 pixel separation between the limbs) and the user is asked to identify the orientation of the shape on four successive occasions.  If the user gives an incorrect response or selects Don't Know for any of the presentations, the size of the target is incremented and the procedure repeated.

The minimum target size will depend on the resolution of the screen.  A score of 100% indicates that the user could resolve the smallest shape possible on the screen.  Otherwise the score is based on the number of incorrect responses, the resolution of the screen and the viewing distance

The test is carried out binocularly (with both eyes) and then using each eye on its own.

Alternate scoring is available :

 5 correct out of 8 attempts (Germany)
Scoring relative to 1,0 (1,0 = 100%,  0,7 = 70%) (6/6 in UK, or 20/20 USA)

Alternate illiterate ‘E’ shape may be used instead of the broken square