The user views a red and a green circle on the screen through the filter 'glasses' provided so that the right eye sees one circle and the left eye the other. 

The user moves one of the circles (the smaller) using the mouse, or cursor keys until it appears to be aligned within the other, then registers the position by pressing the mouse button or the space bar or enter key,  thus giving a direct measurement of any imbalance of the eye muscles (phoria).

This is repeated for five positions on the screen and with each eye fixing, giving a comprehensive assessment of the muscle balance of the eyes while viewing a display.

The colours may be altered to obtain a complete cancellation of the circles so that only the green circle is seen through the red filter and only the red circle is seen through the green filter.  The colour adjustment is made from within the Administration area of the program.