A vision testing system and a method of operating a data processing system for displaying an eye test pattern on a computer display to a patient during an eye examination are disclosed. The method includes causing the data processing system to display the eye test pattern in a chart region of a computer display controlled by the data processing system and characterized by a luminance in regions not displaying the eye test pattern. An intensity of light leaving the computer display is measured in a sensor region outside the chart region using a display luminance sensor. The luminance of the computer display is adjusted to provide a light level within a predetermined range of light levels based on the measured intensity of light. The data processing system can receive the measured intensity and adjust the display luminescence.
