A method displays a single image on a graphical display for diagnostic purpose, without interrupting an observer's perception of the display of a sequence of images. Under that method, a sequence of images is displayed on a graphical display, interrupted only by the diagnostic image that is displayed for a time period not longer than 30 milliseconds. The diagnostic image may be used, for example, for calibrating colors and luminances of pixels in the graphical display. Another use of the diagnostic image may be, for example, determination of the ambient light reflected from the graphical display.