A display for producing either visible or invisible images to the human
eyes comprising a plurality of pixels wherein each of the pixels includes
one or more visible radiation subpixels capable of producing visible
radiations to the human eyes and at least one invisible radiation
subpixel capable of producing invisible radiations to the human eyes; and
means for selectively operating the display to activate either visible or
invisible radiation subpixels to respectively producing visible or
invisible images to the human eyes.