Section III: Computer Screens

Consider printed material

  • Printed characters provide adequate visual information

  • Good contrast from ink color to paper color

  • Sharp transition - good edge definition

  • Our vision system is programmed to understand normal printed material

Computer screen characters

  • Present entirely different kind of image

  • Image is made up of pixels

  • Pixels are darkest in the center and gradually brighter towards the edge

More on pixels

  • If we graph light distribution in a pixel, we see a bell-shaped Gaussian curve

  • Pixel illumination is darkest in center and rapidly falls off away from center

  • Graph shows typical pixel illumination in a computer monitor

  • Green arrow points to transition area where brightness increases towards center of pixel (blue arrow indicates back-ground light level of screen)

Testing Pixel displays

  • Graph on right shows how computer testing device duplicates pixel illumination of a computer

Effect on Vision

  • When viewing computer screens, our focus drifts out beyond the screen

  • This causes a “Focusing mismatch”

  • Eyes and brain try to re-focus at the screen distance

  • This focusing cycle repeats over and over

Illustrating the Effect on Vision

The effect on vision

  • Over time, this additional continual effort to focus becomes exhausting to the eyes

Focusing mismatch

Improved Monitors

  • Desktop computers use cathode ray tube (CRT) technology

  • Newer monitors improve image resolution with higher pixel density but it has little effect on computer eyestrain

  • All monitor pixels exhibit the same poor edge definition

Laptop computers

  • Use totally different technology

  • They use a Liquid Crystal Display (LCD)

  • The monitor characters have similar, poor edge definition

  • Additionally, laptops are typically used under poor light (airplanes, etc.)

To continue, click on Section Four, Lesson One in the left column