Surprisingly interesting to learn how our eyes interact with computer screens…
The human focusing system responds very well to images that have well-defined edges with good contrast between the background and any letters and symbols. The eyes react very well to most printed material that consists of solid black letters on a white background. The eyes react very differently to electronically generated characters than to printed characters on a page. Characters displayed on a computer screen or video display terminal (VDT) are made up of many, many small dots or pixels. Pixels are the result of an electron beam striking the phospor-coated rear surface of the screen. Each pixel is brightest in the center, with the brightness decreasing toward the outer edges. When a light meter with a very small aperture is passed across a pixel, with the light amplitude being charted against the horizontal location, the pixel shows a bell-shaped curve (Gaussian), while the same light amplitude graph of a printed character forms an almost perfect square wave.
The eyes have a very hard time focusing on the pixel characters. They focus on the plane of the computer screen, but cannot sustain that focus. They focus on the screen and relax to a point behind the screen, called the Resting Point of Accommodation (RPA) or dark focus. The RPA is different for every individual, but for almost everyone, it is further away than the working distance to the computer. The working distance is the distance from the computer user’s eyes to the front of the screen. So, the eyes are constantly relaxing to the RPA, and then straining to refocus on the screen. It is similar to raising the arm in a position like when volunteering for something or voting by hand and pumping the fist open and closed 40,000 times. The raised arm would get tired, let alone the hand, which symbolizes the focusing that the eyes must do in an 8 hour day. The following diagram illustrates this: