What causes color fringing around fine lines on a computer monitor?

Color fringing around thin black verticals (photo by chrisdlugosz - CC-BY)

On some computer monitors, color fringes appear around narrow lines, or may appear and disappear as you move the cursor over small text. To see why this happens, we need to consider how the screen is constructed.

Computer screens, regardless of whether they are Liquid Crystal Displays (LCD), Cathode Ray Tubes (CRT) or Light Emitting Diodes (LED), represent colors using tiny colored

dots called pixels. A common arrangement is for each pixel to consist of a group of sub-pixels in the primary colors: red, green and blue (RGB).

If the pixels of the same color are in-line vertically, and part of the display is black (i.e. not lit up), the visual effect will depend on how many columns of pixels are not lit up. If a multiple of three columns is unlit, then each of the primary colors is extinguished to an equal degree, and our eye is likely to perceive a black stripe.

However, if the black part of the display covers something other than a multiple of three columns, the sub-pixels of different colors will be unlit in different proportions. This unbalances the color mixture of the remaining lit sub-pixels. For example, if a thin cursor covers a green column and a blue column but not a red one, our eye is likely to perceive some red fringing at the edge of the cursor.

The effect is not so noticeable on the latest monitors, which usually have their pixels closer together.

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