Convergence Test Patterns
Convergence is a measure of the degree to which the three electron beams in a colour CRT are aligned as they scan the raster. Good convergence means the beams overlap properly. Poor convergence, or misconvergence, results in colour fringing — the appearance of unwanted colour around the edges of images.
Convergence is never perfect in CRT monitors. It is normal to notice slight misconvergence at the corners but there should be little or none near the centre. If your monitor has convergence controls, set them for best results in centre screen. If you can't eliminate convergence your monitor may need internal adjustments.
Convergence can be tested with a convergence test pattern such as the crosshatch examples below. You should not be able to see any colour in these patterns.