The Amsler grid is a simple square chart with a dot in the middle of what resembles graph paper, and can be used to test ones central visual field while at home. The lines are usually black, and the background white, though sometimes the colors are reversed. The test is a diagnostic tool that aids in the detection of visual disturbances caused by changes in the retina, particularly the macula (e.g. macular degeneration, Epiretinal membrane), as well as the optic nerve and the visual pathway to the brain.
When taking the test, the patient will be asked to look at the grid and concentrate on the dot in the middle, using one eye at a time. If the lines near the dot appear wavy, blurred, or are missing, macular degeneration, and the loss of central vision, may be occurring. The distorted image of the grid is not an optical illusion but is evidence that some photosensitive cells in the macula may have been destroyed, obscured (usually by blood) or displaced (as in a retinal detachment).