An afterimage is a false image that occurs when an object or image a person has viewed continues to appear in that person’s field of vision even after he or she is no longer looking at it. This effect can occur in both positive and negative forms, depending on the image or object being looked at. Positive afterimages consist of the image remaining in its original color or form, while a negative effect occurs when the image that remains is opposite in color or tone. An afterimage is often caused by physical properties of the eye, rather than the brain.
Also called a ghost image, an afterimage typically is the result of visual exposure to bright lights or to an image created to act as an optical illusion. When someone stares into a bright light, such as a burning light bulb or the sun, and then looks away from that light source, he or she will typically continue to see the light. Direct visual exposure to intense light, such as the sun, can cause permanent damage to a person’s eyes and should be avoided. When looking at a bright light, the afterimage that remains is also in the form of a bright light, which is referred to as a positive image.
A negative afterimage is typically created due to looking at an image or object that is designed as an optical illusion. These illusions often consist of areas of intense color, which a person stares at for about a minute, and then looks away at a white wall or sheet of paper. The image remains in a person’s visual field even when he or she looks away, and typically appears as a negative image with respect to color or tone. This means that the afterimage of a black object will appear white, while a bright green box will create a red ghost image.
The way in which a negative afterimage occurs is connected to how the human eye perceives colors and tones. Photoreceptors in the eye are responsible for taking in information, such as light and color, and then that information is transformed into visual images that are understood by the brain. When a particular color is received in excess amounts, such as by staring at a green box for a minute, then the photoreceptors become desensitized to the data being received. Once a person looks away from that image, then the other photoreceptors compensate for the ones that have become desensitized, generating a false image until the photoreceptors in the eye adjusts and the afterimage disappears about a minute later.