Ghost image (optics)
An undesired image appearing at the image plane of an optical system. Each surface of an optical system divides the incoming light into two parts: (1) the reflected light, which returns into the first medium, and (2) the refracted light. The reflected light is again divided into two parts when it in turn strikes another dividing surface. The light thus reflected twice forms an image which may be near the plane of the primary image. This may be a false image of the object or an out-of-focus image of a bright source of light in the field of the optical system. Thus a large number of undesired or ghost images may appear. See Optical image, Reflection of electromagnetic radiation, Refraction of waves
If the ghost images are far out of focus, they only diminish the contrast in the primary image, a condition known as flare. But if the ghost images are near the focal plane, they are very disturbing. This effect is especially noticeable if there is a bright light source in the field of the instrument, since the ghost image of the light source may have an even greater brightness than the image of the desired object. The coating of lenses with layers of fluorite and other materials has nearly eliminated ghost images from modern optical systems.
a secondary image that appears in the image plane of an optical system because of the reflection of light from the surfaces of components such as lenses, prisms, and plates. The chief cause of ghost images in lens systems is the multiple reflection of light from the lens surfaces. If brightly illuminated portions of the object being viewed border on sharply delineated shadows, the appearance of ghost images from such portions in the shaded parts of the primary image may noticeably distort the image of the object. Ghost images can be controlled by the use of antireflection coatings on the surfaces of the optical components.