ACHROMATIC LENS, a type of lens that produces images with negligible colored halos or fringes around point or line images. That is, the lens has negligible chromatic aberration.
For a simple lens consisting of a single element, the focal length is determined by the index of refraction of the glass and the radii of the bounding surfaces of the lens. The index I of refraction changes with color (or wavelength) of the incident light. The measure of the change in the refractive index is called the dispersive power of the glass, As a consequence of this dispersive power, the focal length changes with wavelength, causing an appreciable chromatic aberration. It is not possible to eliminate this error if only one type of glass is used. However, the dispersive power of various types of glass differs because each type is made of different materials. This difference makes it possible to construct an achromatic lens.
The simple achromatic lens is a compound lens formed from two simple lenses that are in contact. The dispersive power of the glass in one lens is compensated by the dispersive power of the glass in the other lens. By proper selection of dispersive powers and radii of the bounding surfaces, it is possible to design a compound lens that will image the light from two different colors or regions of the spectrum (for example, red at 6563 A and blue at 4862 A) in the same image plane. When this condition holds, the lens is said to be achromatic. Because the achromatic lens is corrected for only two colors, some residual chromatic aberration will stili be present. For most purposes, this small residual color does not impair the quality of the imagery by a noticeable amount.
Before the discovery of the principle of achromatism, it was not possible to make large-aperture refraeting telescopes for astronomy because the large chromatic error made the imagery useless. Consequently, mirror telescopes were used by Newton and later scientists. After the achromatic lens was invented (1758), many large refraeting telescopes were constructed and used. Achromatic lenses are used in telescopes, microscopes, and binoculars to minimize or eliminate the objeetionable colored fringes that would otherwise be present in the images.
Lenses of high relative aperture used in photographic cameras necessarily are achromatic to ensure high-quality imagery. Color photography would be impossible without good color correction by such lenses.
Some lenses are designed so that three colors are brought to a common focus. These lenses, called apochromatic or process lenses, are used in color process work that requires the exact registry of imagery for three different colors.
Principle of the Achromatic Lens
When parallel white light rays go through a single lens, the different points of focus. This condition is called chromatic aberration. One way to correct for chromatic aberration is to use two lenses of different kinds of glass. When put in contact to form an achromatic lens, the different colors come out parallel and produce a white light.