Spherical lenses, those whose radius of curvature remain constant from edge to edge, are subject to various types of optical aberrations. These aberrations are not caused by defects in materials or manufacturing techniques, but rather by the laws of optics themselves. One common type of aberration associated with light rays passing through the optical axis of the spherical lens is known as chromatic aberration.
Chromatic aberration in spherical lenses
Chromatic aberration occurs because the lens refracts the various colors present in white light at different angles according to their wavelengths. Red light, for example, is not refracted at the same angle as green or blue light so the focal point on the optical axis of the lens is farther away from the lens for red light. Likewise, green light is focused closer to the lens than red light and blue light is focused in a plane that is closest to the lens. This phenomenon is commonly referred to as dispersion and occurs to a certain degree in all spherically shaped lens elements. The inability of the lens to bring all of the colors into a common focal plane results in a slightly different image size and focal point for each of the three predominant wavelength groups. The result is a colored fringe or halo surrounding the image, with the halo color changing as the focal point of the objective is varied.
Correcting chromatic aberration in spherical lenses
Chromatic aberration in spherical lenses is compounded by the difference in image magnification that occurs as a result of the varying focal planes for each color group, an effect termed chromatic difference of magnification. Aberrations of this type can be significantly reduced or eliminated, by making compound lenses that are composed of individual elements having different color-dispersing properties. Esco Optics provides a wide variety of optical glasses that are now available to lens designers. For example, crown glass has dispersive properties that enable it to be paired in a lens doublet with a flint glass element to produce an achromatic doublet lens system that focuses blue and red wavelengths in the same image plane. Additional refinement of an optical system with even more sophisticated glass formulas and shapes can reduce chromatic aberration even further.
Help with eliminating chromatic aberration in spherical lenses
The experienced lens professionals at Esco Optics can work with lens designers to eliminate chromatic aberration in spherical lenses by recommending the best combination of specialized materials and manufacturing techniques for the designer’s specific application.