This point of optimization where chromatic aberration is minimized is called the circle of least confusion. This compound lens can be optimized at a distance of either two wavelengths (achromatic lens), three wavelengths (apochromatic lens), or four wavelengths (superachromatic lens). The ultimate goal is to reduce the chromatic focal shift-the distance between focal points of dispersed wavelengths by a lens-between different wavelengths on the visible spectrum. This is achieved by apposing the two lenses and shaping them so that the chromatic aberration of one is neutralized by the other. Chromatic Dispersion CorrectionĬhromatic aberration can be corrected by combining the crown and flint glass elements to make a compound lens. Dispersive prisms are also often constructed from flint glass to achieve a high angular dispersion. Corrective glass lenses are often made from flint glass as the higher refractive index allows for a lighter weight. įlint glasses are optical glasses with high chromatic aberration (Abbe number below 50) which tend to have a high refractive index (typically greater than 1.55). They are particularly useful for imaging applications as they minimize chromatic aberrations. Many optical components such as lenses, mirror substrates, optical windows, and prisms are made from crown glasses. Crown vs Flint GlassĬrown glasses are optical glasses with low chromatic aberration (Abbe number above 55 or 50) which tend to have low refractive index. The red dots denote each glass type, and the blue labels subcategorize the glass types. The Abbe diagram compares various glass types by plotting its refractive index at a wavelength of 587.6nm and Abbe number. Using the Abbe diagram, glasses are generally categorized as crown glass (last letter “K”) or flint glass (last letter “F”) based on their Abbe number. As demonstrated in Figure 4, there is a general trend of increased dispersion (a lower Abbe number) in high-index glasses. The Abbe diagram is created to compare various glass types based on Abbe number and refractive index for a certain wavelength. A high Abbe number indicates low chromatic aberration, and a low Abbe number indicates high chromatic aberration. The Abbe number is inversely related to the amount of chromatic aberration. Also known as the V-number, the Abbe number combines the refractive indices of a material at three wavelengths (486.1 nm blue from hydrogen, 589.2 nm yellow from sodium, and 656.3 nm red from hydrogen) to approximate the amount of chromatic aberration a material will have when visible light passes through it (Figure 3). In the late 1800s, German physicist Ernst Abbe, a colleague of Carl Zeiss, created the Abbe number, a measure of the magnitude of chromatic dispersion for the visible spectrum. Calculation of the Abbe number to describe the amount of chromatic dispersion for a medium. This can be visualized on images at bright to dark boundaries as “fringes” of color. Each wavelength of light will thus be focused at a different distance away from the lens. In Figure 2, a single lens demonstrates chromatic aberration as different wavelengths of light have different focal lengths after passing through the lens. As a result, the index of refraction for a given medium will vary based on the wavelength of light going through it. When a single beam composed of multiple wavelengths of light passes through a lens, each wavelength of light will change direction and speed to different extents. The interaction of the particular wavelength of light with electrons in the medium gives rise to this phenomenon. Chromatic DispersionĬhromatic dispersion, also referred to as chromatic aberration, is a phenomenon where the speed and direction of light propagation in an optical medium depends on the wavelength of light. Chromatic aberration results in a single beam of light to have a different focal point for each wavelength. Lens Material Properties Index of Refractionįigure 2. Critical lens properties include refractive index, Abbe number (chromatic dispersion), specific gravity, and ultraviolet absorption. Lenses can be produced using a variety of materials and designed with several optical profiles to optimize use in specific applications. Eyeglasses also serve an important role in protecting the eyes from physical trauma and harmful radiation. Both lenses and prisms are also frequently used to improve eye alignment and treat diplopia in strabismus. Corrective spherocylindrical lenses are commonly used to treat refractive errors such as myopia, hyperopia, presbyopia, and astigmatism.
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