4/30/2023 0 Comments Achroma vanitas hescah lecture![]() ![]() As mentioned in the early article, an achromat is formed by using a flint glass and a crown glass. This is, after all, one of the main reasons we will be using an achromatic doublet. The first aberration that we will try to correct for is chromatic aberration. Based on this information, we have several variables that we can manipulate, those are:įour radii of curvature (two for each lens),īased on these variables, we will try to optimize for three aberrations: chromatic, coma, and spherical aberrations Correcting Chromatic Aberration We start by assuming that we have the goals of our designs, that is, we have the focal length, wavelength range, aperture diameter, FOV already defined. What is presented here is just a method we have used in the past with acceptable results. It is, in a sense, an art that benefits from the optical engineer’s technical knowledge. It is a process that requires revisions and feedback. ![]() We will now review the steps necessary to optimize an achromatic doublet.īefore we start, we need to mention that there are different ways to optimize any design there is no “formula” that an engineer can apply to yield the best results. The combination of unique materials (flint glass and crown glass) and the lens shapes of the doublet, an optical engineer can use the science of light to deliver lens solutions without chromatic aberrations. 50MM AChrometic Doublet with minimum chromatic aberration Figure 2, shows a chromatic aberration using these numbers (note: the lens is not optimized).įigure 2. 4, we obtain values of 25.61mm and -50.48 for f1 and f2, respectively. For the flint glass, we choose N-SF8 with an Abbe number of 31.31 and for crown glass, we choose N-BK7 with an Abbe number of 64.17. 1 with an equivalent achromatic doublet, we will set feq to 50 mm and choose two materials for our lenses. Where V1 and V2 are the Abbe numbers for the first and second elements, respectively. Where f1, and f2 are the focal length of the first, and second element, respectively, and feq is the equivalent focal length of the system. Design of DoubletsĪ first approximation to design a doublet uses a two linear-equations system: ![]() Some types of crown glass are: K, SK, BK, LaK, LaSK and many others. Crown glass names usually include a “K” at the end (form the german “kron”, crown). Crown glass has a low dispersion (with an Abbe number above 50) and low refractive index (usually below 1.5), although some glass like BK7 has a refractive index that overlaps with some flint glass. For example N-SF8.Ĭrown glass on the other hand is used to fabricate the positive component. Lead-free flint glass usually has a “N” before their name. Some types of flint glass are: F, LF, SF, KF, BaF, BaLF. Flint glass has some lead and a particular high chromatic aberration (described by an Abbe number below 50), and high refractive index (usually above 1.55). The negative component is usually made of what is called “Flint glass”. The basic idea is that both lenses will compensate their respective dispersions and cancel each other. As mentioned before, the achromatic doublet has two lenses: a negative lens (concave) and a positive one (convex). A way to do this is by using an achromatic doublet. In order to reduce chromatic aberration, we need to find a way to match the lens focal length regardless of the wavelengths we are using. 50MM Chromatic Aberration on a Biconvex Lens That basically means that if we have a biconvex lens made with BK7, it will focus red and blue light at different points, resulting in chromatic aberration. For example, BK7 has a refractive index of 1.5228 at 480 nm (blue) and of 1.5131 at 700 nm (red). Chromatic aberration is the effect caused by the change in refractive index for a given material at different wavelengths. Chromatic Aberrationįirst, we need to understand what is chromatic aberration and why it occurs. Why these shapes and materials? We will try to explain the reason behind this structure. At its most basic, it is a two lens system configuration where one lens is a concave lens, usually made of a flint glass, and the other is a convex element, usually made with crown glass. It is used to reduce chromatic aberrations. One of the most common optical structures is the achromatic doublet. ![]()
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