Anatomy of an Achromatic Lens
An achromatic lens, also referred to as an achromat, typically consists of two optical components cemented together, usually a positive low-index (crown) element and a negative high-index (flint) element. In comparison to a singlet lens, or singlet for short, which only consists of a single piece of glass, the additional design freedom provided by using a doublet design allows for further optimization of performance. Therefore, an achromatic lens will have noticeable advantages over a comparable diameter and focal length singlet.
An achromatic lens comes in a variety of configurations, most notably, positive, negative, triplet, and aspherized. It is important to note that it can be a doublet (two elements) or triplet (three elements); the number of elements is not related to the number of rays for which it corrects. In other words, an achromatic lens corrects for red and blue, independent of it being a doublet or triplet configuration. Refer to Figures 1 — 4 for illustrations of each type.
Figure 1: Positive [View Larger Image]
Figure 2: Negative [View Larger Image]
Figure 3 [View Larger Image]
Figure 4 [View Larger Image]
|Legend for Configurations|
|Dia.||Diameter||R||Radius of Curvature||ET||Edge Thickness|
|EFL||Effective Focal Length||CT||Center Thickness||P||Principle Point|
|BFL||Back Focal Length|