Achromatic or color-corrected lenses for use in the visible portion of the electromagnetic spectrum have been addressed in literature, textbooks, and industry journals as early as the 18th Century. Many of these accounts by scientists and optical designers detail a method of selecting two dissimilar materials to form an achromatic pair or doublet with the ability to greatly counter image blurring resulting from the dispersive nature of refractive optical elements. Whether these tried and true optical formulae produce equally successful results in wavelengths beyond the visible range warrants further examination.

Figure 1. Transmission of Schott BK7 optical glass.

Unlike the more extreme regions of the infrared, shorter wavelengths are transmitted well by the majority of glass materials used to design visible lenses. Figure 1 indicates the transmittance from 300 nm out to about 2.5 microns for one of these types of glasses, the Schott BK7.

This seemingly attractive characteristic of optical glass may perhaps be the major contributor to the common misconception that optics designed for visible imaging, since transparent in SWIR light, must also offer comparable imaging performance in the SWIR.

Figure 2. Achromatic doublet model of positive crown and negative flint elements.