Enhancing Image Clarity in the SWIR

A chromatic 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.

Recent efforts have demonstrated considerable promise in producing lenses and optical systems that deliver images captured in the non-visible wavelengths with enhanced resolution and contrast. These unique optical designs are greatly improving the way in which high-quality imaging in the near-infrared/shortwave infrared (NIR/ SWIR) wavelengths is achieved.

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

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.

Figure 3. Dispersive properties of optical material.

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 4. Achromatic doublet now including defocused SWIR energy.

To understand why this is not the case, one must consider how light “bends” or refracts through a transparent medium. The most commonly known effect of dispersion in optics is the separation of white light into the full-color spectrum created by a prism. This phenomenon is scientifically explained by an equation known as Snell’s law, which describes how the angle of refraction for a particular wavelength of light in a prism depends on the refractive index of the prism material. As the refractive index is dependent on wavelength, the angle by which the light is refracted must also be dependent on wavelength, and therefore not constant. Furthermore, since the degree and consistency by which light is refracted in a medium is dependent on wavelength, a material will, for the sake of simplicity, change the way it behaves depending on the band in which it is operating.

The fact that the refractive index of any material is a nonlinear relationship is an added complexity that is not as commonly known. This means that the angle through which light changes with respect to wavelength is better expressed as a polynomial function rather than a linear one.

Dispersive Effects

Figure 5. Chromatic focal shift with respect to wavelength over the 0.486 to 1.0 micron range.

The challenge for an optical designer is to select materials with differing dispersive characteristics, which when combined, will complement each other to reduce the errors produced with varying wavelengths. Anyone who has designed an optical system can tell you that achromatic correction is one of the most difficult balancing acts to master in quality design space.

Figure 6. Achromat blur in visible (a) and achromat blur in SWIR (b).

The most basic approach to counter dispersive effects is the achromatic doublet. The doublet typically consists of two elements made of different materials — in this case, a crown and flint glass. This pairing, illustrated in Figure 2, reduces the amount of chromatic aberration over a specific range of wavelengths by bringing the extreme wavelengths (A & C) to a common focus.

Figure 7. Visible lens operating in the SWIR (a) and specialized SWIR lens design (b).

Considering the dispersive characteristics of some common optical materials helps to further appreciate why the classic visible achromatic doublet results in degraded performance when employed in SWIR imaging. Figure 3 depicts the dispersive properties of Abbe V-value versus the relative partial dispersion P of a cluster of common optical glasses. This type of comparison depicts the change or slope of a material’s dispersive behavior over a particular spectral range versus the instantaneous change over a finite segment of that spectral range. The data in blue represents materials in the visible spectrum, while those in red indicate the altered states of the same materials when encountering light from the SWIR band.

Since the designer will select materials with dispersive properties defined to a large degree on their geographical positions on this chart, it becomes evident that these same materials can become a poor pairing when there is so drastic a change in dispersive behavior. The extent to which the changing material properties impact something as simple as the achromat depicted earlier is demonstrated in Figure 4. In both this and Figure 2, light from the extreme visible wavelengths designated as A & C remain in sharp focus, while light in the median wavelength designated in the figures as B represents the residual error known as secondary spectrum. Judicious selection of the two materials will reduce the residual error to an acceptable level. Referring again to Figure 4, D represents light beyond the original highest wavelength or in our example, the SWIR region, 0.9 to 1.7 microns. This figure clearly shows that the SWIR wavelengths are significantly blurred when compared to the light from the original visible band.

Another way to represent the performance degradation is to consider a graph of focus shift with respect to wavelength. In the plot shown in Figure 5, the sharp departure in the doublet’s ability to control chromatic aberrations outside the original A to C spectral range can be seen. Not only is the light in this graphic no longer falling on the intended focal plane, but one can now see that at no point beyond the original A to C range will any simultaneous wavelengths be corrected. In other words, achromatic imaging beyond the visible band will not take place.

Figure 6 depicts what the degradation might look like in terms of image blur. This side-by-side comparison shows two optimally focused images of a point source in both the original visible spectrum for which the lens was designed (a) and in the non-visible, SWIR light (b).

The example of degradation in the simple achromat can be considered minor when compared to the degradation one might see in a more sophisticated lens. High-quality lens assemblies produced for visible imaging applications often are meticulously designed to balance critical aberrations over their spectral band. When employed outside the visible band, the balancing act is quick to falter.

A final effort to emphasize these effects is made by observing the comparison of images produced by both a common, high-quality lens designed for visible imaging (Figure 7a) with one specifically designed for SWIR band imaging (Figure 7b).

Conclusion

Shortwave infrared imaging technology has made tremendous progress in recent years. These advancements have lead to improvements in manufacturing technologies and scientific study, and have opened new doors in biomedicine, security, and life sciences.

SWIR imaging advancements are further strengthened with the design and fabrication of specialty optics and lens assemblies specifically intended to perform in these extended wavebands. An optical imaging system is only as strong as its weakest component and in the case of employing visible optics for SWIR imaging, the weakest component just might be the lens.

The work being conducted now on SWIR imaging optics is creating options for high-quality optics for non-visible waveband applications. The results translate into improved image clarity and higher transmission for the non-visible, but more specifically, the NIR/SWIR operating wavelengths. The successful development of these new and cost-effective SWIR lenses is expanding progress in this critical imaging arena.

This article was written by Christopher Alexay, Chief Optical Designer, at StingRay Optics, LLC, Keene, NH. For more information, Click Here .