Enhancing Image Clarity in the SWIR

From the March, 2008 Issue of Imaging Technology

Saturday, March 01 2008

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
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.

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 wave-lengths by bringing the extreme wavelengths (A & C) to a common focus.

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.