Introduction to Infrared (IR)
Infrared (IR) radiation is characterized by wavelengths ranging from 0.750 -1000μm (750 - 1000000nm). Due to limitations on detector range, IR radiation is often divided into three smaller regions: 0.750 - 3μm, 3 - 30μm, and 30 - 1000μm – defined as near-infrared (NIR), mid-wave infrared (MWIR), and far-infrared (FIR). Infrared products are used extensively in a variety of applications ranging from the detection of IR signals in thermal imaging to element identification in IR spectroscopy.
The Importance of Using the Correct Material
Since infrared light is comprised of longer wavelengths than visible light, the two regions behave differently when propagating through the same optical medium. Some materials can be used for either IR or visible applications, most notably fused silica, BK7 and sapphire; however, the performance of an optical system can be optimized by using materials better suited to the task at hand.
As the need for IR applications grows and technology advances, manufacturers have begun to utilize IR materials in the design of plano-optics (i.e. windows, mirrors, polarizers, beamsplitters, prisms), spherical lenses (i.e. plano-concave/convex, double-concave/convex, meniscus), aspheric lenses (parabolic, hyperbolic, hybrid), achromatic lenses, and assemblies (i.e. imaging lenses, beam expanders, eyepieces, objectives). These IR materials, or substrates, vary in their physical characteristics. As a result, knowing the benefits of each allows one to select the correct material for any IR application.
The foremost attribute defining any material is the transmission. Transmission is a measure of throughput and is given as a percentage of the incident light. IR materials are usually opaque in the visible while visible materials are usually opaque in the IR; in other words, they exhibit nearly 0% transmission in those wavelength regions. For example, consider silicon, which transmits IR but not visible light, please refer to the transmission curve below.
Index of Refraction
While it is mainly transmission that classifies a material as either an IR or visible material, another important attribute is the index of refraction (nd). Index of refraction is the ratio of the speed of light in a vacuum to the speed of light within a given material. It is a means of quantifying the effect of light "slowing down" as it enters a high index medium from a low index medium. It is also indicative of how much light is refracted when obliquely encountering a surface, where more light is refracted as nd increases. Please refer to below figure shows the light refraction from a Low Index to a High Index Medium
The index of refraction ranges from approximately 1.45 - 2 for visible materials and 1.38 - 4 for IR materials. In many cases, index of refraction and density share a positive correlation, meaning IR materials can be heavier than visible materials; however, a higher index of refraction also implies diffraction-limited performance can be achieved with fewer lens elements – reducing overall system weight and cost.
Dispersion is a measure of how much the index of refraction of material changes with respect to wavelength. It also determines the separation of wavelengths known as chromatic aberration. Quantitatively, the dispersion is inversely given by the Abbe number (VD), which is a function of the refractive index of a material at the f (486.1nm), d (587.6nm), and c (656.3nm) wavelengths. Materials with an Abbe number greater than 55 are considered crown materials and those with an Abbe number less than 50 are considered flint materials. The Abbe number for visible materials ranges from 20 - 80, while the Abbe number for IR materials ranges from 20 - 1000.
The index of refraction of a medium varies as the temperature changes. This index gradient (dn/dT) can be problematic when operating in unstable environments, especially if the system is designed to operate for one value of n. Unfortunately, IR materials are typically characterized by larger values of dn/dT than visible materials.