Designing optics for index matching requires knowledge of the refractive index of the adjoining optical materials. By convention, the value of the refractive index at the Sodium D line (589nm, ‘yellow’) is usually quoted for optical materials, even when the wavelength of use is not 589nm. Materials that are index matched at 589nm will generally be close to being index matched at other wavelengths. However, for precise index matching, the refractive index should be measured at the wavelength of use.
Also, the refractive index for most polymer materials decreases with temperature at a rate in the range of -1×10-4 to -5×10<-4 per °C. This rate of change of refractive index with temperature, denoted the thermo-optic coefficient, or dn/dT, is typically one to two orders of magnitude lower for most glasses and crystals than for polymers. For this reason it is important to specify the temperature at which the material refractive index is measured. By convention, a measurement temperature of 25.0°C is often used.
Some of the most commonly used optical plastics, glasses and crystals are listed in the table below (Column 1). Under the right hand column reading, “Suitable Index Matching Materials”, ÅngströmLink offers a selection of polymer materials with physical consistencies of Soft Cure, Hard Cure and Fluids (Sub-Columns 3, 4 and 5).
Soft Cure. Soft cures, also known as gels, are non-migrating and so do not require containment seals to be held in the optical interface. Even so, these materials are extremely pliable, with viscoelastic characteristics which allow large values of strain at the optical interface without risk of delamination. Such strain relief is important, for example, when an assembly is subjected to large temperature swings during which the mating optical materials may shift their positions due to differences in their coefficients of thermal expansion.
Hard Cure. These are epoxy like polymers which when cured provide index matching as well as dimensional rigidity. They offer excellent adhesion, and some with versions with limited flexibility for strain relief.
Fluids. These materials are the most convenient to apply, particularly for temporary use in testing or prototyping. However, as true fluids, they will tend to flow out of the optical interface unless properly contained with seals.
