Spectralon

Spectralon reflectance material is a thermoplastic resin that gives the highest diffuse reflectance of any known material or coating over the UV-VIS-NIR region of the spectrum and exhibits highly lambertian behavior. It can be machined into a wide variety of shapes for the construction of optical components such as calibration targets, integrating spheres and laser cavities.

Additional recommended knowledge

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Characteristics

The material has a hardness roughly equal to that of high-density polyethylene and is thermally stable to >350° C. It is chemically inert to all but the most powerful bases such as sodium amide and organo-sodium or lithium compounds. The material is extremely hydrophobic. Gross contamination of the material or marring of the optical surface can be remedied by sanding under a stream of running water. This surface refinishing both restores the original topography of the surface and returns the material to its original reflectance. Weathering tests on the material show no damage upon exposure to atmospheric UV flux. The material shows no sign of optical or physical degradation after long-term immersion testing in sea water.

Spectralon reflectance material gives the highest diffuse reflectance of any known material or coating over the UV-VIS-NIR region of the spectrum. The reflectance is generally >99% over a range from 400 to 1500 nm and >95% from 250 to 2500 nm. Surface or subsurface contamination may lower the reflectance at the extreme upper and lower ends of the spectral range. The material is also highly lambertian at wavelengths from 0.257 µm to 10.6 µm, although the material exhibits much lower reflectance at 10.6 µm due to absorbance by the resin.

The surface and immediate subsurface structure of Spectralon exhibits highly lambertian behavior. The porous network of thermoplastic produces multiple reflections in the first few tenths of a millimeter of Spectralon. Although it is extremely hydrophobic, this “open structure” readily absorbs non-polar solvents, greases and oils. Impurities are difficult to remove from Spectralon; thus, the material should be kept free from contaminants to maintain its reflectance properties. The use of Spectralon should be limited to the UV-VIS-NIR. Spectralon exhibits absorbances at 2800 nm, then absorbs strongly (<20% reflectance) from 5.4 to 8 µm.

Applications

Three grades of Spectralon reflectance material are available: optical-grade, laser-grade and space-grade. Optical-grade Spectralon is characterized by a high-reflectance and lambertian behavior over the UV-VIS-NIR wavelength region. Laser-grade Spectralon offers the same physical characteristics as optical-grade materials but is a different formulation of resin that gives enhanced performance when used in laser pump cavities. Space-grade Spectralon combines high-reflectance with an extremely lambertian reflectance profile and is the material of choice for terrestrial remote sensing applications.

Spectralon's optical properties make it ideal as a reference surface in remote sensing. For instance, it is used to obtain leaf reflectance and bidirectional reflectance distribution function (BRDF) in the laboratory. It can also be applied to obtain vegetation fluorescence using the Fraunhofer lines ^[1]. Basically spectralon allows removing the contributions in the emitted light that are not directly linked to the surface (leaf) properties but to geometrical factors.

References

1. ^ Evain S, Flexas J, Moya I (2004). "A new instrument for passive remote sensing: 2. Measurement of leaf and canopy reflectance changes at 531 nm and their relationship with photosynthesis and chlorophyll fluorescence". Remote Sensing of Environment 91 (2): 175–185.