Spectralon
Encyclopedia
Spectralon is a fluoropolymer
, which has the highest diffuse reflectance
of any known material or coating over the ultraviolet
, visible
, and near-infrared regions of the spectrum. It exhibits highly Lambertian
behavior, and can be machined into a wide variety of shapes for the construction of optical components such as calibration targets, integrating sphere
s, and optical pump cavities
for laser
s.
Spectralon's reflectance is generally >99% over a range from 400 to 1500 nm and >95% from 250 to 2500 nm. However, grades are available with added carbon to achieve various gray scales. 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 257 nm to 10600 nm, although reflectivity decreases at wavelengths beyond the near infrared. Spectralon exhibits absorbances at 2800 nm, then absorbs strongly (<20% reflectance) from 5400 to 8000 nm. Although the diffused reflectance has been shown to increase overall laser efficiency, the material has a fairly low damage threshold of 4 joules per square centimeter, limiting its use to lower powered applications.
The Lambertian reflectance arises from the material's surface and immediate subsurface structure. The porous network of thermoplastic produces multiple reflections in the first few tenths of a millimeter. Spectralon can partially depolarize the light it reflects, but this effect decreases at high incidence angles. 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.
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
.
Basically Spectralon allows removing the contributions in the emitted light that are not directly linked to the surface (leaf) properties but to geometrical factors.
Fluoropolymer
A fluoropolymer is a fluorocarbon based polymer with multiple strong carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases.-History:Fluoropolymers were accidentally discovered in 1938 by Dr. Roy J...
, which has the highest diffuse reflectance
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...
of any known material or coating over the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, visible
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
, and near-infrared regions of the spectrum. It exhibits highly Lambertian
Lambertian reflectance
If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view. More technically, the surface luminance is isotropic...
behavior, and can be machined into a wide variety of shapes for the construction of optical components such as calibration targets, integrating sphere
Integrating sphere
An Integrating sphere is an optical component consisting of a hollow cavity with its interior coated for high diffuse reflectivity , having relatively small holes as needed for entrance and exit ports....
s, and optical pump cavities
Optical pumping
Optical pumping is a process in which light is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion...
for laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s.
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's reflectance is generally >99% over a range from 400 to 1500 nm and >95% from 250 to 2500 nm. However, grades are available with added carbon to achieve various gray scales. 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 257 nm to 10600 nm, although reflectivity decreases at wavelengths beyond the near infrared. Spectralon exhibits absorbances at 2800 nm, then absorbs strongly (<20% reflectance) from 5400 to 8000 nm. Although the diffused reflectance has been shown to increase overall laser efficiency, the material has a fairly low damage threshold of 4 joules per square centimeter, limiting its use to lower powered applications.
The Lambertian reflectance arises from the material's surface and immediate subsurface structure. The porous network of thermoplastic produces multiple reflections in the first few tenths of a millimeter. Spectralon can partially depolarize the light it reflects, but this effect decreases at high incidence angles. 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.
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. Laser-grade Spectralon offers the same physical characteristics as optical-grade material but is a different formulation of resin that gives enhanced performance when used in laser pump cavities. Spectralon is used in a variety of "side pumped" lasers. Space-grade Spectralon combines high reflectance with an extremely lambertian reflectance profile and is used for terrestrial remote sensingRemote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals 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
Bidirectional reflectance distribution function
The bidirectional reflectance distribution function is a four-dimensional function that defines how light is reflected at an opaque surface...
(BRDF) in the laboratory. It can also be applied to obtain vegetation fluorescence using the Fraunhofer lines
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer . The lines were originally observed as dark features in the optical spectrum of the Sun....
.
Basically Spectralon allows removing the contributions in the emitted light that are not directly linked to the surface (leaf) properties but to geometrical factors.