A reflected light is both regular (specular) and diffuse, and sometimes comes both from the bulk material and the surface (Figure 1). Regular reflection is defined as reflection in accordance with the laws of geometrical optics, without diffusion (Figure 2). Every surface reflects light, and the reflectance of any surface is the ratio of the reflected radiant flux to the incident flux.
Figure 1. Diffuse and specular reflectivity. Only specular reflections will be important for any surface which is smooth
Figure 2. With specular reflection, the angle of the reflected component is equal to the angle of the incident beam, with respect to the normal
The reflectivity of any uncoated surface depends on the angle of incidence; the polarization state of the incident light; the refractive index of the material, which differs with the wavelength of the incident light; and the polish or smoothness of the surface. For transmissive optics, the single surface reflectances vary from 3% for calcium fluoride, to 17% for zinc selenide, or even 36% for Gemanium.
Substrates with Dielectric Coatings
The reflectance of a substrate with a dielectric coating depends on the optical properties of both the coating and the substrate. Coatings can be designed as partial reflectors, or can be used to obtain very high reflectance. The wavelength range of any dielectric reflector is rather limited.
Figure 3. Reflectance of freshly deposited metallic coatings
A good example of a broadband specular reflector is a highly polished metal surface. A quality metallic reflector is made by vacuum depositing a thin metal coating on a polished substrate. Aluminum and chrome intermediary layers can be used on glass and gold, respectively. A typical coating is less than 100nm thick. A newly deposited aluminum coating has a reflectance from 200nm to more than 40,000nm. Figure 3 shows the reflectance of newly deposited metallic reflector coatings.
However, freshly deposited metals can tarnish rapidly, due to the formation of oxides and other compounds. As a result, the reflectance drops significantly. In order to prevent considerable loss of reflectance, the metal films can be coated with thin dielectric films.
Dielectric coatings can be designed for a preferred reflectance, including reflectances higher than available from a metal coating. Although high reflectance dielectric coatings are more durable, they are effective only over a narrow range of wavelengths. When compared to metallic coatings, most dielectric coatings are sensitive to polarization and angle of incidence. In contrast, Orion Instruments’ maximum reflection mirrors are exceptionally broadband and not susceptible to angle of incidence or polarization.
Metals are highly conductive in nature and absorb any EM radiation that enters the metal. However, this absorption is accompanied by high reflectance. Since optical radiation is electromagnetic in nature, the conductivity leads to energy loss via Joule heating. Conductivity, and thus reflectance and absorbance, are wavelength dependent.
Effects of Angle of Incidence
Figure 4. Reflectance for s and p polarized light incident at 45° on a freshly deposited aluminum surface
Metal reflectors are effective over a wide range of angles of incidence, unlike dielectric reflectors. There is a phase change on reflection, and this varies p and s polarized radiation, as shown in Figure 4. A linearly polarized ray incident at a high angle will be reflected partly as an elliptically polarized beam because of the phase change and reflectance differences for the p and s components.
Overcoating Metal Reflectors
As mentioned above, silver or aluminum can tarnish quickly and, as a result, the reflectivity drops significantly. Orion Instruments’ metal reflectors are coated with magnesium fluoride or silicon monoxide for improved protection. Magnesium fluoride is better in the ultraviolet, but is more delicate than a silicon monoxide coating. However, any such coating must be half-wave thick, or else the reflectance will be reduced. This is practical for visible or ultraviolet wavelengths, but not for the infrared, because the layer stress increases with thickness. The half-wave coating maintains the reflectance close to that of the bare metal over a spectral range of several hundred nm. Additionally, a multi-layer dielectric coating can be applied to improve the reflectance over a narrower spectral range.
Figure 5. Typical multi-layer dielectric reflective coating, designed for maximum reflectance at normal incidence and at 630nm
Figure 5 shows the reflectance with one quarter wave dielectric layer on a transparent surface. When additional layers of low (1.35) and high (2.2) index of refraction dielectrics are deposited on a transparent substrate, the reflectivity can be improved significantly.
Multi-layer dielectric coatings can be designed to create partial reflectors, narrowband reflectors with very high reflectances (> 0.995), or broadband reflectors. Multi-layer dielectric methods are also utilized to produce cold mirrors, interference filters, and polarizers.
Reflectivity is the reflectance of a material layer of such a thickness that there is no change in reflectance with an increase in thickness. The reflectance for s polarized light becomes different to that of p polarized. For incidences above several degrees, the bandwidth for s polarized light is visibly greater than the p polarized light. Orion exploits this difference to make its dielectric polarizers.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.