SOC's BRDF and Spectral Measurement Facilities

Measurement Methods

Directional Reflectance

UV, Visible, Near-IR

Over the wavelength range 0.3 to 2.0 mm, SOC employs a Cary 14 spectrophotometer fitted with transfer optics and a 9 inch diameter integrating sphere. This instrument is capable of measuring directional reflectance from the normal to angles of 75 to 80° from normal. It is a dual beam instrument operating in the absolute mode.

The Cary 14 employs a lead sulfide detector in the near-IR and a phototube detector in the visible and near-UV. Polarized reflectance measurements are made using a pair of Glan-Thompson prisms.

Mid to Long Wave Infrared (IR)

For mid to long wave IR reflectance SOC utilizes an ellipsoidal reflectometer designed and built by Surface Optics called the SOC-100. The ellipsoidal reflectometer is used instead of an integrating sphere because of the lack of energy throughput from integrating spheres at long wavelengths. The SOC-100 overcomes this problem by focusing the energy from a 1300° F blackbody back down onto either the reference standard or the sample. Energy reflected off the sample is collected with an overhead mirror and steered into an FTIR for deconvolution into the measured spectrum.

Reflectance measurements can be made from 1.5 to 50.0 um at incident angles of 10 to 80° from normal and out to 200.0 um at near-normal incidence. In addition to reflectance, the SOC-100 can be employed to collect transmittance data as well. Operating wavelengths for transmittance are the same as reflectance while incident angles are from normal to 60° from normal.

Specular/diffuse information can be collected using the specular beam blocker attachment on the SOC-100. By blocking the specular reflectance the instrument can gather diffuse reflectance (DDR). The DDR is then subtracted from the HDR to yield information on how specular the material is. This technique is extremely useful in screening samples before more extensive BRDF is utilized.

Bidirectional Reflectance (BRDF)

SOC has two BDR instruments. One instrument is generally employed with a large blackbody or quartz halogen source for measurements in the infrared or visible respectively. This instrument utilizes a cryogenic (liquid nitrogen or liquid helium) cooled detector or phototube detector and synchronous detection for maximum signal-to-noise ratio. It is routinely used to make BDR measurements anywhere in the wavelength range of 0.3 to 20 nm.

Wavelength determination is made by narrow band interference filters. Bidirectional measurements are made over the entire hemisphere subtending the sample, normalizing the results to the directional reflectance or can be made relative to a known reference standard.

The second instrument at SOC is a unique device that measures backscatter within a one degree cone of the incident direction, and is routinely being used to characterize the monostatic BDR of strategic materials. The uniqueness of this instrument resides in its ability to make higher angular resolution measurements (~ 0.4°) including measurements at the exact anti-incident direction for any incident angle from normal to grazing. Speckle resolved measurements are typically made for all four principle polarization combinations of source and detector. Currently measurements are being made at 0.488, 0.6328, 1.06, 3.39, and 10.6 mm, but other wavelength choices can be accommodated.

The importance of the high-resolution retroreflectance measurements, particularly to laser radars, is that some materials exhibit significant enhanced backscatter in the anti-incident direction, even for smooth surfaces (surface asperities and lateral correlation lengths less than the wavelength of the illuminating source). These materials need not necessarily be metallic, but at the present time this phenomenon can only be explained for metals. Furthermore, the phenomenon can only be seen with high angular resolution because the reflected spike is so narrow.

Surface Optics Corporation 11555 Rancho Bernardo Rd.San Diego, CA 92127
Phone: 858.675.7404 Fax: 858.675.2028