Characterization of the Reflectivity of Various Black Materials

Patrick Williams



Total Reflectance Plots

Specular Reflectance Plots

We present total and specular reflectance measurements of various materials that are commonly (and uncommonly) used to provide baffling while simultaneously minimizing the effect of stray light in optical systems. More specifically, we investigate the advantage of using certain black surfaces and their role in suppressing stray light on detectors in optical systems. By sweeping through a broad wavelength range (250-2500 nm), we measure the total reflectance of our samples to observe how they respond in the ultraviolet, visible, and near-infrared regimes. Additionally, we use a helium-neon laser to measure the specular reflectance of the samples at various angles. Finally, we compare these two separate measurements in order to derive the diffuse and specular fractions for each sample.

Total Reflectance Measurements (Diffuse and Specular)

We used the Hitachi High-Tech U-4100 UV-Visible-NIR Spectrophotometer in the Materials Characterization Facility (MCF) at Texas A&M University in order to obtain reflectance profiles for various black materials ranging from anodized metal samples to simple black paints. We also analyzed the reflectivity of everyday tools such as black duct and electrical tapes for their possible use in situations where stray light is unwanted.

The U-4100 dual beam spectrophotometer uses two different lamps to cover a wide range of wavelengths. For the deep UV, the U-4100 uses a deuterium lamp and switches to a tungsten lamp for UV, visible, and near IR measurements. The layout of the U-4100 includes monochromators, beam splitters, mirrors, focusing lenses, and detectors which can be used to analyze liquid or solid samples. Specifically, we obtained percent reflectance values for the wavelength range of 250-2500 nm and used a 5 percent reflectance standard (SRS-05) obtained from Labsphere Inc. to calculate the absolute reflectance of our samples. Below we will give descriptions of our experimental setup as well as the procedure we followed for measuring the reflectance of our samples.

Experimental setup:
Below is the experimental setup of the Hitachi High-Tech U-4100 UV-Visible-NIR Spectrophotometer used at the MCF at Texas A&M. The reference and test sample are placed in the 6 o'clock and 3 o'clock positions of the integrating sphere respectively. The U-4100 and all its experimental parameters are controlled by the program UV Solutions through the computer in the MCF.

Setup

Procedure:
  1. Set up parameters using the UV Solutions menu. Most of our parameters are the same as in the MCF handout for the U-4100 (linked here), but a complete list of our parameters are here.
  2. Collect a baseline measurement. This measurement records the reflectance of a reference substance and uses this as a baseline value to give relative (ratio) measurements of our samples. In our case, the baseline measurement was taken with BaSO4 wafers (~100% reflectance) in both the reference and sample slots of the dual beam spectrophotometer.
  3. Record calibration measurement of SRS-05.
  4. Record measurement of test sample in the 3 o'clock position of the integrating sphere.
  5. Data analysis and computation of absolute reflectance values using the calibration and sample measurements.
Data Analysis:
  1. Using a spreadsheet, we compared the SRS-05 MCF reflectance values to the Labsphere provided values. Take the ratio of the two SRS-05 (Labsphere/MCF) values and divide each sample's MCF reflectance value by this ratio at the appropriate wavelengths.
  2. To see the reflectance profile, plot these new absolute reflectance values versus wavelength.

Specular Reflectance Measurements

In order to measure the specular reflectance for our samples, we used a Helium-Neon laser to reflect off the surface of each of our samples at 10°, 22°, and 44° and used a Gentec Photo-Detector to measure the specular intensity at a specific distance away from the sample.

Experimental Setup:
Below is the setup we constructed on an optics bench in the Munnerlyn Astronomical Instrumentation Laboratory at Texas A&M University. Our setup, shown below, consists of a mounted Helium-Neon laser (top-left) which is reflected off the angled sample (top-right) and sent towards the photodiode detector (middle-left) where an intensity measurement is taken using the Gentec photo-detector (bottom-middle). All measurements were taken in a dark room environment.

Setup

Procedure:
  1. Align the laser using a mirror to make sure it is incident on the center of both the angled sample and the photodiode.
  2. Take a measurement of the initial intensity of the laser. This can fluctuate over a few hundredths of milliwatts.
  3. Take measurements of all the samples at the desired angle while being careful not to bump any components.
  4. Using a mirror, check again that the laser still is centered on the photodiode after the measurement.
  5. Change the angle of the sample if desired and continue taking measurements by repeating these steps.
Data Analysis:
  1. The specular reflectance value is calculated by taking the detected power of the sample and dividing by the initial intensity of the laser.
  2. With the specular reflectance value coupled with the total reflectance values (MCF) we calculated the diffuse reflectance values.
  3. Finally, we took the ratios of the specular and diffuse components to analyze each sample's fraction of specular reflectance. (Shown Here)
Gentec Photodiode: PH100-SiUV (S/N: 181951, datasheet link)

Labeling of Samples:
The labeling of the metal samples was done using a 3-letter ID system separated by periods. The first letter identifies the metal type (C: cast aluminum, A: 6061 aluminum, I: invar, S: stainless steel). The second letter signifies the initial metal treatment (R: raw, P: Polished, M: machined, B: Bead-Blasted). Finally, the last letter identifies the coating treatment of the sample (B: Black-Dye Anodization, H: Hardcoat Non-Dyed Anodization, N: Electroless Nickel Coating).

Example:
C.R.B : Cast Aluminum, Raw, Black-Dye Anodization

Anodizing:
Black Anodization: MIL-A-8625, Type II, Class 2, Black
Hardcoat Anodization: MIL-A-8625, Type III, Class 1, Non-dyed
Electroless Nickel: MIL-C-26074

Polishing:
Polished samples were prepared by sanding the faces of the samples in increasing grit sizes of 500, 1000, 1500, and 2000.

Thick/Thin Invar: Thick invar refers to regular cast invar and thin invar refers to cold rolled sheet invar.

Labsphere SRS-05-100 (Calibration certificate link), data points

 
Astronomy Group
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College Station, TX 77843-4242
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Texas A&M University - College of Science