Differences in reflectivity values

We have a Cary7000 with UMA - we have a method that measures each sample at sample angles 6 degrees through 76 degrees, using scan though each wavelength from 200nm - 2500nm. When we measure in increments of 10 degrees or 2 degrees (for sample angle) we get different reflectivity values for common angles across the range. Why is this? The method is exactly the same, using the same bassline, taken within 24 hours of each other.

Angle 10 Degrees 2 Degrees
6 23.51% 36.81%
16 25.03% 37.85%
26 27.37% 39.21%
36 31.30% 41.08%
46 36.74% 45.33%
56 43.59% 52.17%
66 50.99% 59.82%
76 54.99% 60.03%
    1. Angular Resolution:

      • The UMA provides the ability to move the detector and the sample independently of each other, allowing multi-modal measurements without moving the sample.
      • The angular resolution for setting the sample angle is 0.02°. When you measure in increments of 10 degrees or 2 degrees, you’re effectively rounding the sample angle to the nearest multiple of those increments. This rounding can lead to slight variations in the measured reflectivity.
    2. Sample Surface Characteristics:

      • Reflectance measurements are sensitive to the surface properties of the sample. Even small variations in surface roughness, microstructure, or contaminants can affect the reflectivity.
      • At different angles, the sample surface interacts with light differently. For example, at shallow angles, surface imperfections may scatter light more effectively, affecting the measured reflectance.
    3. Wavelength Dependency:

      • The wavelength range you’re scanning (200 nm to 2500 nm) plays a role. Different materials exhibit varying reflectance behavior across this range.
      • Some materials may have strong absorption or interference effects at specific wavelengths, leading to fluctuations in reflectivity.
    4. Polarization Effects:

      • The UMA uses wire grid polarizers to ensure high polarization accuracy. However, polarization can impact reflectance.
      • Depending on the sample’s anisotropic properties, the reflectivity may vary with polarization direction.
    5. Instrument Stability and Baseline:

      • Although you mention using the same baseline, small instrument drifts or variations can occur over time.
      • Ensure that the instrument is well-calibrated and stable during measurements.
    6. Environmental Factors:

      • Even within a 24-hour window, environmental conditions (temperature, humidity, etc.) can subtly affect measurements.
      • Changes in ambient light or vibrations may contribute to variations.
    7. Measurement Precision:

      • The Cary UMA provides unprecedented data quality in terms of accuracy and precision. However, no measurement system is entirely free from noise.
      • Variations in the sample setup, alignment, or handling can introduce minor discrepancies.
  • It may be sample related. We ordered reference mirrors and will attempt analysis using those

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