Comparing the Performance of a Fiber Optic Probe to an Integrating Sphere

Comparing the Performance of a Fiber Optic Probe to an Integrating Sphere — Technical Note Summary

Significance of the topic

Near‑infrared (NIR) diffuse reflectance is widely used in process and quality control because it enables rapid, non‑destructive analysis of solids and powders. Choice of sampling interface — fiber optic probe versus integrating sphere — directly affects spectral quality, reproducibility, and operational flexibility. Understanding their comparative strengths and limitations is essential for method development, routine implementation, and regulatory applications in FT‑NIR laboratories.

Objectives and overview of the study

• Compare spectral response, throughput, noise and repeatability between a 2‑meter SabIR fiber optic probe and an integrating sphere on a Thermo Scientific Antaris FT‑NIR system.
• Use a standard powder material (KTA‑1920x: talc mixed with rare earth oxides) to minimize sample heterogeneity and isolate instrument/sampling variability.
• Quantify variance for three measurement modes: integrating sphere, stationary fiber probe (in remote stand), and fiber probe repositioned between scans (simulating routine use).

Methodology and procedure

• Instrument: Thermo Scientific Antaris FT‑NIR Method Development System.
• Probe: Thermo Scientific SabIR 2‑meter fiber optic probe (silica fibers, sapphire window).
• Acquisition parameters: 100 scans per spectrum, spectral resolution 16 cm−1.
• Background: same Spectralon reference used for both sampling interfaces to remove background selection as a source of variation.
• Sample: KTA‑1920x wavelength standard placed over the integrating sphere window for sphere measurements and measured with the SabIR in two modes (fixed in stand; moved between acquisitions to allow fiber bundle to swing).
• Software: RESULT software for data collection and TQ Analyst for display and variance spectrum calculation.

Used instrumentation

• Thermo Scientific Antaris FT‑NIR Method Development System.
• Thermo Scientific SabIR 2 m fiber optic probe (silica fiber bundle with sapphire window).
• Spectralon diffuse reflectance standard as background reference.
• KTA‑1920x wavelength standard (talc with rare earth oxides) as sample.
• RESULT data acquisition software and TQ Analyst for variance analysis.

Main results and discussion

Spectrum response and material effects

• Fiber optics built from silica exhibit intrinsic absorption features in single‑beam backgrounds — notably a strong OH second‑overtone band near 7235 cm−1 and reduced response toward 4000 cm−1. These background features reduce effective signal‑to‑noise and can appear as artifacts in sensitive measurements.
• Integrating sphere backgrounds are flatter and show better low‑wavenumber response, resulting in higher usable signal across the spectral range important for many diffuse reflectance analyses.

Throughput and geometry considerations

• Fiber probes collect light within a limited acceptance cone; rays with incidence angles outside that cone are lost. Bending, motion or tight curvature of the fiber bundle changes acceptance and causes variable light losses.
• Increasing fiber diameter can increase throughput but sacrifices flexibility and portability — diminishing one of the fiber probe’s key advantages.
• Integrating spheres collect diffuse reflectance over a wide range of angles and are optimized (diameter and geometry) to efficiently capture sample‑scattered NIR energy, making them less sensitive to small alignment or angular changes.

Noise and signal‑to‑noise

• Under many conditions, sphere and fiber probe modules produce comparable signal‑to‑noise, but spheres are typically modestly superior. Longer fiber runs attenuate the NIR signal further, lowering S/N with distance.

Repeatability and variance analysis

• Integrating sphere measurements produced near‑perfect overlay of ten replicate spectra and very low variance (~400 micro‑absorbance) under the experimental conditions used.
• The SabIR in a remote probe stand (fixed geometry) showed higher variance (~1 milli‑absorbance) and observable spectral offsets compared with the sphere; some low‑wavenumber fringing was attributed to slight repositioning effects and differences between sample and probe windows (stainless steel mounting vs. sapphire).
• When the SabIR probe was repositioned between acquisitions such that the fiber bundle was allowed to swing, variance increased substantially (7–11 milli‑absorbance across the spectral range), with greater variability at higher wavenumbers (shorter NIR wavelengths) where angle‑dependent losses are amplified. Tighter curvature can increase variance more than threefold.
• Internal sphere references (e.g., diffuse gold flag) provide automated, contamination‑protected background collection and further improve reproducibility; this feature was intentionally not used in the study to keep background choice identical between modes.

Benefits and practical implications of each method

• Fiber optic probes: Best when remote or in situ sampling is required. They enable rapid, non‑destructive measurements at the point of interest, supporting process measurements and inline/atline applications. Improved modern fibers improve S/N and mechanical robustness relative to older designs.
• Integrating spheres: Preferable when maximum reproducibility, stability and ease of use are priorities — for example, in regulated QA/QC, method validation, or when analyzing many samples where sample positioning can be standardized (e.g., disposable vials). Spheres are less sensitive to alignment and fiber bending artifacts.
• Operational tradeoffs: If portability and lack of sample handling are critical, fiber probes win; if analytical precision and repeatability matter most, spheres generally perform better.

Future trends and potential applications

• Continued improvements in low‑loss fiber materials and probe head design should narrow the reproducibility gap by reducing intrinsic silica absorption features and bending sensitivity.
• Hybrid approaches and instrument portability (e.g., bringing FT‑NIR analyzers to the sample and using disposable vials over an integrating sphere) enable sphere‑level reproducibility while retaining field flexibility.
• Advanced signal processing, real‑time variance monitoring, and automated internal referencing can mitigate probe variability and expand fiber optic use in more demanding QA/QC contexts.
• Application growth is expected in process analytics, pharmaceutical PAT, and on‑site industrial testing as instruments and sampling accessories become more robust and user‑friendly.

Conclusion

Fiber optic sampling remains a powerful and flexible FT‑NIR approach for remote, rapid measurements, but it carries inherent limitations tied to fiber material absorption, acceptance angle losses, and sensitivity to bending or repositioning. Integrating spheres deliver superior spectral stability and reproducibility and are easier to standardize for routine analyses. Method selection should balance the need for mobility against the required analytical precision; with portable FT‑NIR platforms and careful sample presentation, integrating spheres are a viable and often preferable alternative to fiber probes for reproducible diffuse reflectance measurements.

References

• McCarthy W.J., Lowry S. Comparing the Performance of a Fiber Optic Probe to an Integrating Sphere. Technical Note 51695. Thermo Fisher Scientific, Madison, WI, USA. 2001/2008.