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

Significance of the topic

Fiber-optic probes and integrating spheres are both widely used sampling interfaces for diffuse reflectance near-infrared (NIR) spectroscopy. Choosing the appropriate interface has direct consequences for signal quality, reproducibility, and practical workflow in routine FT-NIR analysis, quality control and process monitoring. Understanding the optical and practical trade-offs between these approaches helps analysts design robust methods and interpret spectral variability correctly.

Objectives and overview of the study

This technical study compared the spectral performance and repeatability of a 2 m SabIR fiber-optic probe against an integrating sphere using a Thermo Scientific Antaris FT-NIR system. The main goals were to characterize background spectral response, throughput and noise behavior, and to quantify repeatability and variance under three sampling presentations: integrating sphere measurement, fiber probe fixed in a remote stand, and fiber probe moved between consecutive acquisitions.

Methodology

• Instrument and general parameters: Thermo Scientific Antaris FT-NIR Method Development Sampling System; 100 co-added scans per spectrum; resolution 16 cm-1.
• Sampling standard: KTA-1920x wavelength standard (talc mixed with three rare-earth oxides).
• Background/reference: identical Spectralon® reference used for both sampling interfaces to eliminate background choice as a variable.
• Acquisition sets: ten spectra collected with the sample over the integrating sphere; ten spectra with the SabIR in a remote probe stand (probe stationary); ten spectra with the SabIR moved between each acquisition (probe allowed to swing freely).
• Software: RESULT used for acquisition; TQ Analyst used to display spectra and compute variance (standard deviation at each wavelength point) for each set of ten spectra.

Instrumentation used

• Thermo Scientific Antaris Fourier transform near-infrared (FT-NIR) spectrometer.
• Thermo Scientific SabIR fiber-optic probe with 2-meter fiber bundle.
• Integrating sphere module compatible with the Antaris system.
• Spectralon diffuse reflectance standard for background measurements.

Main results and discussion

• Spectral background response: Single-beam backgrounds collected with the silica-based fiber-optic probe contained intrinsic spectral features not present in the integrating sphere background. A prominent silica OH overtone band was noted near 7235 cm-1 and the fiber response falls off substantially toward lower wavenumbers (~4000 cm-1), reducing usable signal in that region relative to the sphere.
• Throughput and angle dependence: Fiber optics transmit only rays within the fiber acceptance angle; rays outside this angle are lost. Bending or changing the fiber geometry alters the internal propagation angles and therefore the transmitted intensity. In contrast, integrating spheres collect diffuse reflectance across a broad range of angles and are less sensitive to geometry changes, yielding more uniform collection of diffuse light from the sample.
• Noise and signal-to-noise: Under many practical conditions the S/N of the two interfaces was similar, with the integrating sphere typically offering a modest advantage. Longer fiber lengths exacerbate attenuation and lower S/N for fiber probes.
• Repeatability quantified by variance spectra: The integrating sphere produced very low variance (~400 micro-absorbance) across the spectral range for these experimental conditions. With the SabIR fixed in the remote probe stand variance increased to approximately 1 milli-absorbance. When the probe was moved between each acquisition (allowing the fiber bundle to swing), variance rose substantially—from ~7 milli-absorbance at lower wavenumbers up to ~11 milli-absorbance at ~10,000 cm-1—demonstrating strong sensitivity of fiber probes to positioning and fiber bending.
• Artifacts and small systematic effects: A low-wavenumber wave-like feature appeared in the probe variance spectra, likely due to fringing differences between window interfaces (sapphire window on the SabIR vs the KTA-1920x sample window) and small variations in the sample-to-window air gap. The integrating sphere provided a more reproducible air gap and did not exhibit the same fringing variability.

Benefits and practical implications of the methods

• Fiber-optic probes: provide the key advantage of remote and in situ sampling. They permit flexible measurement geometries and the ability to bring spectroscopy to the sample (e.g., large objects, process streams). Newer low-loss fibers and robust probe designs improve S/N and usability.
• Integrating spheres: offer superior reproducibility, stability and more uniform spectral response, especially at lower wavenumbers where silica fibers have reduced throughput. Built-in references (e.g., internal diffuse gold flags) enable automated background collections and reduce contamination or operator-induced variation.
• Operational trade-offs: For high-throughput QC or methods demanding tight spectral precision and repeatability, integrating spheres are generally preferable. For field work, remote probes and process measurements make fiber optics necessary despite somewhat higher variability that must be managed.

Future trends and potential applications

• Fiber improvements: continued development of lower-loss fiber materials and coatings to reduce intrinsic OH absorption bands and extend usable spectral range.
• Probe engineering: more rigid or strain-insensitive probe and cable designs to minimize bending-induced throughput changes, and integrated reference or auto-background capabilities in probe heads.
• Signal processing and calibration strategies: advanced correction algorithms, calibration transfer and machine-learning approaches to compensate for probe-induced spectral artifacts and improve method robustness across interfaces.
• Portable integrating sphere solutions: compact, field-deployable sphere modules and sample holders (e.g., disposable vial workflows) that combine the reproducibility of spheres with the mobility needed for on-site analysis.
• Process analytical technology (PAT) integration: both interfaces will continue to be used in PAT, with fiber optics dominating in-line/at-line sensing and integrating spheres preferred for repeatable laboratory or portable QC where sample presentation can be standardized.

Conclusion

This comparative evaluation demonstrates that while fiber-optic sampling extends the practical reach of FT-NIR spectroscopy, integrating spheres deliver superior spectral stability and repeatability under controlled sampling conditions. The choice between a fiber probe and an integrating sphere should be driven by the application: choose fiber optics when remote or in situ access is required and accept additional method development to control variability; choose integrating spheres when reproducibility, lower-wavenumber response and simplified standardization are priorities. Advances in fiber technology, probe design and data correction methods are narrowing performance gaps, and portable instruments now make integrating-sphere-based workflows increasingly viable outside traditional laboratories.

References

The study used manufacturer and product-specific materials and methods rather than external literature citations. Key items referenced in the experimental work include the Thermo Scientific Antaris FT-NIR system, the SabIR fiber-optic probe, and the KTA-1920x wavelength standard with Spectralon reference.