When using a spectrometer for transmittance, reflectance, color, or compositional analysis, many users encounter the same problem: even with the same sample and the same instrument, the measured data can fluctuate significantly, resulting in poor repeatability.
The first reaction for most people is to assume that there is something wrong with the sample or the instrument itself. In reality, however, most measurement instability is caused by small details that are easily overlooked.
In Part 1, we will cover the three most common—and most easily overlooked—causes of measurement instability.
1. Probe Position Is Not Fixed
A Slight Difference in Angle Can Make a Big Difference in Results
Whether you are performing reflectance, transmittance, or color measurements, probe positioning is critical. The distance, incident angle, whether the probe is perpendicular to the sample, and whether it is in close contact with the sample can all directly affect the amount of light received.
Handheld measurements may seem convenient, but it is difficult to ensure that the probe is positioned exactly the same way every time. As a result, the measurement data can vary considerably.
To achieve good repeatability:
Fixed mounting > handheld operation
Standardized procedures > relying on operator experience
2. Improper Calibration
Using the Dark Spectrum and Reference Spectrum “Indefinitely”
The dark spectrum is used to eliminate electronic noise and ambient stray light, while the reference spectrum records the spectral distribution of the light source and the overall system response of the optical path. These two steps form the foundation of stable measurement data, yet they are often simplified or overlooked during operation.
For example, if the optical path has been moved but the previous reference spectrum is still used, or if there are changes in ambient temperature, light-source intensity, or fiber-optic condition, the measurement results may change.
To ensure stable results: whenever the environment changes or the optical path is adjusted, recalibration is required.
3. Ambient Light Interference
The “Stray Light Signal” You Cannot See
Workshop lighting, natural light, and fluorescent lamps all contain spectral components. If even a small amount of ambient light enters through gaps around the probe or the edge of an integrating sphere, it can be superimposed on the sample signal. The impact of stray light is particularly significant in reflectance measurements, color analysis, and low-light measurements.
Many users find that measurements taken during the day and at night produce two completely different curves. The root cause may simply be ambient light interference. Simple measures such as blocking ambient light, adding a cover, and avoiding direct exposure to light can significantly improve measurement stability.
In many cases, there is nothing wrong with the spectrometer, and there is nothing wrong with the sample either. The real problem is simply that these basic measurement details have not been properly controlled, causing the test results to repeatedly “fight” with each other.
In Part 2, we will continue with several other equally important—but even more easily overlooked—factors. Stay tuned!
Post time: Sep-10-2026