What is the Resolution of NIRONE Sensors?

The spectral resolution of a NIRONE Sensor is, typically, 1.00%—1.15% of the wavelength. For example, the spectral resolution at 1350 nm for the NIRONE S1.7 sensor is approximately 14 nm, and at 2000 nm for the NIRONE S2.2 sensor the resolution is 22 nm.

Oversampling with NIRONE Sensors

Even though the spectral resolution is always higher than 10 nm for any of the current devices, it is possible to enhance the predictive power and stability of most measurements by oversampling. Oversampling means that the sampling wavelength points are not spaced according to the spectral resolution of the sensor, but rather much more frequently. In research scenarios and first-time testing measurements, we suggest using a wavelength step of either 2 or 5 nm for any NIRONE Sensor.

Sampling at 1 nm rate is typically not sensible for real measurements, as this will result in a very large measurement time, and, typically, no enhancing effect can be seen for any measurement. We would suggest 2 nm for NIRONE S1.7, S2.0, and S2.2 devices.

The figure below demonstrates how sampling at 1, 2, 5, 10, and 20 nm steps with NIRONE S2.2 module, will affect the spectra. A 5 nm step is quite sufficient in plotting out all the major features, but already at the 10 nm step, some of the local minima and maxima can be lost. 1 nm sampling will plot the variation very well; the difference between 1 and 2 nm sampling step is negligible. The time used to sample all of the 401 points at 1 nm step for the whole range could be better spent on further averaging the 2 or 5 nm measurements. Measurements with a 10 or 20 nm step would be very fast.

Figure: Oversampling
Oversampling

Effect of Fiber Core Diameter on Resolution in a Reflection Setup

We created a simple reflection setup, where a single-fiber 200, 400, or 600 µm core fiber was used to collect reflected light from a fixed distance of approx. 20 mm. The sensor dark signal was manually measured. The reference signal was measured from a Spectralon target disc. The setup used the NIRONE S2.0 sensor with SMA connector and ArcOptix ArcLight NIR 20W light source.

The signal levels with the different fibers were quite different, but in all cases very low. Because of this, settings that were used with the acquisition of spectra were 100 point averaging, Buffering mode 90 scans averaged, at 2 nm step for the whole spectral range. No true difference in resolution could be detected. Because of the low level of light, the 200 µm fiber setup spectra still exhibited some noise, which explains some of the sharper features in the spectra.

Figure: Reflection setup with 20 W light source, S20 with SMA connector, and 200/400/600 µm collecting fiber
Reflection setup with 20 W light source, S20 with SMA connector, and 200/400/600 µm collecting fiber
Figure: Raw signal levels from the Spectralon target of the different fiber setups
Raw signal levels from the Spectralon target of the different fiber setups