Spectrometer Success Story

Today’s modular spectrometers have demonstrated key performance gains, explains Rob Morris

The effectiveness of a new CCD-array spectrometer for measuring the absorbance of optical filters and potassium dichromate solution standards was recently evaluated.

The Ocean SR2 spectrometer has a linear CCD-array detector, enhanced electronics for high-speed spectral acquisition (integration times to 10 µs), and excellent SNR (380:1) performance for absorbance measurements, plasma monitoring and other applications. The following are some examples.

Absorbance Of Optical Filters

Using an Ocean SR2 spectrometer with a deuterium-tungsten halogen light source and an optical filter holder, several colour, balancing and UV blocking filters were measured. When compared with a similar model spectrometer for the same measurements, the Ocean SR2 demonstrated higher absorbance values, suggesting lower stray light performance; and better SNR at longer wavelengths, the result of more balanced sensitivity across the spectral range (Fig. 1).

Absorbance Of Potassium Dichromate Standards

Potassium dichromate is a crystalline solid typically used as an oxidising agent. Here, researchers used potassium dichromate solution standards to compare Ocean SR2 measurements to the calibrated absorbance wavelengths at 235nm, 257nm, 313nm, 350nm and 430nm.

The setup comprised the spectrometer with a 50 µm slit, a pulsed xenon light source balanced using a filter holder and diffuser, a 1cm pathlength cuvette holder, and a pair of 600 µm solarisation-resistant optical fibres. The Ocean SR2 demonstrated high accuracy, especially at higher concentration measurements (Fig. 2).

With absorbance measurements, its makers recommend setting the spectrometer integration time so the reference spectrum peaks at 80%-90% of the full scale of counts. This takes advantage of the full dynamic range of the spectrometer, which improves SNR. Also, with a spectrometer that produces balanced output across the wavelength range, users can avoid saturating parts of the spectrum – and having to make unwelcome measurement trade-offs.

The Bigger Picture

With absorbance measurements, spectrometer SNR can be the difference between excellent results and something less than that. The Ocean SR2 outperforms a comparable spectrometer for absorbance measurements, especially in the UV. This is an appealing feature for applications such as molecular diagnostics.

SNR is the signal intensity divided by the noise intensity at a certain signal level, which means SNR can vary from measurement to measurement. Since system noise typically increases as a function of signal due to photon noise, the SNR function is a plot of individual SNR values versus the signal at which they were obtained. Some manufacturers report spectrometer SNR as the maximum possible SNR value obtained at detector saturation. The SNR response curve for each pixel is assumed to be the same.

SNR can be improved by signal averaging, as was done for most of the measurements here. For time-based averaging, the SNR will increase by the square root of the number of spectral scans used. For example, an SNR of 380:1 will become 3800:1 if 100 scans are averaged. For spatially based averaging (boxcar), the SNR will increase by the square root of the number of pixels averaged.

Rob Morris is with Ocean Insight

Recent Issues