Spectrometers > Applications > Benefits of Non-scanning Spectrometers
Scanning Spectrometers: Controlling All Variables Over Time
Scanning spectrometers are often used to collect high resolution spectra to measure very small concentrations of substances. These types of spectrometers provide exceptional dynamic range and spectral resolution; however, both of these come at a significant cost to the collected spectrum. Any change in the collection system over the period of one scan can affect the quality of collected data. For high quality spectra in a scanning spectrometer, it is exceptionally important to control the stability of the illumination source, detector, and of the sample. All environmental variables such as sample temperature or stability also have to be fixed. Most importantly, it is impossible to acquire reliable spectra for dynamically changing systems, as the information obtained at the beginning of a scan corresponds to a different state than when the end of the scan occurs.
Non-scanning Spectrometers: Entire Spectrum Obtained Simultaneously
The most significant advantage of non-scanning spectrometers is that the entire spectrum of a sample is obtained simultaneously. This means that even though the sample might be dynamically changing or the measurement system has time dependent behavior, the variations in signal are collected for all wavelengths at the same time. This is extremely important for any spectral collection for dynamically changing systems such as biological tissues or on-line processes.
Although it is widely believed that non-scanning spectrometers cannot provide sufficient spectral information or spectral resolution to compete with scanning spectrometers, this is not true for high performance spectrometers developed by P&P Optica. Our systems use high quality components and transmission based, high efficiency volume phase holographic diffraction gratings to provide very high spectral resolution and dynamic range in a non-scanning spectrometer.
Simulation Comparing Scanning to Non-scanning Spectrometers
To better illustrate the effects of time dependent processes on both scanning and non-scanning spectrometers, a simulation was performed. The absorption spectrum for acetone vapor was measured by a non-scanning system. First, raw spectral data was obtained for both acetone vapour in a cuvette, and a reference spectrum for air filled cuvette as shown below:
Raw spectra obtained by non-scanning system for both acetone vapor and air filled cuvettes
Simulated time dependent variation of the light source. The amplitude changed by 1% periodically over time. A somewhat different period was assumed for acquisition of the acetone vapor spectrum and for the air spectrum.
The small time-dependent amplitude change was then applied to raw spectra. No visible differences exist in the "scanning system" spectra:
Simulated spectra obtained with "scanning spectrometer." The raw spectra obtained by the non-scanning spectrometer were multiplied by simulated periodically varying "source" signal shown above. There is almost no visible effect on the raw data when compared to the actual non-scanning spectrometer measurement.
Finally, absorption was calculated using data for both scanning and non-scanning spectrometers:
Acetone vapor spectra obtained by both a non-scanning and a scanning system which underwent time dependent changes.
As can be seen from the above results, time dependent changes over the time of a scan can have significant effects on the resulting absorption spectra when a scanning system is used. In scanning systems, it is imperative to control both the measurement system and the sample so that no dynamic changes occur over the scan time. No such problems exist when a non-scanning system is used.
