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Spectroscopy Questions & Answers
- Is the C12666MA micro-spectrometer capable of accomplishing Raman spectroscopy measurements?
- Conventional mini-spectrometers for Raman for 785 nm excitation source
- Mini-spectrometer C13054MA
- Mini-spectrometer C14214MA
- What is spectral resolution?
- Why use a spectrometer for color measurement? Can’t my RGB sensor do that for cheaper?
Is the C12666MA micro-spectrometer capable of accomplishing Raman spectroscopy measurements?
Typically, the C12666MA micro-spectrometer is not suitable for most Raman applications for two primary reasons: resolution and sensitivity.
Resolution
The C12666MA is a very small, versatile, and relatively low-cost spectrometer. However, achieving these design objectives required trade-offs in spectrometer performance such as spectral resolution. Per the C12666MA data sheet , the maximum resolution of the spectrometer is around 15 nm. This does vary over wavelength (see Fig. 1) with the best resolution occurring around 450 nm.
Figure 1: C12666MA’s spectral resolution
In much of Raman spectroscopy, it is important to have better than 1 nm resolution typically—and sometimes much less than 1 nm—to resolve molecular peaks that may be close in spectral proximity. It is also often important that the spectrometer’s FWHM resolution provide enough granularity and detail in the spectral peaks to be able to properly determine molecular identity and concentration. Generally, the C12666MA’s resolution is not sufficient to resolve closely spaced molecular peaks when analyzing Raman spectral information.
Sensitivity
Typical Raman analysis also needs relatively high sensitivity. Because Raman scattering is a low level, inelastic scattering effect and since measurements usually use just the Stokes scattered light, only a portion of all the scattered light is captured during measurements. Most Raman analysis applications deal with very low light levels, so the spectrometer used should have relatively high sensitivity to detect the very low light levels.
The C12666MA is based upon CMOS image sensor technology. It offers good sensitivity for its physical size, cost, spectral range, and resolution but usually does not offer sufficient sensitivity to accomplish most very low light Raman analysis. Fig. 2 shows the C12666MA’s relative sensitivity, which varies across its spectral response range with peak sensitivity occurring around 500 nm. This indicates that Raman signals near the peak sensitivity may be measurable, but perhaps signal peaks at other wavelengths may not be observable.
Figure 2: C12666MA’s relative sensitivity
Spectrometers suitable for Raman
Although the C12666MA is not typically suitable for Raman analysis, a few customers have achieved low-resolution Raman analysis for certain applications using the C12880MA micro-spectrometer data sheet.
The C12880MA has a slightly wider spectral response range (340 to 850 nm, with resolution peaking at around 425 nm) and a higher sensitivity based upon its APS-CMOS (Active Pixel Structure CMOS) image sensor. The APS-CMOS image sensor yields a significant improvement in spectral sensitivity (peaking at around 500 nm) over the sensitivity of the C12666MA, which uses a different CMOS image sensor. The spectral response plots for the C12880MA are shown in Fig. 3 and 4.
Figure 3: C12880MA’s spectral resolution
Figure 4: C12880MA’s relative sensitivity
The Raman applications that used the C12880MA were unusual since the performance requirements were loose enough to accommodate the operational specifications of the micro-spectrometer, but this is generally not expected to be the case in typical Raman measurement.
The C12666MA and C12880MA micro-spectrometers will typically not provide sufficient spectral sensitivity and resolution to accommodate most Raman applications. But do review the requirements of the application just in case your implementation may be the exception to this generalization.
Also, Hamamatsu does offer small mini-spectrometer models that were specifically designed for Raman applications. They have the higher performance characteristics typical of Raman spectrometers, including relatively high resolution and high sensitivity.
Conventional mini-spectrometers for Raman for 785 nm excitation source
- APS-CMOS based
- Small, thin, flat form factor
- Low cost
- Spectral range: 790 - 920 nm
- APS-CMOS based
- Small, thin, flat form factor
- Low cost
- Spectral range: 790 - 1050 nm
What is spectral resolution?
Resolution should always be considered when selecting a spectrometer, as many applications require a certain resolution to be successful. If you imagine a random spectrum in your head, I bet there are a bunch of peaks large and small. Resolution is essentially the ability to discriminate peaks of the same intensity within a certain wavelength window. A good example would be to think of your typical camera and its ability to focus on certain aspects of an image.
One popular method to define spectral resolution uses the Rayleigh criterion in DIN (Deutsches Institut für Norming) standards, for which the valley between the two peaks must be lower than 81% of the peak value.
On the other hand, a practical alternative for defining spectral resolution is the spectral half-width or FWHM (full width at half maximum). This is the spectral width at 50% of the peak value and directly defines the extent of spectral broadening. The more broadening you have, the more information you may be losing within the peak. The spectral resolution defined as FWHM is approximately 80% of the resolution defined by the Rayleigh criterion. The spectral resolution of Hamamatsu mini-spectrometers is defined by FWHM.
Figure 5a: Resolution defined by Rayleigh criterion
Figure 5b: Definition of FWHM
Another thing to note is that your spectrometer’s resolution may have wavelength dependence. More resolution is especially important in cases of chemical fingerprinting, where minute changes in purity and identification of functional groups depend on the ability to find small differences in interaction.
Why use a spectrometer for color measurement? Can’t my RGB sensor do that for cheaper?
There is nothing wrong with an RGB sensor + photo IC solution, but certain color measurement applications require a higher resolution (finer detail) than a simple RGB solution can provide. There may be considerable information within even the smallest change in light level and/or color. Food safety/quality, fluorescence, medical applications, and oil/fluid quality are all prime examples of the potential applications that require such a level of resolution. These applications of the future are simply beyond the capability of RGB sensors. A sensor with the ability to detect such small changes opens up many possibilities for new markets, and in terms of price, you would be surprised how competitive our spectrometers are. One example of the C12666MA’s prowess can be found here: https://www.nature.com/articles/srep32504
If you have a technical question you’d like to see answered on this page, email us.
Meet the engineers
As an Applications Engineer in the New Jersey office of Hamamatsu, Dana Hinckley’s primary technical focus is on the mini- and micro-spectrometers and MEMS-FPI spectral sensors. He especially enjoys the wide range of applications and the daily dose of education in science, engineering, and spectroscopy, along with the myriad implementations in which our customers employ the spectral analysis tools. In his off-hours, Dana enjoys a wide palette of interests: spending time with family and friends, guitar playing, and music are some of his more predominant pastimes.
Gary Spingarn is a Product Manager in the New Jersey office of Hamamatsu, where he focuses on business development for certain products and exploring new applications. Leveraging his chemistry expertise, Gary supports the mid-infrared (MIR) product lines with a particular knack for process monitoring, gas analysis, and environmental applications. In his spare time, Gary hones his chess skills as well as partakes in strength sports and world travel.
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