Spectroscopy Questions & Answers

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 (https://www.hamamatsu.com/resources/pdf/ssd/c12666ma_kacc1216e.pdf), 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.

C12666MA micro-spectrometer’s spectral resolution plot
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.

C12666MA micro-spectrometer’s relative spectral sensitivity plot
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 link: https://www.hamamatsu.com/resources/pdf/ssd/c12880ma_kacc1226e.pdf). 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.

C12880MA micro-spectrometer’s spectral resolution plot
Figure 3: C12880MA’s spectral resolution
C12880MA micro-spectrometer’s relative sensitivity plot
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. Below is a list of the models arranged by the type of Raman analysis and excitation source.

Conventional Raman
for 532 nm excitation source
Conventional Raman
for 785 nm excitation source
SERS Raman using built-in
785 nm laser excitation source
C11713CA
• CCD based
• Spectral range: 500 – 600 nm
C13054MA
• APS-CMOS based
• Small, thin, flat form factor
• Low cost
• Spectral range: 790 – 920 nm
C13560
• Compact SERS Raman spectrometer
• APS-CMOS based
• Small, thin, flat form factor
• Low cost
• Spectral range: 790 – 920 nm
C14214MA
• APS-CMOS based
• Small, thin, flat form factor
• Low cost
• Spectral range: 790 – 920 nm
 
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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.

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