Handheld Raman spectrometer

The size of a spectrometer is driven mainly be the focal lengths of the optics used. The focal length (L_F) of a spectrometer is related to the detector length (L_D), the grating groove density (G), the wavelength range ( λ₂ – λ₁) and the diffraction angle ( β) of the spectrometer by the following approximate equation:

From this equation it is clear that a small spectrometer can be obtained with the following methods:

The width of the detector and the wavelength range may indirectly be determined by the resolution requirement.  As an example, let us say you want 0.5 nm resolution over a wavelength range of minimum 800 – 1100 nm. You need at least 2 – 3 pixels to sample your 0.5 nm resolution with reasonable accuracy, which means your detector should have a total of 1200 – 1800 pixels for the full range. Detectors for spectroscopy typically have pixel width in the range from 7 to 25 microns so the smallest possible detector width (L_D) in this case would be 1200 * 0.007 mm = 8.4 mm.

The numerical aperture of the spectrometer also influences the size. Normally, you would want as high a numerical aperture as practically possible in order to obtain a very sensitive spectrometer. But unfortunately, this leads to a wide and tall beam going through the spectrometer leading to a large size of the spectrometer. So, for handheld Raman spectrometers the numerical aperture is often limited to 0.07 – 0.16.

Schematic diagram of diffractive spectrometer with definitions of relevant parameters: L_C = focal length of collimation mirror; L_F = focal length of focus mirror; L_D = length of detector.

When choosing a spectrometer for a handheld instrument you need to pay attention to the environmental stability of the spectrometer. Temperature changes, vibrations, and shocks can potentially bring the spectrometer out of calibration, so you want a spectrometer that is robust against such effects.

One important aspect to consider is that transmissive optics are much less influenced by small angular variations than reflective optics. If you consider the difference between a mirror and a window this becomes apparent. If you are looking at something in a mirror and someone rotates the mirror a few degrees you will see the mirror image move. However, if you look through a window that is being rotated nothing happens. This is the main reason spectrometers build from transmissive optics are much more robust.

The only electrical power consuming component in a spectrometer is the detector. Especially, if the detector is cooled it can consume several Watts. Therefore, handheld Raman spectrometers should be designed around non-cooled detectors.

The objective of cooling the detector for (benchtop) Raman instruments is to reduce the dark charge level which builds up over a long integration time. So, using a non-cooled detector typically means that the integration time should be limited below 1 second.