Miniature and Portable Spectrometers

Why?

There are straightforward motivations for miniaturizing a spectrometer. 

If an instrument can be made smaller, it will often also consume less power, enabling it to be portable and eventually hand-held. This allows the spectrometer to be taken to the sample, as opposed to the sample to the spectrometer; in other words, the laboratory is moving to the field. Recent experience has shown that this capability is transformative – it changes the way in which people work by enabling on-site analysis that heretofore could only be performed in the laboratory.

What are they used for?

In broad terms, portable elemental analyzers are used for elemental analysis of bulk samples and their major components, and the field until recently was dominated by x-ray fluorescence (XRF).  Recently portable laser-induced breakdown spectroscopy (LIBS) instruments have become available, as have Raman, mid-infrared (typically Fourier transform, FT-IR) and near infrared (NIR) spectrometers for the molecular analysis of bulk materials and their major components.  For complex samples, complex matrices and trace analysis, gas-chromatography/mass-spectrometry (GC/MS) instruments are now being deployed in a portable form .  All airport travelers who are spectroscopists have probably observed ion mobility spectrometers (IMS) in the security areas, interrogating swabs for traces of explosives and related compounds.  Other spectroscopic instruments are being miniaturized (e.g., nuclear magnetic resonance (NMR)) but not deployed in the field.  Finally, a recent development is the use of smartphones as colorimetric spectrometers, typically for field clinical applications.    

Multispectral sensors can now not only be incorporated into ‘white goods’ like washing machines and dryers, but also into ‘fitness’ products like smart watches and sports watches, and as photonic miniaturization increases, into ‘wearables’ like smart rings, providing the user with health information.

How has this field grown?

With the exception of applications in teaching laboratories, the growth of the commercial handheld spectroscopy market has been driven by economic and security factors, primarily the desire for rapid screening of materials in the field, without the need to take a sample to the laboratory for analysis. The application areas can be broken down into four broad categories: rapid value determination in the field (e.g., mineral exploration and recycling); rapid safety screening or threat detection (e.g., homeland security, airport screening, first response, environmental, home and workplace safety); law enforcement (e.g., identification of narcotics and other illegal materials) and rapid purity or authenticity determination (product safety and adulteration detection, product authenticity and anti-counterfeiting,  point-of-use testing in processes, and raw material identification). Potential future areas for optical spectrometers are clinical diagnostics and biomedical applications, especially in “low resource” areas of the world.

We are about to see the next step in portable spectroscopy, as optical spectrometers and multispectral sensors appear in consumer goods, all the way from washing machines and vacuum cleaners to smoke detectors, ‘wearables’ like sports watches, and into smartphones.  These may not be immediately obvious to the customer, but they will deliver intelligence to the product itself to optimize its operation, or to the customer informing them of their health.

Silicon photonics and photonic integrated circuits (PICs), produced en masse using semiconductor manufacturing techniques, will be a further step.

Spectroscopy is becoming pervasive.

Design Considerations

The instrument designer has to consider use of the instrument by a non-spectroscopist to generate answers: use in the field under various lighting conditions (sunlight, fluorescent lights), temperature and humidity ranges; short ‘warm-up’ and measurement times, with no or minimal sample preparation and handling -  ideally a ‘point-and-shoot’ device; use with mixtures and potentially fluorescent samples; compliance to x-ray and laser safety regulations; and battery-powered, with small size and low weight. (In military terminology, this is known as SWaP – Size, Weight and Power.). In the most extreme applications (chemical, biological, radiological and nuclear [CBRN] hazards), the spectrometer must be able to survive decontamination. In these environments the operator will likely be wearing a protective suit that limits visibility, while wearing heavy gloves that can preclude a touch-screen interface, and therefore demand an easy-to-operate ‘large-button’ control mechanism.  The challenge for the instrument developer and manufacturer is to ensure that the resulting spectra are ‘good enough’ for the intended purpose, which may be screening, confirmation or identification only, not a spectroscopic study.

How big is the market?

The nature of these portable and field instruments is that they have very ‘applied’ applications, and comparatively few papers are written on these topics; they are somewhat ‘invisible’ to the traditional scientific community.  However, substantial numbers of these instruments are sold every year, and instrument companies have expended a lot of effort in their development.  Both the portable elemental (XRF and LIBS) and molecular (Raman, FT-IR and NIR) markets are estimated at between $200 and $300 million per year.   As examples of the numbers of instruments sold, the US government procurement of IMS-based personal chemical weapons detectors is greater than 100,000.  In a recent paper, Thermo Fisher Scientific disclosed that they (and their predecessor companies) have sold more than 35,000 portable elemental analyzers since 1995, and more than 10,000 portable molecular analyzers since 2005.  Today, there are probably more portable Raman instruments in use than laboratory Raman instruments.   

The market report company, Tematys stated that the compact spectrometer systems market grew from $655 million in 2016 to $922 million in 2019.  They classify spectrometers by their physical volumes: Mini (10000 cm3 – 100 cm3); Micro (100 cm3 – 1 cm3) and Chip-S (< 1 cm3), where CHIP-S means ‘chip-sized’.  They see CHIP-S spectrometers as the major growth area, costing less than $100, and appearing in embedded devices used by non-experts.  They foresee these devices as appearing both in consumer and medical applications, and their revenues exceeding other types of spectrometer modules by 2024.  They do caution, quite correctly, that hardware is not the only important part of such systems --- application knowledge and data processing are also required.

J. Kulakowski, B. d'Humières. "Chip-size spectrometers drive spectroscopy towards consumer and medical applications," Proc. SPIE 11693. 2021. Photonic Instrumentation Engineering VIII, 116931A. doi: 10.1117/12.2591048

J. Kulakowski, B. d’Humières. “Compact Spectrometers: Technologies, Market Trends and Needs”. PhotonicsViews. 2021. 1/2021: 28-30.  Wiley-VCH GmbH

 The Economist recently estimated that the market for wearables has grown from $8 billion in 2015 to $29 billion in 2021, accelerated by COVID-19. In the U.S., the publication said that smartwatches are catching on as rapidly as early mobile phones did. The noticeable health benefits from exercise tracking have convinced some U.S. health insurers to give fitness trackers and smartwatches to customers. Market reports suggest that around 40 million smartwatches are sold every quarter of the year (i.e., about 160 million per year).

The Quantified Self”, The Economist (London), May 7, 2022, https://www.economist.com/technology-quarterly/2022-05-07 .

Spectrometer Performance

A typical portable spectrometer tends to have “poorer performance” than its laboratory counterpart, in terms of resolution, range, signal-to-noise, sampling versatility, etc. However, it also tends to be specially adapted to field use, especially by non-spectroscopists, being mechanically hardened and producing specific answers (identity, validity, concentration, pass/fail, etc.) so that the reduced spectroscopic performance is ‘good enough’. In fact, the overall analytical performance for the operator can be better, as judged by the specific information it produces, at that point of use.

Use Cases

In a handheld instrument, used in the field, by non-specialists, there are three typical cases: Identification (What is this?), Authentication (Is this what it claims to be?) and Screening (Does this contain a material of interest?). These applications typically require specialized spectral libraries, and also algorithms tailored to the particular case and need.  Some instruments include specific hardware features for certain applications. For instance, those used in raw material verification can be equipped with a barcode reader. Here, the barcode is read first, informing the instrument of the probably identity of the sample. The spectrum can then be compared against a library spectrum and a verification algorithm employed.

My comprehensive (50 pages, 582 references) review article on portable spectroscopy has just been published in Applied Spectroscopy as a ‘Focal Point’ article. Focal Point articles are ‘open access’, and thus are free to download.

See: https://doi.org/10.1177/0003702818809719

This is the December 2018 issue of the journal. Applied Spectroscopy, 2018, Vol. 72(12) 1701–1751.