Smartphones Poised to Shake Up Spectroscopy

JAMES SCHLETT, CONTRIBUTING EDITOR, james.schlett @photonics.com

This June will mark the 10-year anniversary of the release of Apple’s iPhone. It did not take long for this mobile operating system to earn its reputation as a disruptive innovation, with that disruption being most palpable in the digital-camera market. While worldwide shipments of digital still cameras with built-in lenses peaked in 2008 — one year after the iPhone’s launch — at 110 million, they have been rapidly declining since then, plummeting to 23.3 million in 2015, according to the Camera and Imaging Products Association in Japan.

Headlines have screamed about smartphones “slowly killing the camera industry¹” and how camera makers’ “hope lies in pricey devices for the few².” Now, with the introduction of several smartphone plug-in and add-on devices that rely on light to determine the chemical makeup of materials, the iPhone is positioned to disrupt another market: spectrometers.

“Potentially, spectrometers in smartphones can replace traditional diagnostic devices in areas where accuracy is less important and only warning signs are required. Generally speaking, a handheld spectrometer is not a substitute for a lab-grade one,” said Dror Sharon, CEO and co-founder of Tel Aviv, Israel-based Consumer Physics, maker of the iPhone- and Android-compatible handheld SCiO* near-infrared (NIR) spectrometer, which can, among other things, detect the nutrient and caloric content of food products and the presence of contaminants in them. “However, we can already see that with the advent of accessories, there is a possibility to re-create some of the measurement conditions and thus replace some of the lab-grade tests.”

Limitations

Technically, there currently are no commercially available “smartphone spectrometers”; there are only spectroscopy-based accessories that utilize mobile operating system cameras, computing power and access to cloud databases housing mass quantities of spectral information. Without these accessories, smartphones are ill-equipped for spectroscopy. Ryan Bean, an applications scientist with StellarNet in Tampa, Fla., said smartphones are limited by their light source, which is typically an inexpensive white LED without uniform output across the entire visible spectrum.

“If you are measuring absorbance of a sample, you ideally want to have the most signal at every wavelength, something that requires tungsten halogen or other more expensive lamps,” said Bean.

Even more, he added, UV and NIR ranges are largely undetectable to out-of-the-box smartphones, and fluorescence and NIR chemometrics are important to biological and chemical research. Such a wide spectral range is expected for research-grade spectrometers, and so is subnanometer spectral resolution. Most iPhone plug-in and add-on devices, however, can only achieve several nanometers of resolution.

Cost and size are two other limitations for smartphone spectrometers, according to Andy Low, co-founder of frinGOe, which offers spectrometers that are integrated into the protective case of certain iPhone models. He said existing production models cannot bring the cost of spectrometers down to a level appealing to smartphone makers.

“Consumers are used to smartphones being slim and lightweight,” Low added. “The addition of traditional grating-based smartphone spectrometers increases the thickness by several centimeters — something that is not acceptable by current consumer standards.”

One of the simplest, commercially available, smartphone-compatible spectrometer add-ons is the Foldable Mini-Spectrometer by the Public Laboratory for Open Technology and Science (Figure 1). As its name suggests, this add-on is made of paper folded into a trapezoidal prism with a slit at one end and a wider opening at the other, where a diffraction grating — cut from a DVD-R with its reflective coating peeled off — is placed. The grating end can then be mounted to a smartphone camera.

Figure 1. The Foldable Mini-Spectrometer by the Public Laboratory for Open Technology and Science. Courtesy of the Public Laboratory for Open Technology and Science.

The availability of cheap sensors has largely put mid-IR and UV wavelengths beyond the reach of this add-on, though by removing the NIR “hot mirror” filter found in most smartphone CCD cameras, users can capture spectra out to approximately 900 or 1000 nm, according to Public Laboratory Research Director Jeff Warren. The frinGOe is also largely limited to the visible light range of 400 to 700 nm, though longer and shorter wavelengths are accessible, depending on the detector used.

“The spectral range of frinGOe is highly dependent on the camera sensor used. If the camera is sensitive to UV, then frinGOe can measure UV with a small change to the components used. Similarly for NIR, a typical CMOS camera with [the] IR filter removed can have spectral range extended till nearly 1000 nm, and frinGOe can be used with that, too,” said Low.

Figure 2. A micro-spectrometer with smartphone applications. Courtesy of Hamamatsu Photonics.

Toward smartphone-spectrometer integration

A more advanced approach to this type of smartphone spectroscopy is seen in a MEMS-based spectrometer with a mobile phone for skin cancer screening, developed by researchers at the Massachusetts Institute of Technology’s Media Lab in Cambridge, Mass. This device used an Arduino Bluetooth module to connect to a Hamamatsu Photonics micro-spectrometer with a 10-nm spectral resolution and a 340- to 780-nm wavelength range (Figure 2). The integrated spectrometer included an 8× telephoto lens system, a beam splitter, polarizers and UV and white LED light sources³.

“There are trade-offs that must be made in performance when moving from large, expensive lab implementations to small, inexpensive mobile implementations,” said Hamamatsu Applications Engineer Dana Hinckley. “Some of the primary trade-offs currently include characteristics such as resolution and sensitivity, but other specifications such as repeatability, wavelength accuracy and noise may also be less stringent in the small mobile spectrometers versus the research grade instruments.”

Figure 3. Consumer Physics’ mobile spectrometer uses near-infrared spectroscopy to measure the quality, content and composition of foods and drugs. Courtesy of Consumer Physics Inc.

Consumer Physics Inc. recently partnered with Analog Devices Inc. in Cambridge, Mass. to embed its SCiO technology into smartphones, wearables, home appliances, industrial devices and medical applications. The two companies will collaboratively develop a sensor-to-cloud personal and industrial Internet of Things platform for the rapid analysis of food and drug content, quality and composition (Figure 3).

Sharon said SciO is the “only spectrometer that can eventually be reduced in size and cost to fit inside a smartphone” (Figure 4). But Isabel Hoffmann, CEO and founder of Tellspec in Toronto, pointed out that silicon chips, such as those used in the SCiO, can only take mobile spectroscopy so far. Her company worked with the prototype of a small silicon-based spectrometer integrated into a mobile phone, similar to the MIT setup. Its silicon chip enabled Tellspec to perform simple detection tests, but Hoffmann said its narrow spectral range was “very limited and not very accurate.” That prompted Tellspec to adopt an indium, gallium and arsenide (InGaAs) detector for its pocket spectrometer.

Figure 4. Progressively smaller versions of Consumer Physics’ smartphone-compatible NIR spectroscopy devices (a). The mobile spectrometer’s optical head (b). Courtesy of Consumer Physics Inc.

Short- and mid-wavelengths

While the SCiO scanner covers the short-wavelength NIR (SW-NIR) range of 700 to 1100 nm — the wavelength cutoff for silicon image sensors — the Tellspec scanner covers a portion of that range, from 900 to 1100 nm, as well as from 1100 to 1700 nm in the mid-wavelength NIR (MW-NIR) range. Like the SCiO, the Tellspec scanner can analyze consumer food products (Figure 5). Hoffmann said the benefits of the wider spectral range are apparent in melamine detection applications. Melamine has been linked to the death and illness of dogs, cats and infants through their consumption of adulterated animal foods and milk formula, but a spectral range of 1450 to 1550 nm is needed to detect this toxic organic compound (Figure 6).

Figure 5. Tellspec’s smartphone-compatible NIR spectroscopy scanner. Courtesy of Tellspec.

“The MW-NIR has long been established as a much better choice for analysis of foods,” said Hoffmann. “SW-NIR gives much less information. SCiO offers a narrower range because SW-NIR can be done with a cheap silicon detector, while MW-NIR requires a detector made of an expensive alloy of indium, gallium and arsenic.”

The development of micro-spectrometers that reach deeper into the IR range will pose challenges for cooling devices, which will also need to be miniaturized. For example, Hamamatsu’s ultralight MEMS-FPI (Fabry-Perot Interferometer) spectrum sensor, which is built around an InGaAs PIN photodiode, has a spectral range of 1550 to 1850 nm, and the inclusion of a temperature control mechanism could open new spectral ranges for this product line. Hamamatsu is also pursuing micro-spectrometers with short wavelength options for UV applications, Hinckley said.

Figure 6. A comparison of the NIR spectra of pure milk and milk contaminated with melamine. Courtesy of Tellspec.

Gratings

Additionally, miniaturization creates challenges for diffraction gratings, the optical components used to split light and measure its wavelengths. While both research-grade and handheld spectrometers use transmission or reflection gratings, those for the former usually measure larger than 1 × 1 in. and those for the latter measure smaller than 0.25 × 0.25 in.

However, traditional grating technology may not work for a smaller spectrometer integrated into a smartphone. Arrayed waveguide gratings, which are commonly used in optical communications, grating-on-chip and Fourier transform infrared spectroscopy (FTIR) technology, may be more applicable for this application because of their ability to substantially increase transmission capacity and also reduce the size; developing technology such as quantum dots would further enhance the performance of smaller spectrometers, according to Brian Lin, a product manager for II-VI Photop.

New Opportunities

Given the limitations and proliferation of mobile silicon-based spectrometers, such as the SCiO, Hoffmann said they are “tools that cannot be used for serious research; they should be seen as fun tools.” She believes that, given time, “spectrometers will come down in size and price, much the same way cellular phones did.” Although she does not foresee smartphone spectrometers supplanting traditional spectrometers in labs for “quite a few years,” she sees the Tellspec scanner making inroads toward that end. In fact, the company benchmarked its scanner with other life sciences scanners and she said its performance was “equal if not better.”

While there is little doubt in the spectroscopy industry that smartphone spectrometers will eventually reach commercial markets and achieve performance levels on par with today’s larger research-grade instruments, manufacturers view this trend more eagerly than anxiously. After all, unlike digital cameras, there was not a sizable consumer spectrometer market prior to the launch of the iPhone.

“Rather than replace traditional spectrometers, I think the work we’re doing may open up spectrometric techniques to a much broader set of applications where they’d formerly have been cost-prohibitive. Even with lower sensitivity or limited wavelength range, the ability to design an experiment which incorporates ongoing real-time monitoring with multiple instruments, potentially in the field, can make spectrometry far more versatile,” said Warren at Public Laboratory.

Even without higher resolution and sensitivity, Hinckley at Hamamatsu said biophotonic applications not requiring extreme spectroscopic performance will likely migrate to small, portable, inexpensive smartphone-type devices.

“These types of technological improvements will hopefully lead to great improvements in the biophotonics areas such as pathogen identification and analysis,” said Hinckley. “This alone will likely lead to great benefits to humanity. Large, expensive, high-performance research grade spectrometer systems will play an important role in biophotonic spectroscopy for the foreseeable future.”

Low at frinGOe agreed that the research-grade spectrometer will not be dethroned in the lab by its smartphone counterpart “due to its superior performance that some applications require.” But he does see smartphone spectrometers making headway in “areas of applications which are less stringent on performance factors.” These areas include the spectroscopic monitoring of crop growth, testing for fake or spoiled food, blood-glucose monitoring and the selection of biomedical cosmetic products compatible to a consumer’s skin or of personal hygiene products suitable for his or her use.

“We foresee the proliferation of smartphone spectrometers will happen within the next two years, as smartphone makers have come to realize the power of a mobile spectrometer and the possibilities it presents to consumers,” he said.

References

1. L. Savvides, How smartphones are slowly killing the camera industry. CNET, Oct. 29, 2012.

2. S. Shankland, For camera makers, hope lies in pricey devices for the few, CNET, Sept. 25, 2014.

3. A. Das et al. (2015). Mobile phone based mini-spectrometer for rapid screening of skin cancer. Proceedings of SPIE, 9482.

*ScioInspire in Pittsburgh last March filed a trademark infringement lawsuit in U.S. District Court for the Western District of Pennsylvania against Consumer Physics and Verifood over use of the term “Scio.”