Miniature Optical Spectrometers, Part III: Conventional and Laboratory Near-Infrared Spectrometers

In contrast to the mid-infrared, where Fourier transform (FT)-IR spectrometers are the standard, and the UV–vis, where silicon photodiode-array (PDA) and charge-coupled-device (CCD) spectrometers dominate, there are a plethora of different technologies employed in laboratory near-infrared (NIR) spectrometers (3–5). Stark and Luchter (6) quote a total of 60 manufacturers, and the survey by Workman and Burns (3) lists close to 100 different instruments. There are several reasons for this. First, Fourier-transform instruments do not dominate NIR spectroscopy because commercial scanning grating instruments have been highly optimized for low-resolution applications, and FT spectrometers lose their multiplex advantage in this region, as they are generally shot-noise limited. FT spectrometers do, however, retain their throughput advantage and laser-based wavelength scale referencing in the NIR. Second, both one- and two-dimensional photoconductor array detectors are available at reasonable costs, although they are expensive compared with the PDAs and CCDs used in the visible and UV regions. Finally, many other wavelength-selective devices are available, including liquid-crystal tunable filters (LCTFs) and acousto-optic tunable filters (AOTFs). Each of these approaches has their particular advantages and disadvantages.

There are at least three distinct threads in the evolution of NIR spectrometers. Research-grade instruments for molecular spectroscopy studies of liquids and gases, characterization of optical filters, and other applications, with the ability to do kinetic measurements at single wavelengths, have evolved from the Cary 14, introduced in the mid-1950s. Low-resolution instruments, for analytical solid-phase studies of heterogeneous food and agricultural products, following the work of Norris (7), started with linear variable filters and evolved via modification of a Cary 14 (8) to scanning filter and scanning grating instruments. Handheld and portable instruments generally are used for qualitative and quantitative analysis of solid materials, and therefore follow the latter thread. Third, with the development of small NIR spectrometers and small fiber-optic probes, application areas have opened up in biomedical spectroscopy: noninvasive diagnoses (9), minimally invasive diagnoses (10), and in-vivo studies (11). Optical coherence tomography (12) (OCT) is a closely related field that employs some of the same fiber-optic and NIR source technologies.

Laboratory NIR spectrometers have been reviewed extensively (3–5,13–18). Some of the key components and approaches will be covered here briefly, as an introduction to miniature spectrometers, but the reader is referred to those reviews for further details about these instruments. Stark and Luchter (6) noted the high diversity of NIR technologies available a few years ago, and McClure and Tsuchikawa (5) classified these spectrometers as follows: diodes (emitting diode arrays, laser diodes, and diode-array detectors); filters (fixed, wedge, tilting, acousto-optic, liquid crystal); prism; grating; FT-NIR, and Hadamard.

For technological reasons, there is another interesting division in the NIR spectrum, between the shortwave region (~740–1100 nm) and the conventional or midwave region (~1100–2500 nm). Silicon-based detectors are sensitive in the shortwave region, while lead sulfide (PbS), lead selenide (PbSe), and indium-gallium-arsenide (InGaAs) detectors work in the midwave region. The third overtone bands found in the shortwave region are considerably simpler and weaker than those observed in the midwave region (19,20). A weaker band (lower absorption coefficient) is not necessarily a disadvantage though, because it allows long pathlengths (for example, in transmission), or long effective pathlengths (for example, in diffuse reflectance or interactance) to be employed, and the simpler bands do not prevent measurements of properties (for example, octane (21)) or broad chemical constituents such as sugars (5) from being made. These long pathlengths discriminate against surface effects in transmission cells and against the skin or peel of natural products, such as in measurements on whole fruits. For example, the low cost and small size of PDA and CCD detectors enables handheld spectrometers, operating in the shortwave region, to be used for Brix (soluble sugar) measurements on intact apples and melons (5,22).

In this article, we will first look at the critical elements of an NIR spectrometer (sources, wavelength-selective devices, detectors, and array detectors), multiplex techniques, and how diffuse reflectance sampling affects spectrometer design. Then we will survey existing small NIR spectrometers.

Sources

The ubiquitous NIR source is a quartz-halogen light bulb, which operates around 3000 K (23), and whose emission peaks in the near-infrared; these are low-cost, reliable, very bright sources. Light-emitting diodes (LEDs) are an example of a semiconductor light source (24), and we will look at these in some detail because semiconductor light sources of various types show promise in NIR spectroscopy, especially for small, portable, and dedicated-function instruments. Historically, a small number of instruments (4,5,18,25) have used LEDs as sources, typically in the shortwave NIR region, the original telecommunications range. Because of their wide bandwidth (>40 nm), they have been used in conjunction with narrow-band filters.

Schubert has covered the history, technology, and applications of LEDs (26). NIR (870–980 nm) LEDs were developed starting in the 1960s, using GaAs, followed by visible LEDs using GaAsP, deployed first as indicator lights on printed circuit boards. Yellow and green LEDs were developed in the 1970s, using materials such as GaP. In the late 1960s and 1970s, market drivers for these devices were indicator lights and numeric displays, in items such as telephones, calculators, and digital watches. Resonant cavity LEDS (RCLEDs) were developed in the 1990s using AlGaAs/GaAs materials; these have high infrared and red emissions from microcavities and are used widely in remote controls and telecommunications. Blue GaInN/GaN LEDs came on the scene in the 1990s, and today AlGaInP is the dominant system for high brightness LEDs used in traffic lights and automotive lighting. In telecommunications, AlGaAs/GaAs devices emit at 870 nm, and with the switch to the 1300-nm region, InGaAsP/InP LEDs were developed. Infrared LEDS are available commercially at wavelengths as long as 7 μm, based upon GaSb systems (27); however, their full width at half maximum (FWHM) increases at longer wavelengths and is around 120 nm at 1600 nm and almost 1 μm at 5 μm. Typical shortwave NIR wavelengths available are 940, 970, and 770 nm, with halfwidths (FWHM) of about 40–60 nm.

White LEDs were developed in the mid- to late 1990s and usually are based upon a blue LED combined with a longer-wavelength phosphor; for instance, a GaInN/GaN LED and a cerium-doped yttrium aluminum garnet (Ce³⁺:YAG) phosphor. These have been used as miniature broadband visible sources in some experimental studies (28,29). Their output is far from flat; there is typically a "double hump" — one at short wavelengths (~450 nm) corresponding to the output of the blue LED and a second at longer wavelengths (~550 nm) due to the phosphor.

New mass-market applications (for example, camera autofocusing) for LEDs emerge rapidly, so that there is continuing development in this area. Today, LEDS are widely available at low cost from a variety of manufacturers (30), and the worldwide LED market was estimated at $4.2 billion in 2006, growing to $11 billion in 2011. Progress in solid-state sources is very rapid, and we shall see in Part IV the use of superluminescent light-emitting diodes (SLEDs), leading to the development of broadly tunable NIR lasers (31).

Wavelength-Selective Devices

Given the many review articles on conventional NIR instruments (3–6,14–18), this paper will not describe the wide variety of wavelength-selective devices available in the near-infrared. Instead, a few of the technologies that could yield, or have yielded, handheld devices will be discussed briefly.

Fixed-filter NIR analyzers continue to be sold into a large number of process applications, ranging from gases (CO₂, SO₂, and so forth), to moisture in almost anything: building materials, foods, textiles, paper, and tobacco (32). Wetzel and colleagues (15) estimated that 85% of all NIR analyses are performed with fixed-filter instruments, but tilting-filter spectrometers have largely disappeared from the marketplace. Two tunable-filter technologies have been used: acousto-optical tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs). These also have been reviewed in detail by Wetzel and colleagues (15). These two technologies have found different niches: AOTFs for scanning NIR instruments, and LCTFs for imaging instruments, both in the NIR and visible, especially in fluorescence microscopy. Commercial instruments based upon AOTFs are available, including a number of portable systems (33–35). As an example, one of these (Infrared Fiber Systems' PlastiScan-C) covers 1200–2600 nm, with an optical head size of 9 in. × 6 in. × 2 in., weighing 2.1 lb. This spectrometer is run from a PC via USB.

Tunable filter technologies developed for telecommunications include linear variable filters and Fabry-Pérot tunable filters with a variety of tuning mechanisms. Telecommunications Fabry-Pérot tunable filters are described later, while linear variable filters (LVFs) were described in Part I of this series (2), and have been reviewed by Wilks (36). LVFs were the key component in one of the first miniature spectrometer "engines" to be seriously marketed to system integrators and instrument companies, as opposed to endusers. This was the MicroPac from Optical Coating Laboratory, Inc. (now JDSU [37]); it was described by Coates (38) and featured prominently in an early review of miniature spectrometers in 2000 (39). It was 0.63 in × 1.4 in. × 1.4 in. in size, and was offered, as an engine, for $799, in two versions: covering 400–700 nm with 5 nm resolution, or 600–1100 nm with 7 nm resolution. At that time, OCLI had very ambitious plans for this device, including driving the cost down so that it could be included in every color printer and copier as a measurement and correction sensor. This vision did not come to pass, but the concept of using an LVF and an array detector in a very low cost spectrometer survives today in the mid-infrared instrument from Wilks Enterprise (40) and the recently introduced series of visible and shortwave NIR spectrometers from microSpectralSensors (41,42).

Detectors

The responsivity of a particular detector usually is defined in terms of the noise equivalent power, or NEP (43). Simply put, this value is the power incident on the detector so that a signal-to-noise ratio (S/N) of unity is achieved. NEP is dependent upon the area of a detector, with larger detectors having larger noise. NEP is measured in units of W Hz^–1/2, and is defined formally for a particular modulation frequency, wavelength, and effective noise bandwidth; the lower the NEP, the better the detector. To compare detector material, independent of area, the specific detectivity, or D* (D-star), is defined as:

D* = A^1/2 / NEP

and is measured in units of cm Hz^1/2 W^–1. The higher the D*, the better S/N the detector will have. Formally, the D* is the S/N at a particular electrical frequency, in a 1-Hz bandwidth, when 1 W of radiant power is incident on a 1-cm² active area detector.

Scanning grating and filter-based systems traditionally have used photoconductive lead sulfide (PbS) detectors (44), with a range of 1–3.5 μm, a D* of about 5 × 10¹⁰, and a time constant of about 1 ms. Photoconductive lead selenide (PbSe) detectors have a longer wavelength cut-off (~4.8 μm), a typical D* of ~2 × 10⁹, and a time constant of ~2 μs. The telecommunications industry has driven the development of InGaAs PIN photodiode detectors because of their high sensitivity and low noise in the 1.3- and 1.55-μm regions, and their very fast response times — as low as 1 ns. These detectors are now used widely in FT-NIR and other fast data rate spectrometers. A PIN photodiode is a semiconductor positive–negative (P-N) structure with an intrinsic (I) region sandwiched between the other two regions (45).

InGaAs detectors typically have a range from 0.8 μm to 1.7 μm, but extended InGaAs detectors are available with typical cut-off options of 1.9, 2.2, 2.4, and 2.6 μm (46). The D* for these detector materials drops steadily as the wavelength cut-off is extended, and typical room temperature values are 5 × 10¹² for 1.7 μm, 2 × 10¹¹ for 2.2 μm, and 5 × 10¹⁰ for 2.6 μm, all in units of cm Hz^1/2 W^–1, and measured at 1 KHz. Large-area (up to 5 mm diameter) InGaAs detectors also are available, and these find applications in diffuse reflectance and integrating sphere applications. In the short wavelength, or third overtone, region of the near-infrared, silicon PIN photodiode detectors are used. These have a range of 300–1100 nm, a D* of about 10¹⁴, and a time constant of a few nanoseconds.

Array Detectors

In the shortwave NIR region, shorter than 1100 nm, silicon PDAs and CCDs are used. Stark (16) points out that while CCDs are used for low light level work, for instance in Raman spectroscopy, PDAs are used for analytical NIR transmission and reflection. Shortwave array detector costs have been driven down by widespread commercial applications such as cameras, cell phones, and scanners. There are many commercially available, compact, midwave NIR instruments, based upon spectrographs containing InGaAs linear array detectors (16). A few instruments employ lead sulfide and lead selenide linear arrays (47–49), with up to 256 elements. Detector arrays usually are specified differently from single-element detectors because of their different readout and amplification. A typical linear array has between 128 and 1024 detector elements, or pixels, and these are typically 25–50 μm wide and 50–500 μm high — designed for use with slit-based grating instruments. The pixel spacing is referred to as the pitch, and efforts are made to minimize any gaps between pixels. The ratio of total pixel width to length of the array is termed the "fill factor," and for modern arrays, this is close to 100%. One or more analog-to-digital (A/D) converters typically are located on the array package itself. These are commonly 14-bit, but recently, some 16-bit converters have been used (50). The key parameters that govern the S/N of an array are its dark current, the current through the detector pixel under normal operating conditions with no incident radiant power, and its read noise. In most situations, the dark current is the dominant noise. The typical dark current in InGaAs arrays increases from 0.08 pA for a 1.7-μm array to 1000 pA for a 2.6-μm array, which is a factor of 12,500 in increased noise (46). The dark current can be reduced by cooling the array, and a recently available spectrometer (46) using a 2.2-μm array and a 16-bit A/D quotes 800 counts dark current out of 65,536 (2¹⁶) total counts.

InGaAs arrays with 128 pixels were employed initially, but arrays with 256, 512, and 1024 pixels are now available (51,52). Although it is logical to assume that 256 pixels covering the range of 1120–2100 nm would imply a resolution of 4 nm, the practical resolution attained is actually at much worse — around 12–15 nm. This is for two reasons: at least three points are required to define and resolve a band, and these devices are not perfect; there can be leakage or crosstalk between pixels. That is, the signal on one pixel can affect the signal on its neighbors, with possible deleterious consequences in qualitative and quantitative analyses.

Multiplex Techniques

Harwit and Sloane divided multiplex spectrometers into two classes (53): the first, which uses interference techniques and Fourier transforms, and the second, which uses masks and discrete transforms. The Michelson interferometer, which delivers both a throughput and a multiplex advantage, falls into the first class, and Hadamard-transform spectrometers into the second. Part I of this series (2) described why Fourier-transform spectrometers (Michelson interferometers), with their throughput and multiplex advantages, came to dominate mid-infrared instrumentation: because source temperatures are limited; suitable tunable sources do not exist; and detectors are noisy. It is important to note that the multiplex advantage applies because mid-infrared instruments are detector-noise limited; in the case of a shot-noise-limited spectrometer, the multiplex advantage is canceled by the increase in shot-noise. Hadamard spectrometers (53–55), which also have both a throughput and a multiplex advantage, have been applied in the mid-infrared, but historically, the practical construction difficulties relegated them to a footnote. For particulars of the theory of Hadamard-transform spectroscopy and a detailed analysis of noise for various designs, the reader is referred to the book by Harwit and Sloane (53).

As multiplexing devices, Hadamard-transform spectrometers can be expected to produce higher signal-to-noise spectra than a sequential scanning instrument with the same optical dimensions and measurement time, when the system is detector-noise limited. Mid-infrared spectrometers are almost always detector-noise limited, while visible spectrometers are shot-noise limited. NIR spectrometers are an intermediate case, with instruments operating at low light levels, and with longer wavelength radiation, possibly detector-noise limited, while instruments operating at high light levels and shorter wavelengths (and with shorter-wavelength cut-off detectors) being shot-noise limited. Therefore, in some circumstances, a multiplex NIR spectrometer will have an S/N advantage; however, given the costs of NIR array detectors, there may well be a significant cost advantage in a Hadamard design.

In a typical scanning dispersive spectrometer, light from a source and sample passes through a slit, is dispersed by a grating, and passes through a second slit onto a detector. Hadamard spectrometers replace one or both slits with encoding masks (56). In a typical single mask design, light passes through an entrance slit, is dispersed by a grating, then falls on the mask, returns to the grating, and is de-dispersed, and is then focused on a single element detector. If there are n elements in the mask, and n points in the spectrum, then a signal-to-noise gain of 1/2√n can be obtained.

The work done in the 1970s concentrated largely on single mask designs, because the masks used at that time were mechanical and array detectors were in their infancy. However, with the widespread availability of array detectors, construction of a doubly encoded spectrograph became possible (57). Here, one takes a conventional grating spectrograph, equipped with an array detector, and replaces the entrance slit with a Hadamard mask. The array detector acts as the second mask in this case. The spectrograph already enjoys a multichannel advantage, and the mask, with its increased area over the slit, provides a throughput advantage, potentially providing a total signal-to-noise gain of n/4. In a 1993 paper (57), this was applied to the weak emission from applications like aurora spectroscopy, which were detector-noise limited, even with a CCD detector. The authors noted that the technique assumes that the coded aperture mask (entrance aperture) is illuminated uniformly, at least in the dispersion direction: this is a key caveat with important sampling consequences for analytical spectrometers. Today, the combination of array detectors and MOEMS devices enable the fabrication of doubly encoded Hadamard spectrometers in many spectral regions (58).

The Implications of Diffuse Reflectance Sampling

Analytical NIR spectroscopy has been dominated by the study of natural products, especially in the food and agriculture industries (59). This has some very important implications in spectrometer and accessory design. These samples are heterogeneous, and therefore, to obtain an average spectrum of the sample, large sample sizes are required. Further, because these samples are not pure materials, and because each chemical constituent might be present in a wide variety of environments, the spectral features in these samples are very broad, typically many tens of nanometers halfwidth. Pure materials (hydrocarbons, fine chemicals, pharmaceuticals, crystalline materials, and synthetic polymers) have much narrower bands in the near-infrared, especially in the combination and first overtone regions.

The broad features from natural products enabled grating instruments to be designed for these analyses with 10-nm resolution (7), and the much lower resolution optical-filter-based instruments to work well for moisture analysis, for example. These grating instruments, therefore, have much greater throughput than a high-resolution (~1 nm) laboratory spectrometer designed for molecular spectroscopy. When a solid material is examined in diffuse reflectance, as is commonly the case, the sample itself essentially becomes the source — and a large area, diffuse, dim source at that. This is because the sample scatters the incident light over a 2π solid angle. The problem is now to throughput match this large area source to the rest of the optics in the spectrometer. Therefore, the use of a large detector area is a distinct advantage. This is straightforward in predispersive (wavelength selection before the sample) spectrometers and Fourier-transform spectrometers. The solution to this problem devised by Norris (7) was to use a predispersive spectrometer with a very compact sampling accessory, equipped with two large area detectors inclined at 45° to the optical axis, and instruments using this concept are still available today (60). For this type of measurement, modern FT-NIR spectrometers use an integrating sphere, equipped with a large-area extended InGaAs detector.

A spectrometer based upon a spectrograph and a linear array detector is, by definition, a postdispersive instrument. In an array-detector–based spectrograph, the individual pixels now become the limiting aperture of, and define the limiting angle in, the optical system. The trend in linear array detector development has been to increase the number of pixels and to decrease their size. There is an obvious mismatch between the detector pixel area with its acceptance angle, and the area of the sample (that is, the effective source) and the solid angle over which it reflects. This makes linear diode-array spectrometers fundamentally less suited to diffuse reflectance measurements than predispersive instruments.

Conventional Small Spectrometers

Some of the approaches to, and issues with, miniature spectrometers have been reviewed (61,62). In this section, we will describe some of the existing small or miniature spectrometers, fabricated using conventional techniques, before examining the impact of new technologies in this field in Part IV.

Spectrometers Using LED Sources

Rosenthal has reviewed the use of LEDs as sources in small, portable NIR spectrometers, but focused on a few products only (18); if the definition of a spectrometer is broadened to include specific analyzers and sensors, then it is apparent that LED-based spectrometers are used routinely today in applications as varied as gas sensing, alcohol in wine, and blood oxygen saturation.

In the products that Rosenthal reviewed, he noted that, because of their large bandwidth and the variability of LED sources, a bandpass filter, ~12 nm width, is placed in front of each LED. Although these are not true spectrometers, they are NIR analyzers, and LEDs have some advantages, especially in the construction of a portable, battery-powered device. They have low power consumption; a long life; enable a no-moving-parts system because they can be switched on and off rapidly; are "bright" sources (high power output per unit area); are optically efficient when their light output is confined to a narrow range; and because they are discrete wavelength sources, they enable the use of a low-cost, single-element detector in the analyzer. However, Rosenthal also notes that, "As of this writing, all IRED-based commercial NIR analyzers are individually calibrated." This is due to the variability of commercial LEDs and the lack of a suitable internal wavelength and power calibration system in these analyzers. However, he notes that, using short-wave NIR LED technology, it was possible to develop an NIR analyzer for body fat analysis with a sale price of less than $100, and that 100,000 of these were sold. In the 1990s, with longer wavelength, smaller, and more powerful LEDs, Zeltex (63) developed a portable octane analyzer that incorporated as many as 39 LED–filter sources. These instruments do highlight the possibilities of using semiconductor light sources in spectrometers, and taking advantage of telecommunications technology.

Two 1998 papers surveyed the possibilities at that time for LED-based spectrometers for handheld sensors and process monitoring (64,65). They highlight the change in an LED's output, both power and wavelength shift, as a function of temperature, and means to compensate for that. They also describe visible-NIR LED-based device designs for the pulp and paper industry, and a prototype shortwave NIR spectrometer with LEDs operating at 850, 900, 935, 950, and 1020 nm.

In 1995, Fateley and colleagues (66) described a different concept for an LED-based spectrometer. This consisted of a number of LEDs, each of which could be modulated at a different frequency. In this way, the intensity of each channel could be recovered via a Fourier transform. However, there will come a time when all the diodes are "on" simultaneously, and therefore, the detector signal will show a "centerburst." This can lead to dynamic range problems, and so the authors proposed a Hadamard-type scheme in which the modulation frequencies for each diode are the same but their phases are different, which eliminates the centerburst (67).

McClure and colleagues (68) described a handheld NIR spectrometer for measuring moisture and chlorophyll. This used three unfiltered LEDs as sources, emitting at 700, 880, and 940 nm, was powered by four AA batteries, measured 10 cm × 19 cm × 5 cm, weighed 364 g, and cost around $300 to produce. The emphasis in this work was to produce a device for some basic, but critical, agricultural measurements, that would be affordable in developing countries.

An instrument for measuring alcohol in wine from Anton Parr (69) uses an LED source and a small detector array. It operates between 1170 nm and 1200 nm (70): the C-H stretch second overtone region. Light from an NIR LED passes through a sample cell (with a Peltier thermostat control), then through a slit, is dispersed by a grating, and falls on a nine-element array detector. A relative baseline can be determined using the water-alcohol isosbestic points in this region. There are no moving parts in the spectrometer, and this is another example of a dedicated spectroscopic analyzer using an LED as a light source.

A miniature carbon dioxide sensor from Vaisala (71) employs an LED light source, transmitting light through a miniature gas cell, and a miniature Fabry–Pérot interferometer, and onto a detector. The Fabry–Pérot can be set to transmit two wavelengths: one for a baseline and the other on a carbon dioxide absorption band. The company's data sheet claims that the sensors are so stable that they only require a calibration check every five years. A complete sensor on an electronics board measures 96 mm × 60 mm; the sensor module itself, including the gas cell, is approximately 20 mm × 20 mm.

The pulse oximeter (72) also employs LEDs as light sources. Human blood contains oxyhemoglobin (HbO₂), reduced hemogloblin (Hb), and usually small quantities of methemoglobin and carboxyhemoglobin. In a clinical situation it is important to know the oxygen saturation in arterial blood, defined as the oxygen content divided by the oxygen capacity. HbO₂ absorbs strongly in the 900–1000 nm region and weakly in the 600–700 nm region, and the converse is true for Hb. The complication in this analysis arises from the human pulse; this causes the arteries to expand and contract, altering the pathlength under study. However, this pulsation also produces a modulated spectroscopic signal, and this is the key to the elegance of the modern pulse oximeter. Two LEDs typically are used, at 660 nm and 910 or 940 nm, and a photodiode detector. Four signals are measured, essentially the constant (or DC) component at each wavelength and the pulsatile (or AC) components. The AC component is only due to the change in absorbance due to HbO₂ as the pathlength alters. A ratio R is calculated, which is therefore proportional to the oxygen saturation:

R = (AC₆₆₀/DC₆₆₀)/(AC_NIR/DC_NIR)

where NIR is the signal at 910 or 940 nm. A stand-alone pulse oximeter can now be purchased by the public (on Amazon.com, for example), and costs around $200. It is therefore an excellent example of photonic components being used in a miniature, dedicated function analyzer, in a high-volume, low-cost market. Because it is a small, low-cost device, it can be integrated with other clinical analyzers (such as cardiac defibrillators), deployed in the field (by monitoring firefighters and first responders), and wirelessly networked for remote monitoring. Frost and Sullivan (73) estimated the market for pulse oximeters for the U.S. alone at $600 million in 2006, or hundreds of thousands of spectrometers. This dwarfs the unit sales of laboratory spectrometers.

Array-Based Spectrometers

Stark has reviewed all the technical aspects of NIR array spectrometers in detail (16), and other authors have reviewed their performance (74,75). Ocean Optics (Dunedin, Florida) pioneered commercial development of miniature UV–vis and NIR spectrometers, starting in 1992 with its model S1000 (76), and they have sold about 100,000 spectrometers since that time. The development of this product line fits the model of a disruptive technology (1). Today, many other companies participate in this field, with PDA-, CCD-, and InGaAs-array-based spectrometers, in small form factors; a review published in 2000 covered 10 vendors, and more have appeared since then (39). A typical Ocean Optics UV–vis spectrometer (USB4000 UV-VIS) has dimensions of 89.1 mm × 63.3 mm × 34.4 mm and weighs 190 g. It includes a 3648-pixel linear CCD array, and covers 200–850 nm with a resolution of ~1.5 nm, with a cost of less than $2800. NIR versions, with linear InGaAs array detectors, are more expensive, and range from $15,000 to $23,000, because of the higher cost of InGaAs arrays.

In Japan, a number of portable, dedicated, NIR analyzers are available, operating in the short-wave region (780–1100 nm) (22,77). These predict the sweetness of many fruits, including apples, peaches, and melons, and some are used by consumers in Japanese supermarkets. FANTEC (78) has developed a Fruit Quality Analyzer handheld instrument (FQA-NIR Gun), in a cordless drill form-factor, for use in the field. This instrument also has the capability to connect to a cellular phone to transmit spectra and results.

Multiplex Spectrometers

Fourier-Transform Spectrometers

Small FT-IR spectrometers have been reviewed recently (79), and were covered in Part I of this series (1). In general, a small FT-IR spectrometer developed for mid-IR applications can be converted to the NIR range with a suitable source, beamsplitter, and detector. The size of an FT-NIR spectrometer is therefore essentially the same as a mid-infrared FT-IR spectrometer, and specific FT-NIR spectrometers for laboratory applications do not employ novel technologies. One exception appears to be InLight Solutions (80), which has developed a small FT-NIR spectrometer for noninvasive clinical measurements (81). This has been applied to noninvasive alcohol testing (82), operating in a very narrow range (4225–4625 cm^–1), corresponding to ethanol C-H combination bands, and a commercial system is now available, dedicated to this application (83). Other applications include bioreactor monitoring and blood glucose measurements (84).

Hadamard-Transform Spectrometers

Hadamard-transform spectrometers failed to make an impact in the mid-infrared, but MOEMS advances have provided off-the-shelf technologies for Hadamard masks that make this technique practical in the near-infrared. The first MOEMS device applied in this area was the Digital Light Projector (DLP), described here; the Aspectrics (85) spectrometer was described in Part I (2); and more recently, an active grating is the key component in a handheld NIR spectrometer, which will be described in Part IV.

Fateley and coworkers have described the use of a DLP as a Hadamard mask device (86,87). Their early work used liquid crystal stationary optical masks, but these had two limitations as Hadamard devices: a slow transition time between on and off, coupled with a less-than-ideal modulation efficiency (that is, imperfect "0" and "1" modulation). DLP devices eliminate these problems, with a switching time of less than 20 μs and almost perfect modulation efficiency. They identified a number of other attractive features of the DLP: highly reflective aluminum mirrors; high position reliability; ability to program different patterns, allowing variable spectroscopic resolution; and high optical radiation tolerance.

The DLP chip was built into a grating spectrograph, which could be operated either as a Hadamard spectrometer or as a conventional scanning spectrometer. By matching the data collection parameters carefully, the authors were able to verify that they achieved an S/N improvement that was close to that predicted by the multiplex advantage. In this case they had 811 resolution elements, and the predicted improvement was 1/2√811, or about 14. In practice, they observed an improvement of approximately 12–15, varying slightly across the spectrum.

However, there were also some limitations in the material and antireflection coating of the DLP's glass cover plate, the size of its circuit board, the small size of the pixel micromirrors (16 μm), and the overhead of the video processing electronics. Some of these are due to Texas Instruments' initial decision not to sell DLPs as components, but this position has changed since Fateley's initial work was done, and well-documented starter kit, boards, and chipsets are now available (88). A variety of window materials for different spectral ranges is also available. A typical DLP chip supports 1024 × 768 resolution, with 13.6 μm square pixels, for a total array size of 14 mm × 10.5 mm.

Conclusions

We have summarized some of the conventional approaches to miniaturizing NIR spectrometers. In Part IV, we will see how MOEMS and telecommunications photonics technologies (2) are poised to revolutionize NIR spectroscopy with a new generation of miniature instruments.

Richard A. Crocombe is with ThermoFisher Scientific in Billerica, Massachusetts.

References

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