Patent Publication Number: US-9841322-B1

Title: Spectral imaging with multiple illumination sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application 62/006,480 filed Jun. 2, 2014, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This document concerns an invention relating generally to molecular spectroscopy, and more specifically to non-contact optical analysis wherein a sample is imaged using several different discrete-wavelength light sources, in particular, laser sources. 
     BACKGROUND OF THE INVENTION 
     Molecular spectrometers are sometimes used to analyze the composition of materials. Such spectrometers emit light having known wavelength(s) and intensity onto the material to be analyzed, and then capturing the light scattered from (and/or transmitted through) the material, with the difference between the emitted and measured light providing information regarding the characteristics of the material. For example, near-infrared spectrometers direct different near-infrared wavelengths of light onto a material, either simultaneously or sequentially, and detect the intensity of the reflected or transmitted light at each wavelength. (Other wavelength ranges are possible, with ultraviolet, visible, or mid-infrared ranges—or some combination of ranges between 200 nm to 25,000 nm—being common.) The resulting spectrum returned by the material—that is, the intensity at each measured wavelength—can provide information regarding the composition of the material (or regarding other material characteristics, such as thickness, porosity, prior heat treatment, etc.) at the illuminated area. Further details can be found in, for example, Davies, A. M. C. et al: Near Infrared Spectroscopy: The Future Waves, NIR Publications (1996); and in Burns, D. A, et al: Handbook of Near-Infrared Analysis, Practical Spectroscopy Series, Marcel Dekker, Inc. (1992). 
     The spectrum returned by the illuminated area effectively represents the average material composition over the illuminated area; for example, if the illuminated area on a composite material has both fibers and polymeric material, the spectrum for this area will effectively be a combination of the spectrum for the fiber alone, plus the spectrum for the polymeric material alone. It is often more useful to know the spatial distribution of materials across a sample, rather than just measuring the “bulk” composition of the sample. Thus, a method known as spectral imaging—or hyperspectral imaging if a large number of wavelengths is analyzed—obtains spectra at sub-areas or “pixels” across the surface of the sample, where each pixel contains its own distinct spectral information. This (hyper)spectral imaging can be performed, for example, by uniformly illuminating a sample area with a sequence of wavelengths, and detecting the distribution of the composition at each pixel on the area using a camera sensitive in the wavelength range of the illumination. 
     (Hyper)spectral imaging devices require uniform and reproducible illumination across the sample area to be analyzed. Direct illumination using incandescent lamps is commonly used for cost-effectiveness, with quartz halogen lamps being popular owing to the wide wavelength range of quartz halogen sources. However, illumination from an incandescent lamp is nonuniform due to the structure of the lamp&#39;s filaments and bulb/enclosure, and due to any reflectors used to direct the light as needed. The use of multiple lamps introduces further nonuniformity in the distribution of light intensity and color due to the differences between the individual lamps, and due to the geometry of their relative placement. 
     Solid-state light sources such as LEDs and lasers are powerful light sources which produce a fraction of the heat of incandescent sources, and which typically have lifetimes extending to tens of thousands of hours. However, they are typically monochromatic sources—they emit in only a single wavelength, or in a narrow band—and typically a wider range of wavelengths is desired for spectral illumination. Thus, considerations for selecting and using solid-state sources are the availability and cost of the light sources capable of providing the desired wavelengths, and how to efficiently couple their light output to the sample area in a uniform manner. For example, a common method of seeking uniform output illumination from multiple input sources—whether solid-state or incandescent—is to couple the sources to fiber optics wherein the fibers are randomized from their input ends to their output ends. The large number of fibers and their random arrangement homogenizes the light, and provides a relatively uniform light spot at a distance from the output end. Such devices are commercially available from optical catalog companies, such as Edmund Optics (Barrington, N.J.). Each input end in these devices is usually a round fiber bundle, and the output end has fibers arranged in a round or linear pattern. This method works acceptably well for incandescent lamps, but for highly directed light sources such as lasers, the light coupling into the fiber is insufficiently uniform, and there is often a distinct laser speckle pattern on the sample being illuminated. Additionally, because the randomization of the fiber bundle is usually imperfect, there can be variations in intensity across the output side. A variation of this method uses a fiber bundle with one or more round input sides, and an output side wherein the fiber ends are arrayed in a ring-like configuration, with the fiber ends being oriented such that light output therefrom is oriented towards a common location. Here too illumination tends to be nonuniform at the output ends when highly directional light sources, such as LEDs and lasers, are used. Spatial uniformity of illumination is critical to the quality of spectral measurements, and each input wavelength needs to be projected on the area of interest with uniform intensity across the area. 
     Integrating spheres have previously been used to detect multi-directional reflection off of diffuse surfaces, as well as for measuring the absolute intensity of light-emitting devices. Companies such as Labsphere, Inc. (North Sutton, N.H.) produce differently-sized integrating spheres made from, or having their inner surfaces coated with, highly reflective materials. Input and output ports on the sphere provide input and output of light with a good degree of directional homogenization of the light due to the multiple reflections occurring inside the sphere. Integrating spheres have previously been used in near-infrared analyzers, such as the InfraAlyzer IA 450 (Bran and Luebbe, Norderstedt, Germany). Highly uniform illumination of the sample was achieved only if the sample was in contact with the output port of the integrating sphere; non-contact arrangements result in unsuitably nonuniform illumination. A small space between the illuminating device and the sample can be achieved by a half integrating sphere, such as the Hemilite™ Vision Illuminator (StockerYale, Inc., Salem, N.H.). This device has built-in LEDs and improves uniformity of illumination, but is not applicable to highly directional illumination, such as that provided by multiple lasers, because the light does not undergo the many multiple reflections that it experiences in complete integrating spheres. 
     SUMMARY OF THE INVENTION 
     The invention, which is defined by the claims set forth at the end of this document, is directed to devices and methods which seek to provide greater illumination uniformity on a sample area from several different solid-state directional light sources (e.g., lasers and LEDs) for the purposes of (hyper)spectral imaging. A basic understanding of some of the features of exemplary versions of the invention can be attained from a review of the following brief summary of preferred versions of the invention, with more details on these and other versions being provided elsewhere in this document. To assist in the reader&#39;s understanding, the following review makes reference to the accompanying drawings (which are briefly reviewed in the “Brief Description of the Drawings” section following this Summary section of this document). 
       FIG. 1  provides a schematic view of a spectrometric device  100  for analyzing the composition of a sample  10  by capturing spectra from an area on the sample  10 . The device  100  includes two or more monochromatic light sources  102 , such as LEDs or diode lasers, for illuminating the sample area  10 ; a camera  104  configured to capture a two-dimensional image of the sample area  10 ; a control system  106 / 108  configured (at  106 ) to activate the light sources  102  to emit light (preferably by sequentially activating the light sources), and to activate the camera  104  (at  108 ) to capture two-dimensional images of the sample area  10  (at least at times when the light sources are activated); and a light transmission assembly which transmits the light from the light sources to the sample  10  (and homogenizes the light as it does so), with the light transmission assembly in  FIG. 1  including an integrating sphere  110 , a source optical transmitter  112  for transmitting light from the light sources to the integrating sphere (with the source optical transmitter  112  in  FIG. 1  being provided by source optical fibers), and an output optical transmitter  114  for transmitting light from the integrating sphere to the sample  10  (with the output optical transmitter  114  in  FIG. 1  being provided by an output optical fiber bundle). 
       FIG. 1 &#39;s source optical transmitter  112 —the source optical fibers—have input ends optically coupled to the light sources  102  (i.e., affixed in relation to the light sources  102  in such a manner that they receive light from the light sources  102 ), and output ends optically coupled to sphere input ports  116  on the integrating sphere  110 . Preferably, each source optical fiber  112  is in fact an optical fiber bundle, with the fibers in each bundle being randomized between their input and output ends. More preferably, the source optical transmitter is a fiber combiner  200  ( FIG. 2 ) having two or more combiner input ends  202 , each being optically coupled to one of the light sources, and a single combiner output end  204  optically coupled to one of the sphere input ports  116 . 
     Apart from the one or more sphere input ports  116  which receive light from the light sources  102 , the integrating sphere  110  of  FIG. 1  has one or more output ports  118 . These include at least one “standard” sphere output port  118  optically coupled to the output optical transmitter (the output optical fiber bundle  114  of  FIG. 1 ) for transmitting integrated light from the integrating sphere  110  to the sample  10 , and optionally a sphere reference port  120  optically coupled to a reference detector  122 . The reference detector  122  is configured to measure the light intensity within the integrating sphere  110 , and thereby provide a reference intensity measurement for any light sources  102  emitting into the integrating sphere  110  at the time the reference measurement is made, which can be useful where light sources  102  may have intensity which varies over time. 
     The output optical fiber bundle  114  of the output optical transmitter of  FIG. 1  has its fiber input ends optically coupled to the sphere output port  118 , and its opposing fiber output ends, which may directly illuminate the sample  10 , are preferably randomized for improved light homogeneity. More preferably, the fiber output ends are configured as a “ring light”  124  (see also  FIG. 3B ): the fiber output ends are arrayed along a closed path—preferably a circular path—extending about a central passage  126 , and are aligned to direct light from the light sources  102  to the sample area  10  (that is, the beams from the various fiber output ends intersect at a “focal spot” at which the sample  10  can be located). In this instance, as depicted in  FIGS. 3A-3B , the camera  104  is preferably situated to image the sample area  10  through the central passage  126  defined between the fiber output ends of the output optical fiber bundle  114 . 
     For ease of use, the camera  104  and ring light  124  (or other sample illuminator) is preferably configured as a compact and easily manipulated measurement head allowing a user to easily situate it at locations where sample analysis is desired, while the remaining components can be provided in a supply unit which may be placed in a nearby out-of-the-way location.  FIG. 4  shows an arrangement of this nature, wherein the camera  104  and ring light  124  of  FIG. 1  are provided in a gun-like measurement head  402  connected to a supply unit  404  by an elongated flexible cable  406 . The measurement head  402  has a muzzle  408  bearing the camera  104  and the illuminator  124  of  FIG. 1 , and a grip  410  bearing a trigger  412  which actuates spectroscopic imaging, with the measurement head  402  being flexibly tethered to the control system in the supply unit  404  by the cable  406  (which includes the output optical fiber bundle  114 , power lines, etc. needed for operation of the measurement head  402 ).  FIG. 1  illustrates how the  FIG. 4  arrangement may be applied to the components of  FIG. 1 , with certain components being provided in the measurement head  402  and others being provided in the supply unit  404 , with the cable depicted at  406 . 
     Additional features can assist in enhancing data quality and ease of use. Initially, because the intensity of the light sources  102  and/or the sensitivity of the camera  104  (more particularly, its CCDs or other photosensitive detection elements) can change over time, it is useful to include calibration features in addition to, or instead of, the reference detector  122  noted above. Referring to  FIGS. 3A-3B , a mask  300  may be mounted in fixed relationship to the camera  104  at the camera&#39;s focal plane, with the mask  300  being intended to rest atop the sample  10 , and having an aperture  302  through which the camera  104  is to image at least a portion of the sample area  10 . The mask  300  has a reference area  304  defined adjacent the aperture  302 , whereby the camera  104  images at least a portion of the reference area  304  when capturing the image of the sample area  10 . The reference area  304  has a known spectral response, and by imaging the reference area  304  while imaging the sample  10 , the response of the reference area  304  can be used as a datum by which drift in the camera  104  (e.g., variation in the sensitivity of its CCDs or other imaging elements), and/or variation in the intensity of the light source(s)  102 , can be detected by the control system (more particularly by its data processor  108 ) and corrected for. Further, the mask  300  is preferably mounted at the desired distance from the camera  104  by a rigid spacer tube  306 , wherein the aperture  302  of the mask  300  (and its reference area  304 ) is imaged through the interior of the tube  306 . The spacer tube  306  beneficially maintains the mask  300  at the proper distance from the camera  104 , and also serves to prevent unwanted incident light from illuminating the sample area  10  while it is being imaged. The spacer tube  306  and mask  300  can beneficially be provided as components which can be attached to the measurement head  402  ( FIG. 4 ) bearing the camera  104  and illuminator  124 , and which can be removed and replaced when desired. 
     In the device  100  schematically illustrated in  FIG. 1 , the control system  106 / 108  for the device  100  is illustrated as having two parts: an illumination controller  106  configured to activate the light sources  102  to emit light (preferably by sequentially activating the light sources  102 ), and a data processor  108  configured to activate the camera  104  to capture two-dimensional images of the sample area  10  (at least at times when the light sources  102  are activated), and to process the captured images. To better avoid variations in illumination and imaging from image to image, the control system  106 / 108  preferably activates the light sources  102  and the camera  104  in synchrony with an AC line voltage powering the control system  106 / 108  (see  FIG. 7 ). 
     Further versions, features, and advantages of the invention will be apparent from the remainder of this document in conjunction with the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating components of the exemplary spectrometric device  100 . 
         FIG. 2  is a schematic diagram of a fiber combiner  200  suitable for use in lieu of the multiple source optical fibers  112  of  FIG. 1 , for coupling into a single sphere input port  116  of the integrating sphere  110 . 
         FIGS. 3A and 3B  are respectively assembled and exploded (disassembled) perspective views of an exemplary spectrometric measurement head for use in the device  100 , with sample illuminator (ring light)  124  and imaging camera  104 . 
         FIG. 4  is a view of an exemplary form in which the spectrometric device  100  of  FIG. 1  might be provided, with a measurement head  402  flexibly tethered to a supply unit  404  via a cable  406 . 
         FIG. 5  is a perspective view of an exemplary spectrometric measurement head resembling that of  FIGS. 3A-3B , but modified to have a spacer  506  resting forwardly of the camera  104  which better allows spectrometric analyses of materials situated along inside corner seams of structures. 
         FIG. 6  is a schematic view of another exemplary spectrometric measurement head having a spacer  606  with an internal mirror  614  allowing imaging of surfaces situated at right angles to the camera&#39;s line of sight. 
         FIG. 7  is a chart showing exemplary timing for illumination and imaging operations in the spectrometric device  100  of  FIG. 1 . 
         FIGS. 8A and 8B  are hyperspectral images of the moisture content of a fresh lettuce leaf ( FIG. 8A ), and of the leaf after several hours of drying ( FIG. 8B ). 
         FIG. 9  is a chart showing the thickness of thermosetting resin across a portion of the surface of a carbon fiber-resin composite. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED VERSIONS OF THE INVENTION 
     To review the exemplary spectrometric device  100  of  FIG. 1  in greater detail, several light sources  102 , such as LEDs or lasers, are connected to source optical fibers  112 , which are in turn coupled to the sphere input ports  116  of the integrating sphere  110  by fiber couplers (e.g., simple SMA connectors or light collimators). The light sources  102  can have the same or different formats (e.g., B mount, C mount, TO-can, fiber coupled, and/or lens coupled lasers), and their wavelengths can be chosen as desired, preferably at wavelengths spaced at intervals across some range of interest (e.g., 200 nm to 25,000 nm). Often, the desired laser wavelengths are only available in different formats, making it necessary to use different types of light sources  102 . If a light source  102  has an appropriate format, it could alternatively be directly optically coupled to a sphere input port  116 , as with the light source  102 A. In any event, the sphere input ports  116  are preferably placed symmetrically about, and equidistantly from, the sphere output port  118  so that their input light is directed onto a diffusely reflective surface of the integrating sphere  110 , so that the input light from each source experiences substantially the same degree of internal reflection before reaching the sphere output port  118 . The internal reflections within the integrating sphere  110  homogenize the light from the various light sources  102 , rendering it more uniform in intensity and directionality by the time it reaches the sphere output port  118 , and also serving to reduce or eliminate any laser speckle. Without the integrating sphere  110 , it is difficult to couple several directed light sources  102 , such as diode lasers, into an optical fiber bundle or other light transmitter and achieve uniform wide-angle illumination at the output end of the light transmitter. 
     The sphere gain (the efficiency of the integrating sphere  110 ) is reduced by the total area of the ports  116 / 118 / 120  on the sphere  110 , as these reduce the available reflective surface. Thus, it is preferable to have the ports  116 / 118 / 120  occupy as little of the area of the sphere  110  as possible. To reduce the area occupied by the ports  116 / 118 / 120 , it is useful to pre-combine the light from at least some of the light sources  102  using the fiber combiner  200  ( FIG. 2 ), wherein the light from the various combiner input ends  202  is channeled to a single combiner output end  204  (e.g., a standard SMA  905  connector) optically coupled to one of the sphere input ports  116  (with the integrating sphere  110  preferably having only a single input port  116  in this instance). It has been found that use of a fiber combiner  200 , and combining the output of as many light sources  102  as possible into as few sphere input ports  116  as possible, significantly helps with spectral data quality, likely because providing fewer input ports  116  on an integrating sphere  110  provides more interior sphere reflecting surface (thereby allowing more interior light reflection, enhancing the homogeneity of the light illuminating the sample  10 ), and also because light sources  102  illuminating the same sphere input port  116  will have greater similarity in the optical conditions that they encounter while traveling to the sample  10  (thereby reducing the effect of potential spatial differences in light intensity between different light sources  102 ). It is also possible to have one or more sphere input ports  116  each be supplied by multiple light sources  102  via respective fiber combiners  200 , with one or more other sphere input ports  116  each being supplied by its own individual light source  102 . (It is further possible to combine all light sources  102  using a fiber combiner  200 , and then use the fiber combiner  200  as an output optical transmitter  114  for illuminating the sample  10  without the use of an integrating sphere  110 , but as discussed above, the use of an integrating sphere  110  has been found to substantially improve the uniformity of the output light.) 
     As discussed above, apart from the sphere output port  118 , the integrating sphere  110  may also include a secondary sphere output port—a sphere reference port  120 —optically coupled to a reference detector  122 . This is useful because semiconductor light sources  102  such as diode lasers are known to have pulse to pulse variations in light intensity, which limits measurement repeatability and spectrometric data quality. Some laser light sources  102  have built-in reference detectors to measure a signal proportional to the output of the laser, and this signal can be used for calibration and feedback purposes. However, as the number of light sources  102  used by the device  100  increases, it can become more difficult to compare the light output between the sources  102  because the intensity signals from the multiple reference detectors  122  can be difficult to fully normalize owing to their different characteristics: because the different reference detectors  122  may be of different detector materials, may be positioned differently relative to the solid-state light emitting components, may experience different temperatures when in use, etc., the different detectors  122  can behave differently from light source  102  to light source  102 . Thus, it can be useful for the integrating sphere  110  to include the sphere reference port  120  and reference detector  122 , enabling the device  100  to compare the actual light output from all light sources  102  after homogenization in the integrating sphere  110  (most preferably after encountering the same homogenization, where the light sources  102  all enter a single sphere input port  116  via the fiber combiner  200  of  FIG. 2 ). The intensity signal from the detector  122  can be amplified, converted to a digital signal, or otherwise be processed before supplying it to the control system&#39;s data processor  108 . 
     The light from the sphere output port  118  can then be used to illuminate the sample  10 , preferably via an output optical transmitter  114  (an output optical fiber bundle or other light-transmitting element). The preferred approach is to have an output optical fiber bundle  114  wherein the many fibers within the bundle are randomized, and their fiber output ends are arranged to surround the central passage  126  ( FIG. 3B ) in the ring light  124  with their illumination directed at an angle towards the axis of the ring. The resulting light is diffuse, multi-directional, and uniform, particularly where the ring light  124  is supplied by both the integrating sphere  110  and fiber combiner  200  ( FIG. 2 ). The sample  10  can then be situated at the location where the emitted light beams from the fiber output ends converge, where the uniformity and strength of the light is optimal for measurement. 
     During analysis, the best data quality results if the ring light  124  or other illuminator (fiber combiner output end, fiber bundle terminus, sphere output port, etc.), and the camera  104 , are maintained at a steady (and optimal) focal distance from the surface of the sample  10 . To this end, it is useful if the illuminator  124  and the camera  104  are mounted in fixed relation to each other as a unit, referred to herein as a measurement head (with an example shown at  402  in  FIG. 4 ), and if a rigid spacer tube ( 306  in  FIG. 3 ) is removably affixed forwardly of the illuminator  124  and camera  104  so that the light from the illuminator  124  is directed through, and the imaging of the camera  104  occurs through, the interior of the tube  306 . (Throughout this document, it should be understood that a “removable” attachment is one that can be attached and detached by hand, or with the use of simple tools such as a pliers or screwdriver.) The length of the spacer tube  306  is sized such that situating its end against the sample  10  situates the sample  10  at the camera&#39;s focal plane, and also at the location at which the illuminator  124  provides optimal location (e.g., the location at which the beams from the ring light  124  converge). Thus, by situating the spacer tube  306  against and about the desired sample area  10 , the spacer tube  306  automatically situates the sample area  10  at the proper location for analysis. Additionally, so long as the spacer tube  306  is tightly fit against the sample  10  and lacks locations for light entry, the sample area  10  will be isolated from ambient illumination that may interfere with accurate spectral imaging. 
     Referring again to  FIGS. 3A-3B , a particularly preferred arrangement is to provide a mask  300  at the end of the spacer tube  306  wherein the mask  300  has an aperture  302  exposing the sample area  10  to the illuminator  124  and camera  104 , with the aperture  302  being at least partially bounded by a reference area  304 . Because the light sources  102  can have pulse-to-pulse variations in intensity, and because the imaging elements in the camera  104  can vary in sensitivity over time, image-to-image differences in spectral measurements can occur, interfering with data quality. By providing a reference area  304  with known spectral characteristics and illuminating and imaging a portion of it while also doing so to the sample area  10 , the portion of the image from the reference area  304  can be used as a datum allowing for calibration of the image of the sample area  10 . As an example, the reference area  304  may be situated at one or more locations about the aperture  302 , and may be formed of a diffuse grey material which is at least substantially spectrally flat across the wavelengths of interest, so that it does not introduce new spectral artifacts to the measurement. Additionally, the reference area  304  is preferably thin and situated as close to the sample  10  as possible so it does not cast a shadow; is preferably diffuse and free of defects so that angular effects are minimized (e.g., tendency to preferentially reflect along certain angles); is preferably placed in such a way that the light reflected from the reference area  304  is representative of the amount of light which illuminates the sample area  10 ; is preferably consistently positioned between images so that the processing algorithms/software can easily identify the reference area  304  within the image; and preferably has a reflectivity similar in magnitude to that of the sample of interest. By including the reference area  304  within the image, the independently-varying output of the light sources  102  and the sensitivity of the camera  104  are characterized in one measurement, eliminating the need for the reference detector  122 . 
     Where samples  10  with nonplanar surfaces are to regularly be analyzed, masks and/or spacers having shapes specially configured to closely fit against the nonplanar surface can be devised. As an example,  FIG. 5  illustrates a measurement head (camera  104  and ring light illuminator  124 ) with a combined mask  500  and spacer  506 —formed as a unit—which can be used to analyze interior corner seams. The tubular spacer  506  has a wedge-like form with opposing planar sides  508  which join at a curved vertex  510 , from which the interior passage  512  of the spacer  506  extends. The mask  500 , which is integrally formed with the end of the spacer  506  which is to abut the sample, has reference areas  504  which extend inwardly from opposing sides of the interior passage  512  at the vertex  510 .  FIG. 6  then illustrates a measurement head  602  with a tubular spacer  606  configured to allow imaging of a wall adjacent a corner, with the spacer  606  having an interior mirror  614  which directs illumination from, and passes an image of the wall to, the measurement head  602 . 
     To further review the operation of the device of  FIG. 1 , the data processor  108  coordinates imaging along with the illumination controller  106  via a digital signal line  128 . When illumination from a light source  102  is needed, the illumination controller  106  turns on a current driver  130  for the desired duration (and at the desired current, and thus the desired intensity), and instructs a multiplexer  132  to select the desired light source  102 . Where the light sources  102  are laser diodes, the use of the integrating sphere  110 , and the output optical fiber bundle  114  and ring light  124 , provides uniform high-quality illumination of the sample  10 , particularly where the light sources  102  are coupled into the integrating sphere  110  using the fiber combiner  200  ( FIG. 2 ). When the trigger  412  ( FIG. 4 ) is pressed by a user, the camera  104  is activated via camera control line  134  to image the sample  10  when it is being illuminated by the selected light source  102 . The camera  104  images the sample  10  through a hyperspectral lens  136 , which is preferably free of distortion and chromatic error across the range of wavelengths used by the light sources  102 . The image is transmitted through a camera signal line  138  to a data port  140  on the data processor  108 ; depending on the make and model of camera  104  used, the data port  140  may be a Camera Link, USB, Gigabit Ethernet, or other connection. 
     As discussed below in greater detail, the light sources  102  are multiplexed in succession, so that an image is acquired at the wavelength of each light source  102 , allowing the data processor  108  to construct a multispectral image over time. A “dark” reference image of the sample  10  is also acquired with all of the light sources  102  turned off, to provide a calibration image which can be used to characterize any ambient broadband light falling on the sample  10  and reflected back to the camera  104 . The reference area  304  ( FIG. 3B ) included in each image is used to capture the relative illumination intensities of each of the wavelengths for each image. The multispectral image—i.e., the foregoing images—is processed to create an image of the sample  10  illustrating the locations of the sample&#39;s physical/chemical features. With suitably fast components and switching, a complete multispectral image can be constructed in under a second. 
     Other components of the device include a power supply  142  connected via power lines  144  to the illumination controller  106 , current driver  130 , camera  104 , and data processor  108  (and its accompanying input and output devices, e.g., an input device  146 —such as the trigger  412  of  FIG. 4 —for activating illumination and imaging, and a touchscreen  148  for display of results and adjustment of settings). Regarding the control system (the illumination controller  106  and data processor  108 ), these can be provided as separate components or as a unit within the measurement head  402  and/or in the supply unit  404 , and can be provided as a special-purpose computer (e.g., an ASIC), a general-purpose computer (e.g., a conventional personal computer, microcontroller, or the like), combinations of these devices, and/or in other forms having equivalent functionality. 
     A preferred version of the device resembled that of  FIG. 4 , with a 9-foot cable  406  joining the measurement head  402  to the supply unit  404 . The light sources  102  were laser diodes emitting in the 1000-1700 nm near-infrared region, and all optical fibers were glass or low-OH glass-type fibers, which efficiently transmit light in this wavelength region with minimal optical absorption. A fiber combiner  200  ( FIG. 2 ) received the output from the light sources  102  and supplied it to the sole input port  116  of the integrating sphere  110  via an SMA connector. The integrating sphere  110  had a 1.5 inch diameter, and was internally coated with diffuse gold (LaserGold™ by Epner Technologies, Brooklyn, N.Y.). No sphere reference port  120  or reference detector  122  was used, and a reference area  304  ( FIG. 3 ) on the measurement head  402  was used instead. Additionally, a reference station  414  of known composition was included on the supply unit  404 , whereby the device&#39;s functionality could be checked by placing the measurement head  402  against the reference station  414  and imaging it to see whether the expected results were returned. 
     To review the illumination and imaging timing in greater detail,  FIG. 7  shows a timing diagram. Measurement is initiated by the illumination controller using a measurement trigger signal shown at the bottom of  FIG. 7 , which might be generated by the user pushing the trigger  412  on the measurement head  402  of  FIG. 4 . The trigger signal initiates a sequence of measurement periods wherein all light sources  102  are turned off in the first period to record a reference image of the dark signal (or, in the case where ambient light is present, to record that light level), to be used later for correction (e.g., subtraction of the background signal). In each measurement period, the camera  104  is triggered to image for a predetermined imaging time, and is turned off for the rest of the measurement period. The light sources  102  are each turned on in succession, with each one being illuminated in its own measuring period, and with the light sources  102  each being turned on for a stabilization period before the camera  104  is triggered to allow the light source output to stabilize before imaging occurs. After the stabilization period, each light source  102  is kept on for the duration of the camera imaging time. Once the camera  104  has captured images using all light sources  102 , the device may either stop and await another trigger signal, or it may repeat the cycle shown in  FIG. 7  one or more times, and then average the images from each light source  102  (and also average the reference image) before performing further analyses of the measurement. 
     During measurement, it is important that the camera exposure—and thus the image—is not influenced by ambient light. Assuming ambient light changes slowly over time, the foregoing scheme is effective, as the dark/ambient reference image characterizes the ambient light level on the sample  10 . However, where ambient light changes rapidly—for example, where measurements are being taken of a sample  10  under conventional fluorescent lights operating at standard 50-60 hz line voltage—it&#39;s useful to compensate for the effect of such background variation. Assuming the measurement scheme of  FIG. 7  is used, the camera imaging is always initiated at the same time during the line voltage cycle. This approach assures that the same amount of external light is included in the dark/ambient reference image as with the images taken using each light source  102 . After recording all images, the following formula may be used to generate the corrected signal for each pixel in the images:
 
 I   i,j,k =−log {( S   i,j,k   −D   i,j )/( R   i,j,k   −D   i,j )}
 
     Where I i,j,k  is the corrected absorbance signal at pixel i,j of the image for light source k; S is the sample measurement; R is the light level recorded on the reference area, and D is the dark measurement, with all light sources  102  turned off. Once the corrected signals are calculated for all pixels in all images, a hyperspectral image can be represented by appropriately weighting the intensity at each wavelength:
 
 Y   i,j   =B   0   +B   1   *I   i,j,1   +B   2   *I   i,j,2   B   3   *I   i,j,3 + . . . .
 
Y i,j  can then be displayed for all points in the image, or can be saved or otherwise used.
 
     To illustrate a potential use of the invention, the device  100  was used to analyze the moisture content of fresh lettuce leaves. Test spectra taken from fresh lettuce leaves, and from those that lost moisture from drying, show a peak around 1450 nm which is indicative of the moisture content of the leaves. The test spectra were taken at one-hour time intervals while air was blown over the leaves to dry them, with full 1000-2500 nm spectra being recorded using a push-broom hyperspectral SWIR camera (SWIR-LVDS-100-N25E by Specim Ltd, Oulu, Finland). 
     The leaves were then analyzed by a device  100  having three lasers  102 , one having a wavelength near the moisture peak, and the others having wavelengths at valleys above and below the moisture peak. Using the 1450 nm analytical wavelength and two baseline indicator wavelengths at 1350 and 1550 nm, the image of the moisture content and its distribution can be calculated as follows:
 
 M (%)= B   0   +B   1 ( I   1450 −½( I   1350   +I   1550 )) or  M (%)= B   0   +B   1   *I   1450   +B   2   *I   1350   +B   3   *I   1550  
 
     Where the optical intensity, I n , is the absorbance at wavelength n.  FIG. 8  shows an exemplary image of the moisture distribution in a fresh (high moisture) leaf (A) and one after drying (B). (Because the original image was in color, and presented areas of higher moisture with darker red and areas of lesser moisture with darker blue—with these darker colors both presenting as darker grey in the accompanying  FIG. 8 —interpretation of  FIG. 8  is not as easy as for the original full-color image.) The device  100  would be useful for measurement of the moisture content of not only plants, but for any other materials where moisture content is of interest. 
     To illustrate another potential use of the invention, a device  100  as described above was constructed for determination of resin thickness on the surface of a carbon fiber-resin composite. Resin pockets—that is, resin pooling—on the surface of 3D woven composites is an indicator of wrinkles in the composite material. Wrinkles may affect the structural integrity of the composite part, but without a good method for detecting such wrinkles, it is difficult to study those effects. The existing non-destructive techniques for characterizing resin pockets (e.g., ultrasound methods) are not sensitive enough to identify shallow pools (less than 60 mils). The constructed device  100 , used in the near-infrared range, was able to measure surface resin features on composites with much higher sensitivity than prior non-destructive technologies. 
     When near-infrared spectra of different resin thicknesses are obtained, the different spectra display the same peaks, but the peaks grow more exaggerated as resin thickness (and thus absorbance) increases. By selecting wavelengths that correspond (at least roughly) to these peaks, and using a set of standard calibration samples with known resin thicknesses, a calibration equation was developed for the device  100  whereby the images taken at the different wavelengths were converted into thickness maps. The thickness of a surface resin feature of an unknown sample could then be measured by placing the hand-portable measuring head  402  ( FIG. 4 ) on the part of the sample to be analyzed, and pressing the trigger  412  on the handle  410 . The device  100  produced a thickness map of the resin thickness on a 320×256 pixel LCD screen provided on the measurement head  402 . Images obtained with all lasers off, and with each laser on, were processed by use of the calibration equation to generate the thickness map shown in  FIG. 9 . Beyond the thickness map, the data processor of the device  100  was able to provide cross-section graphs, width and depth of resin features, and various measurement statistics. 
     While the device  100  has been described as operating in the near-infrared wavelength range (typically 1000-2500 nm), generally the same components and methods apply to operation in the ultraviolet (200-400 nm), visible (400-800 nm), VNIR (400-1000 nm), and mid-infrared (2500-25000 nm) regions as well. 
     When constructing a device as described above, performance is enhanced if consideration is given to the materials which the device is intended to analyze, and if the wavelengths (and number of wavelengths) chosen are appropriate for the materials. It is useful to compile a sample calibration set consisting of perhaps 2-100 different samples representing all the variations that are expected to be encountered, with material concentrations ranging between the expected extremes (e.g., in the case of moisture determination, the driest and wettest samples that are expected to be encountered). The samples can then be analyzed using conventional UV-visible, near-infrared, mid-infrared, spectral, or hyperspectral devices. Conventional statistical and chemometric practices, such as Principal Component Analysis, Partial Least Squares, Science-Based Calibration, or Multivariate Curve Resolution, can then be used to determine the component spectra (i.e., the materials present in the samples). These spectra can then in turn be used to help select the optimal wavelengths to be chosen for the device, with the candidate wavelengths being chosen from those for which light sources  102  are available from different manufacturers. 
     Two exemplary methods that can be used to select the optimal wavelengths, and thus the light sources  102  for use in the device  100 , are forward selection and backward elimination. In forward selection, the first wavelength selected from the list of all available light source wavelengths is the one with the strongest correlation with the analyte of greatest interest, or the strongest correlation with a physical feature of interest (such as layer thickness). Fixing this first wavelength, the second best wavelength is selected, the calibration calculations are repeated, the statistical parameters for this new set is established and this procedure is repeated to add new wavelengths. The criteria for selection could be based on the standard error of calibration (SEC) or if there are sufficient number of samples, the standard error of prediction (SEP). 
     In backward elimination, all available wavelengths are used to establish the correlation between the spectra and the analyte. The wavelength with the weakest contribution to the correlation is eliminated, and the correlation is recalculated. This process is continued until the correlation gets significantly weaker, or when the SEP falls below the desired level. 
     Regardless of the method chosen, choice of wavelengths (and thus light sources  102 ) will also require economic and engineering considerations, e.g., cost of the candidate light sources  102 , their ease of installation, calibration, and use, and their mechanical, cooling and electrical requirements. 
     It should be understood that the versions of the invention described above are merely exemplary, and the invention is not intended to be limited to these versions. Rather, the scope of rights to the invention is limited only by the claims set out below, and the invention encompasses all different versions that fall literally or equivalently within the scope of these claims.