Patent Description:
The present disclosure relates to calibration of a transmission spectroscopy device.

Transmission spectroscopy is widely used for quantitatively measuring components of gaseous, liquid and solid substances. A transmission spectroscopy device can direct light into a sample, and determine properties of the sample based on how much light emerges from the sample, as a function of wavelength.

In some examples, a transmission spectroscopy device can base its calculations on the Beer-Lambert law, which relates the attenuation of light to properties of the material through which the light propagates. One way to express the Beer-Lambert law mathematically is A = e × b × c. Quantity A is the absorbance of the sample, which also equals log (<NUM> / transmittance of the sample). Quantity e is the molar absorptivity of a compound of interest in the sample. Quantity b is the optical path length traversed by light in the sample. Quantity c is the concentration of the compound of interest in the sample. In order to produce an accurate value for the compound concentration, c, one should have an accurate value for the optical path length traversed by light in the sample, b. The Beer-Lambert law is but one example of how a transmission spectroscopy device can perform its calculations. Other calculation techniques can also be used. For each of these calculation techniques, it is beneficial to know or measure the optical path length traversed by light in the sample as accurately as possible.

An article entitled "Infra-Red Spectroscopy of Solids and Solutions" to be found at https://web. org/web/20140912182700if_/http://web. edu:<NUM>/marasing/ CHEM380/Labs/380PLABL/IRLAB. pdf describes an experiment for obtaining an IR spectrum from a sample in a demountable cell holder. Calibration, as a precursor to the use of a newly mounted cell, is described, using an interference fringe model.

<CIT> also describes a process for measurement of near infra-red spectra using a demountable NIR transmission cell. The process comprises: (a) measuring etalon fringes that arise when NIR light passes through the NIR cell in the absence of a liquid sample, (b) using this to calculate the pathlength of the NIR cell, (c) introducing the sample to be analysed into the NIR cell, and (d) measuring the NIR spectrum of the sample.

Infrared spectroscopy: fundamentals and applications by Barbara Stuart, describes, at a section titled "<NUM>. <NUM> Pathlength Calibration" also details the counting of interference fringes as a method for determining pathlength of a cell.

In two articles by <NPL>" and "<NPL>" the author(s) describe(s) the use of computer programs to determine optical constants (real and imaginary refractive indices) from transmission measurements of a sample.

The invention is a transmission spectroscopy device and method as defined in the appended claims.

In a first example, a method for determining a size of a cell of a transmission spectroscopy device includes the features according to independent claim <NUM>.

In a second example, a transmission spectroscopy device includes the features according to independent claim <NUM>.

Corresponding reference characters indicate corresponding parts throughout the several views. Elements in the drawings are not necessarily drawn to scale. The configurations shown in the drawings are merely examples, and should not be construed as necessarily limiting the scope of the claims.

In transmission spectroscopy, it is beneficial to know or measure the optical path length traversed by light in the sample as accurately as possible. For gaseous or liquid samples, a transmission spectroscopy device can use a cell or cuvette to contain the sample. The cell can include two opposing, transparent walls. During a measurement, light enters the cell through one of the transparent walls, passes through the sample, and exits the cell through the other of the transparent walls. The size of the cell defines the optical path length traversed by light in the sample. In some examples, the size of the cell can refer to a cell width, a cell length, or another suitable cell dimension. To ensure accuracy in the measurements, it is beneficial to calibrate the transmission spectroscopy device by measuring the size of the cell periodically or as needed.

To measure the size of the cell, the device performs a transmission spectroscopy measurement of a known substance, such as pure water, to produce a measured absorbance spectrum of the known substance. The device subtracts a known absorbance spectrum of the known substance from the measured absorbance spectrum to form an oscillatory fringe pattern. The fringes can be Fabry-Perot fringes, caused by interference from reflections from two parallel surfaces. The fringes can be present in the measured absorption spectrum, but masked by larger absorbance effects. Subtracting the known absorbance spectrum from the measured absorbance spectrum can remove or reduce the absorbance effects, and can emphasize the fringe pattern in the measured absorption spectrum. The device determines the size of the cell from a period of the fringe pattern. This is but a summary of a technique to measure the size of the cell; the technique is discussed below in more detail, following a description of a transmission spectroscopy device.

<FIG> shows a schematic drawing of an example of a transmission spectroscopy device <NUM>, in accordance with some embodiments. The transmission spectroscopy device <NUM> is but one example of a device that can use the technique described herein to measure the size of the cell; other devices can also be used.

A cell <NUM> has a first transparent wall <NUM> and a second transparent wall <NUM> opposing the first transparent wall <NUM>. The cell <NUM> is fillable with a sample to be measured. The sample is not part of the transmission spectroscopy device <NUM>. The cell is drainable to remove the sample and replace the sample with air. In some examples, the first transparent wall <NUM> and the second transparent wall <NUM> can be formed from glass, calcium fluoride, or other suitable materials selected to be insoluble with respect to the sample. In some examples, the first transparent wall <NUM> and the second transparent wall <NUM> can have a nominal separation between <NUM> and <NUM>.

In some examples, the separation between the first transparent wall <NUM> and the second transparent wall <NUM> can vary slightly over time. For example, the separation can increase slightly if the first transparent wall <NUM> and the second transparent wall <NUM> are eroded by continual use. Likewise, the separation can decrease slightly if samples cause a buildup of material on the first transparent wall <NUM> and the second transparent wall <NUM>. The technique described herein to measure the size of the cell <NUM> can track the cell size variation over time, and can allow the transmission spectroscopy device <NUM> to properly account for cell size and perform accurate measurements. The cell size measurement technique can be used as a calibration routine, which can be executed periodically, such as once a day, or once for every thousand measurements.

Plumbing <NUM> delivers the sample to the cell <NUM> and drains the sample from the cell <NUM>. The plumbing <NUM> can include one or more pipes, hoses, valves, pumps, and other elements can direct fluids or gases as needed. The plumbing <NUM> can include one or more connections to the cell <NUM>. In some examples, the plumbing <NUM> can direct the sample into or out of the transmission spectroscopy device <NUM>, and can connect to one or more elements outside the transmission spectroscopy device <NUM>. In some examples, the plumbing <NUM> can deliver air to the cell <NUM>. In some examples, the plumbing <NUM> can pump air to the cell <NUM>, to drain the cell of a sample.

A light source <NUM> can illuminate the sample through the first transparent wall <NUM>. In some examples, the light source <NUM> can be a broadband light source, such as a blackbody source, an incandescent source, one or more light emitting diodes, or other suitable light sources. In some examples, the light source <NUM> produces a collimated output beam. In some examples, the light source <NUM> includes one or more collimating lenses, which can collimate light from a diverging source, such as a light emitting diode, to produce a collimated output beam. In some examples, the light source <NUM> can emit light in the mid-infrared portion of the spectrum. In some examples, the light source <NUM> can emit light with wavelengths between <NUM> and <NUM>, or, equivalently, inverse wavelengths between <NUM>-<NUM> (corresponding to <NUM>) and <NUM>-<NUM> (corresponding to <NUM>). Other wavelength ranges can also be used, including the visible portion of the spectrum, the near-infrared portion of the spectrum, or the far infrared portion of the spectrum.

A detector <NUM> can detect light transmitted through the sample through the second transparent wall <NUM>. In some examples, the detector <NUM> is sensitive in the wavelength range emitted by the light source <NUM>.

A controller <NUM> operably controls the light source <NUM>, operably receives at least one signal from the detector <NUM>, and operably controls the plumbing <NUM>. During execution of the cell size measurement technique, the controller <NUM> fills the cell <NUM> with the sample, produces a measured absorbance spectrum of the sample, and drains the sample from the cell <NUM>. In some examples, the controller <NUM> can include a processor and memory, including instructions that, when executed on the at least processor, cause the processor to execute the technique to measure the size of the cell.

The transmission spectroscopy device <NUM> can further include a spectrometer <NUM> that can receive light emitted from the light source <NUM>, and can analyze the received light as a function of wavelength. In some examples, the spectrometer <NUM> can include a diffraction grating, a prism, or another optical element capable of spatially or angularly dispersing light as a function of wavelength. In some examples, in a configuration referred to as dispersive spectroscopy, the spectrometer <NUM> has a selectable output wavelength (or narrow band of wavelengths), which can be controlled by the controller <NUM>, and can vary over time. The controller <NUM> can correlate the wavelength output of the spectrometer, over time, with the signal received at the detector <NUM>, to measure a sample in the cell <NUM> as a function of wavelength. In other examples, the spectrometer can analyze the wavelength dependence of the transmitted light signal by generating an interferogram produced by the varying the optical path length of two interfering light beams. The Fourier transform of this interferogram results in a spectrum of intensity vs. wavelength (or energy, often in units of <NUM>/wavelength). This technique is referred to as Fourier Transform Spectroscopy, and has several advantages over dispersive spectroscopy in terms of optical throughput and wavelength multiplexing. Positioning the spectrometer <NUM> between the light source <NUM> and the cell <NUM> is but one configuration. Other configurations can also be used, such as positioning the spectrometer <NUM> between the cell <NUM> and the detector <NUM>, using a multi-element detector to simultaneously capture light at different wavelengths for the dispersive technique, and others. The spectrometer <NUM> can receive and output collimated light.

The transmission spectroscopy device <NUM> can further include a lens <NUM> that can focus light from the spectrometer <NUM> onto the cell <NUM>, and a lens <NUM> that can focus light from the cell onto the detector <NUM>. The lens <NUM> can allow the transmission spectroscopy device <NUM> to use a relatively small cell <NUM>, which can advantageously take measurements of samples having a relatively small volume. The lens <NUM> can allow the transmission spectroscopy device <NUM> to use a relatively small detector <NUM>, which can advantageously reduce noise associated with the detector <NUM> and can increase the speed of the detector <NUM>.

The transmission spectroscopy device <NUM> can further include a housing <NUM>, which can ensure that the cell <NUM> operates in an environment that is stable over time. For example, the housing <NUM> can control the temperature and pressure of the sample in the cell <NUM>. The housing <NUM> can maintain a constant composition of the gases, such as water vapor and carbon dioxide, around the cell <NUM> and the optical elements. The housing <NUM> can be sealed from the ambient environment. The housing <NUM> can be temperature-regulated. The housing <NUM> can include a desiccant to reduce water vapor to a relatively low and stable level. Stabilizing the environmental variables of temperature, pressure and the composition of the gases in the optical path can increase a precision of the measurements. Stabilizing these parameters can stabilize the optical alignment of the spectrometer, which can increase the precision of the measurements.

<FIG> shows a flowchart of an example of a method <NUM> for determining a size of a cell of a transmission spectroscopy device <NUM>, in accordance with some embodiments. The method <NUM> can be executed on a transmission spectroscopy device, such as device <NUM> (described above with reference to <FIG>), or on other suitable transmission spectroscopy devices. The method <NUM> is but one example of determining the size of the cell.

At operation <NUM>, the device <NUM> performs a transmission spectroscopy measurement of a known substance in a cell <NUM> to produce a measured absorbance spectrum of the known substance. The measured absorbance spectrum of the known substance is formed from a ratio of a first emittance scan of the cell <NUM> to a second emittance scan of the cell <NUM>. The first emittance scan is taken when the cell <NUM> is filled with the known substance. The second emittance scan is taken when the cell <NUM> is empty (e.g., filled with air).

At operation <NUM>, the device <NUM> subtracts a known absorbance spectrum of the known substance from the measured absorbance spectrum of the known substance to form a fringe pattern. In some examples, the known absorbance spectrum can be stored locally and can be access via a lookup table. In other examples, the known absorbance spectrum can be accessed remotely through a wired or wireless network. In some examples, the known absorbance spectrum can be obtained from an earlier transmission spectroscopy measurement of a particular sample (e.g., not necessarily a sample having a tabulated absorbance spectrum).

The fringe pattern is caused by interference between opposing first and second transparent walls <NUM>, <NUM> of the cell <NUM>. The fringe pattern peaks when there is constructive interference between the first and second transparent walls <NUM>, <NUM> of the cell <NUM>, which occurs when the roundtrip optical path between the first and second transparent walls <NUM>, <NUM> equals an integral number of wavelengths. In general, the fringe pattern is smaller in amplitude than the features in the measured absorbance spectrum, so that subtracting the known absorbance spectrum of the known substance from the measured absorbance spectrum can enhance the fringe pattern. The fringe pattern can be oscillatory in amplitude with respect to inverse wavelength. In some examples, the device <NUM> can scale an amplitude of one of the known absorbance spectrum or the measured absorbance spectrum to match an amplitude of the other of the known absorbance spectrum or the measured absorbance spectrum.

At operation <NUM>, the device <NUM> determines a size of the cell <NUM> from a period of the fringe pattern. In some examples, the device <NUM> can calculate a separation between opposing first and second transparent walls <NUM>, <NUM> of the cell <NUM> to equal <NUM> / (<NUM> × p), where quantity p is a period of the fringe pattern. In some examples, the device <NUM> can determine inverse wavelength values at which the fringe pattern peaks, and calculate the period to equal a separation between adjacent determined inverse wavelength values. In some examples, the device <NUM> can fit the determined inverse wavelength values to a linear fit, determine a slope of the linear fit, and set the period equal to the determined slope.

In some examples, performing the transmission spectroscopy measurement of the known substance in the cell <NUM> can include the following. The device <NUM> can fill the cell <NUM> with the known substance. The device <NUM> can illuminate the known substance through a first transparent wall <NUM> of the cell <NUM>. In some example, the illuminating light has a broad spectrum. The device <NUM> measures a first light from the known substance through a second transparent wall <NUM> of the cell <NUM>, opposite the first transparent wall <NUM>. The device <NUM> produces, from the first light, the first emittance scan. The device <NUM> can drain the known substance from the cell <NUM>. The device <NUM> can fill the cell <NUM> with air. The device <NUM> can illuminate the air-filled cell <NUM> through the first transparent wall <NUM> of the cell <NUM>. In some example, the illuminating light for the air-filled cell can have the same broad spectrum as the illuminating light used for the known substance. The device <NUM> measures a second light from the air-filled cell <NUM> through the second transparent wall <NUM> of the cell <NUM>. The device <NUM> produces, from the second light, the second emittance scan. In some configurations, the device <NUM> can measure the sample before measuring the air-filled cell; in other configurations, the device <NUM> can measure the sample after measuring the air-filled cell.

In some examples, draining the known substance from the cell <NUM> can include, repeatedly: pumping air through the cell <NUM>; illuminating the cell <NUM> through the first transparent wall <NUM> of the cell <NUM>; measuring a third light through the second transparent wall <NUM> of the cell <NUM>; and comparing the third light to a previous measurement of the third light, until a difference between successive measurements of the third light is below a threshold. As the cell <NUM> dries out, the amount of light passing through the cell reaches a constant level.

In some examples, the known substance can be pure water, although other known substances can also be used. For these examples, the broad spectrum of the illuminating light can include at least one peak or valley in the known absorbance spectrum of water. For example, in the near-infrared and mid-infrared wavelength ranges, liquid water has absorption bands around <NUM>-<NUM> (corresponding to a wavelength of <NUM>), <NUM>-<NUM> (<NUM>), <NUM>-<NUM> (<NUM>), and <NUM>-<NUM> (<NUM>). It is convenient to use values of inverse wavelength, rather than wavelength, because the fringes in the fringe pattern are equally spaced with respect to inverse wavelength.

<FIG> shows an example of a plot of a measured absorbance spectrum <NUM> of water, a known absorbance spectrum <NUM> of water, and a fringe pattern <NUM>, in accordance with some embodiments. The measured absorbance spectrum <NUM> and the known absorbance spectrum <NUM> overlap significantly; the element numbers <NUM> and <NUM> follow the respective plots in <FIG>. In practice, the fringe pattern <NUM> is present in the measured absorbance spectrum <NUM>, but with a significantly smaller amplitude than the absorption features. Subtracting the known absorbance spectrum <NUM> from the measured absorbance spectrum <NUM> of water (optionally with scaling of one or both absorbance spectra) can enhance the fringe pattern <NUM>. In this example, the fringe pattern extends from about <NUM>-<NUM> to about <NUM>-<NUM>, with a discontinuity in the fringe pattern around water's absorption band at <NUM>-<NUM>.

The period of the fringe pattern can be determined in many suitable manners, including Fourier transforming the fringe pattern and locating a peak in the Fourier transform, finding peaks in the fringe and determining separation between the peaks, finding valleys in the fringe pattern and determining separation between the valleys, finding zero-crossings in the fringe patterns and determining separation between the zero-crossings, and others.

One suitable way to calculate the period includes: determining inverse wavelength values at which the fringe pattern peaks; fitting the determined inverse wavelength values to a linear fit; determining a slope of the linear fit; and setting the period equal to the determined slope.

<FIG> shows an example of a plot of the inverse wavelength values at which the fringe pattern of <FIG> peaks, plotted along a linear scale by dimensionless peak number. The inverse wavelength values are fit to a linear fit, according to the equation: y [in cm-<NUM>] = <NUM>. 43x + <NUM>. The slope of the linear fit is <NUM>-<NUM>, which equals the period of the fringe pattern. In this numerical example, the separation between opposing first and second transparent walls <NUM>, <NUM> of the cell <NUM> equals <NUM> / (<NUM> × <NUM>-<NUM>), or <NUM>. This is but one example; other suitable numerical examples can also be used.

It is beneficial to consider a set of experimental data that shows the effectiveness of periodically measuring the size of the cell, as with the method <NUM>. Data was taken for a milk inspection system over the course of seven months. In this example, the milk inspection system inspected raw milk samples using the mid-infrared portion of the spectrum (e.g., with wavelengths between <NUM> and <NUM>). The system used chemometric calibration methods to relate the measured milk spectrum to a component concentration, for components, such as fat, protein, and solids. The system operated in a relatively demanding environment, running more than <NUM> samples per day, with relatively high accuracy, and relatively high potential for cell wear from the high sample volume. The system measured the samples with an accuracy relative standard deviation of less than <NUM>% and a repeatability relative standard deviation of less than <NUM>%. In this example, the inspection system tested a set of thirteen reference milk standards over a course of seven months. <FIG> shows a relative fat error, in percent, for measurements in which the cell width was periodically measured (<NUM>), and corresponding measurements in which the cell width was not periodically measured or updated (<NUM>). <FIG> shows a relative protein error, in percent, for measurements in which the cell width was periodically measured (<NUM>), and corresponding measurements in which the cell width was not periodically measured or updated (<NUM>). <FIG> shows a relative solids error, in percent, for measurements in which the cell width was periodically measured (<NUM>), and corresponding measurements in which the cell width was not periodically measured or updated (<NUM>). In each of <FIG>, the relative error grew with time when the cell width was not periodically measured or updated, but remained relatively small and relatively constant over time when the cell width was periodically measured and updated.

The method <NUM> for determining the size of the cell <NUM> of the transmission spectroscopy device <NUM> has significant advantages over other approaches for determining the size.

For example, a first approach of determining a cell size takes a measurement of a sample of a precisely known composition, measures its absorption at particular wavelengths, and determines the optical path length traversed in the sample from the Beer-Lambert law and from a tabulated molar absorptivity of the compound. This first approach is subject to errors caused by producing, maintaining and delivering samples of precisely known composition. The method <NUM> discussed herein is not subject to producing, maintaining and delivering samples of precisely known composition, as is required in the first approach.

As another example, a second approach of determining a cell size removes the cell from the device, measures the cell size externally using an interferometer or other suitable measurement device, then returns the cell to the device for future use. This second approach is time-consuming and disruptive, especially for devices that operate in a tightly controlled environment with regulated temperature, pressure, and humidity. The time lost for cell size measurements can be especially problematic for high-volume applications. The method <NUM> discussed herein can be performed in situ, with a relatively short times required for the measurement.

Claim 1:
A transmission spectroscopy device for performing plural consecutive sample measurements, the transmission spectroscopy device comprising:
a cell (<NUM>) having opposing first and second transparent walls (<NUM>,<NUM>), a separation of the first and second transparent walls defining a size of the cell, the cell being fillable with a sample to be measured, the cell being drainable to remove the sample and replace the sample with air;
plumbing (<NUM>) for delivering the sample to the cell (<NUM>) and for draining the sample from the cell; and
a controller (<NUM>) configured to operably control the plumbing (<NUM>) to fill the cell with the sample and to drain the sample from the cell,
the controller (<NUM>) further configured to produce a measured absorbance spectrum of the sample by operably controlling a light source (<NUM>) configured to provide illumination to the sample and operably receiving at least one signal from a detector (<NUM>) configured to detect light transmitted through the sample, thereby determining properties of the sample based on how much light emerges from the sample and on the size of the cell, and
to calibrate the transmission spectroscopy device, when the sample is a known substance sample of a known substance, by:
producing a measured absorbance spectrum of the known substance sample of the known substance, the measured absorbance spectrum of the known substance sample formed from a ratio of a first emittance scan of the cell (<NUM>) to a second emittance scan of the cell (<NUM>), the first emittance scan taken when the cell (<NUM>) is filled with the known substance sample, the second emittance scan taken when the cell (<NUM>) is air-filled;
subtracting a known absorbance spectrum of the known substance from the measured absorbance spectrum of the known substance sample to form a fringe pattern (<NUM>), the fringe pattern being oscillatory in amplitude with respect to inverse wavelength; and
determining the size of the cell (<NUM>) from a period of the fringe pattern;
wherein the controller (<NUM>) is further configured to produce a measured absorbance spectrum of the sample for plural consecutive samples, the size of the cell being updated through periodic calibration to account for variation in the size of the cell over time.