Patent Description:
This application further claims priority from <CIT>.

The present disclosure generally relates to spectrophotometer calibration methods and systems.

Spectrophotometers are instruments used for the analysis of samples to identify the presence of or determine the concentrations of materials or substances (e.g. molecules, elements, or compounds) of interest, i.e. the analyte. Spectrophotometers are configured to direct electromagnetic energy in the form of light in the ultraviolet (UV), visible or infrared (IR) range from a source to a sample. For example, a method of UV-visible spectroscopy exposes samples to light in the UV-visible range. By measuring the characteristics of the resultant light following interaction with the sample (for example, the light intensity and/or wavelengths of light transmitted, absorbed, scattered or emitted by the sample), a type of analyte or amount of analyte can be evaluated. For example, an amount of optical absorption associated with the sample can be related to an analyte concentration.

Flash lamps are often used as a light source for performing spectroscopy as they typically produce uniformly bright and spectrally broad light emissions. A flash lamp is a type of gas-filled electrical arc lamp comprising a pair of opposed discharge electrodes contained in an envelope of gas through which a current pulse is passed to create an electrical arc. During a period of electrical discharge, gas inside the volume of the arc is heated and ionized to create a plasma. Light emitted from the arc is a mixture of discrete emissions from exited atoms and ions and broadband emissions from the hot plasma between the electrodes.

<CIT> discloses a spectrometric instrument comprising a flashing light source for emitting a flashing source beam of light. <CIT> discloses a photometric system operable to determine certain characteristics of a fluid sample.

Spectrophotometers generally require calibration to confidently obtain accurate wavelength and spectral bandwidth measurements. Generally, a source different to the light source routinely employed in the spectrophotometer is used for calibration purposes. For example, a Mercury arc lamp is sometimes used for calibration due to the number and sharpness of emission lines it produces.

According to the claimed invention, there is provided a method of calibrating a spectrophotometer comprising a flash lamp, the method comprising:.

The range of wavelengths may be selected to be substantially centred about a wavelength associated with the self-absorption feature from the known spectral profile of the flash lamp.

In some embodiments, configuring the monochromator to progressively transmit the received light at each of a plurality wavelengths of a selected range of wavelengths comprises configuring the monochromator to scan at a relatively high resolution. For example, at least some of the plurality of wavelengths of the selected range may be spaced apart at a wavelength in the range of about <NUM> to about <NUM>.

Embodiments are described in further detail below, by way of example, with reference to the accompanying drawings briefly described below:.

Described embodiments relate to methods of calibrating spectrophotometers using flash lamps. It has been recognised by the inventors that in addition to showing emission lines, output spectral profiles of flash lamps also show self-absorption features (notches). These self-absorption features originate in the low lying energy levels of the atoms of the gas in the flash lamp and because the gas responsible for the absorption is much cooler than that responsible for the emission, the absorption lines are relatively narrow and are located within the emission profile of flash lamp. As flash lamps comprising particular gases have known spectral profiles, a comparison of the emission profile of the flash lamp, and in particular, the absorption features of the emission lines, with the known spectral profile for that particular type of flash lamp, the spectrophotometer can be calibrated.

By using flash lamps for calibrating spectrophotometers, the need for a different light source (for example, one that emits narrow spectral emission features at well-known spectral positions) to be used for calibration of the spectrophotometer than is used for performing spectroscopy is eliminated. In addition to performing a comparison of the flash lamp's spectral profile or measured spectra with segments of a predetermined reference spectrum, the approach of the invention considers the self-absorption features / notch of the flash lamp as an inverted peak and further evaluate the peak centre. Such an approach desirably does not require a previously identified or stored reference spectrum segment to compare against but instead utilizes knowledge of the flash lamp's self-absorption features / notch wavelength in spectral evaluation.

Referring to <FIG>, a spectrophotometer <NUM> is shown comprising a flash lamp <NUM>, such as a gas-filled electrical arc lamp, a monochromator <NUM>, at least one sample holder <NUM> and at least one detector <NUM>. The sample holder <NUM> is configured to receive and reproducibly position a sample cell (not shown) in a fixed location. The sample cell is configured to receive a sample to be analysed in the spectrophotometer <NUM>.

The flash lamp <NUM> is configured to produce and provide light <NUM>, such as broadband light, to the monochromator <NUM>. The monochromator <NUM> is configured to disperse or split the light into constituent wavelengths and to provide substantially monochromatic light <NUM> to the sample holder <NUM> to be detected by the detector <NUM>. The monochromatic light <NUM> may, for example, have a relatively narrow bandwidth in the range of <NUM> to <NUM>. During calibration, the sample holder <NUM> tends to be left empty, i.e., no sample is provided in the sample holder <NUM>.

In some embodiments, the spectrophotometer <NUM> and/or the monochromator <NUM> may be controlled by a computing device <NUM>. The computing device <NUM> may comprise a processor <NUM> and a storage medium <NUM>. The storage medium <NUM> may be a non-volatile storage medium such as a hard disk drive or a solid-state memory device. The processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to cause the computing device <NUM> to record the intensity of light <NUM> detected by detector <NUM>. The processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to cause the monochromator <NUM> to vary the wavelength of light being provided to the sample holder <NUM>.

The processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to progressively configure the monochromator <NUM> to cause the monochromator <NUM> to transmit light at each of a plurality of selected wavelengths within a range of wavelengths by progressively varying (scanning) the selected wavelength and to determine, detect and/or measure the respective light intensity or light power received at the detector <NUM> for each selected wavelength. The range of wavelengths is associated with a self-absorption feature according to a known spectral profile of the flash lamp. In some embodiments, the monochromator <NUM> is scanned at a relatively narrow range of wavelengths (high resolution) in order to accurately detect one or more of the self-absorption features in the spectral profile of the flash lamp <NUM>.

In some embodiments, the monochromator <NUM> may be configured to transmit light based on calibration parameters stored in the storage medium <NUM> of the computing device <NUM> or retrieved from an external database, for example. The calibration parameters may comprise instrument settings or configurations that when applied to the monochromator <NUM> cause the monochromator <NUM> to transmit light at the selected (predetermined) wavelengths. For example, the instrument settings may relate to orientations, rotation angles and/or positions of optical elements, such as a diffraction grating <NUM> (<FIG>), within the monochromator <NUM>.

The flash lamp <NUM> may be a relatively high output flash lamp. In some embodiments, the flash lamp <NUM> is capable of producing a pulsed light output with an energy of up to about <NUM> J per pulse (flash) at a rate of up to <NUM>. The flash lamp <NUM> may produce light over a range of wavelengths. In some embodiments, the flash lamp <NUM> may be configured to produce light over a select range of wavelengths between about <NUM> to about <NUM>. The flash lamp <NUM> may be a short-arc flash lamp comprising electrodes contained in pressurised Xenon gas. For example, the flash lamp <NUM> may comprise a <NUM> series FX-<NUM> short-arc flash lamp, such as that produced by Excelitas Technologies or a similar flash lamp produced by Hamamatsu.

The spectrophotometer <NUM> may further comprise a temperature sensor (not shown) connected to the processor <NUM> for measuring the temperature within the spectrophotometer <NUM>. As explained in further detail below, in some embodiments, the sensed temperature may be used in determining a calibration parameter for the spectrophotometer <NUM> and/or when calibrating the spectrophotometer <NUM>.

Referring to <FIG>, there is shown a monochromator <NUM> of the spectrophotometer <NUM>, according to some embodiments. As illustrated, the monochromator <NUM> may comprise a plurality of optical elements including a first mirror (collimation mirror) <NUM>, a wavelength dispersive element (such as a diffraction grating <NUM>) and a second mirror <NUM>. The monochromator <NUM> may also comprise an entry slit <NUM> between the flash lamp <NUM> (<FIG>) and the diffraction grating <NUM> for assisting in calibration and/or collimation of light from the flash lamp <NUM>.

The monochromator <NUM> may further comprise one or more drive components <NUM> coupled to one or more of the optical elements. The one or more drive components <NUM> may be configured to selectively adjust the orientation and/or rotation angle of the one or more optical elements. For example, drive component <NUM> may be coupled to the diffraction grating <NUM> and configured to selectively adjust the rotation angle of the diffraction grating <NUM>. The drive components <NUM> may comprise a direct drive component such as a stepper motor or DC servo motor. Alternatively, the drive components <NUM> may comprise an indirect drive component with a mechanical linkage such as a sine bar (lever arm) drive from a micro-meter or a pinion and sector gear.

By adjusting the rotation angle of the diffraction grating <NUM>, the monochromator <NUM> may be configured to transmit light at different selected wavelengths. In some embodiments, the processor <NUM> is configured to execute instructions to cause the one or more drive components <NUM> to adjust the rotation angle of the one or more optical elements to cause the monochromator to transmit light at select wavelengths. For each selected rotation angle of the one or more optical components, light at associated wavelengths is dispersed and each wavelength is focused by the second mirror (focusing mirror) <NUM> to a different location at a focal plane <NUM>.

In some embodiments, the processor <NUM> may be configured to select a set of rotation angles for the one or more optical elements, which may correspond to evenly spaced wavelength increments, to thereby select a range of evenly spaced wavelengths of light. By detecting the light at the focal plane with detector <NUM>, progressive variation of the rotation angle of the one or more optical elements results in progressive variation of the wavelength of light detected. Therefore, a plurality of detected light intensities each corresponding to selected wavelengths over a range of wavelengths can be recorded, for example in the storage medium <NUM>, in order to determine or measure a scanned light intensity spectrum.

In some embodiments, the rotation angle (θm) of the diffraction grating <NUM> is related to a selected wavelength (λ) according to the grating equation: <MAT>.

In some embodiments, the monochromator <NUM> further comprises an exit aperture <NUM> located at or adjacent the focal plane <NUM> to transmit substantially monochromatic light. For example, light may be transmitted at a relatively narrow bandwidth may be in the range of <NUM> to <NUM>. The exit aperture <NUM> may be adjustable. For example, the exit aperture <NUM> may comprise a slit or an iris to select the relatively narrow bandwidth of wavelengths transmitted. The monochromators may further comprise an entrance aperture (not shown) which may be adjustable. For example, the entrance aperture (not shown) may comprise a slit or an iris to select the relatively narrow bandwidth of wavelengths transmitted.

Referring to <FIG>, an example of an intensity spectrum <NUM> of a Xenon-filled flash lamp is shown. Spectrum <NUM> may be an output spectrum obtained using the spectrophotometer <NUM> fitted with a Xenon-filled flash lamp <NUM> for use as the light source and an empty sample holder <NUM>.

Spectrum <NUM> comprises a plurality of discrete emission peaks <NUM> and a broad continuum emission background. The discrete emission peaks <NUM> arise from transitions between discrete energy levels of the atomic gas and may be relatively sharp intensity features at relatively low plasma temperatures and pressures. The broad continuum emission arises from the heated plasma within the electrical-arc of the flash lamp <NUM>. The discrete emission peaks <NUM> are characteristic of the gas within the flash lamp <NUM>. For a flash lamp <NUM> with a known gas, such as Xenon, a plurality of emission peaks will be present at predetermined and well-known corresponding wavelengths. In principal, by comparing the wavelengths corresponding to the emission peaks <NUM> in the obtained spectrum <NUM> to the predetermined wavelengths of the characteristic emission peaks, as published in the literature, for example, the National Institute of Standards and Technology (NIST) Atomic Spectra Database (https://physics. gov/asd), the spectrophotometer <NUM> can be calibrated. In some cases, peak broadening and shifting induced by the electric arc and high discharge temperature may add uncertainty to their use in a calibration procedure. Thus, in some embodiments, the measured spectrum may be compared with a spectrum obtained from a low pressure light source to identify which peaks have shifted and by how much. Calibration methods are described in further detail below.

<FIG> also shows a relatively narrow range of wavelengths <NUM> which is a narrow range of wavelengths relative to a wide range <NUM> of wavelengths. As shown in <FIG>, the narrow range of wavelengths <NUM> may be a subset of wavelengths within the wide range <NUM>. However, in some embodiments, the narrow range <NUM> may comprise at least some wavelengths that are not within the wide range <NUM>. In some embodiments, the narrow range of wavelengths <NUM> may comprise wavelengths between about <NUM> to about <NUM>. In some embodiments, the wide range of wavelengths <NUM> may comprise wavelengths between about <NUM> to about <NUM>. In some embodiments, the wide range of wavelengths <NUM> may comprise wavelengths between about <NUM> to about <NUM>. In some embodiments, the wide range of wavelengths <NUM> may comprise wavelengths between about <NUM> to about <NUM>.

Referring to <FIG>, there is shown a spectrum <NUM>, which is a close-up view of the intensity spectrum <NUM> over the relatively narrow range of wavelengths <NUM>.

As shown, the intensity spectrum <NUM> comprises a self-absorption feature <NUM> at a corresponding predetermined wavelength (feature wavelength). The self-absorption feature <NUM> appears in the spectrum <NUM> as a narrow trough in intensity superimposed over a broadened emission peak <NUM>. For example, there may be gas that forms an envelope of cooler gas around the volume of hot gas heated by the electrical arc in flash lamp <NUM>, and the self-absorption features <NUM> may be present as a result of some of the emitted light from the hot gas in the flash lamp <NUM> being absorbed by cooler gases surrounding the hot gas. The light absorption leading to self-absorption is the reverse process to light emission described above and may also be due to electron transitions between energy levels of the gas in the flash lamp <NUM>.

The self-absorption features are seen primarily in transitions originating in the <NUM>st excited state of the Xe. This low lying excited state of the Xe atom is not radiatively coupled to the ground state due to Quantum Mechanics. The low lying excited state may be a metastable state. Energy deposited into the Xe atoms by the discharge leaves some of them in this metastable state. These metastable first excited state Xe atoms around the periphery of the discharge give rise to this self-absorption effect. These metastable state Xe atoms around the periphery of the discharge being at a much lower temperature than the rest of the arc absorb light from the discharge over a narrower absorption bandwidth than the emission bandwidth from the arc and at an emission line centre wavelength that is closer to the accepted published values. Thus, the self-absorption feature may be used to calibrate the spectrophotometer as discussed in more detail below.

These features will not necessarily be common to all atomic species used in electric arc flash lamp discharges but will depend on the configuration of the low-lying energy states of the atoms involved. However, any short arc noble gas flash lamp with transverse or axially aligned electrodes would be suitable. Noble gases are typically used in flash lamps because, being inert, they tend to provide longest life for the lamp. Due to the transitions originating in the first excited state of atomic Xe as described above, Xe is a suitable atomic species, and further provides good efficiency at a relatively low pressure. Argon and Krypton may also be used. In some embodiments, mixed-gas continuous short arc high power lamps may be used.

As shown in <FIG>, the spectral bandwidth <NUM> of the self-absorption feature <NUM> is narrower than the bandwidth <NUM> of the emission peak <NUM> that the self-absorption feature <NUM> is superimposed over.

Although methods for calibrating a spectrophotometer <NUM> are described with reference to example spectra <NUM>, <NUM>, the methods are not intended to be restricted to calibrating with the specific example spectra <NUM>, <NUM> and reference is only made to spectra <NUM>, <NUM> for illustrative and descriptive purposes.

Referring to <FIG>, there is shown a process flow-diagram for a method <NUM> of calibrating a spectrophotometer <NUM> comprising a flash lamp <NUM>, according to some embodiments. The processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to perform the method <NUM>.

Light <NUM> emitted from the flash lamp <NUM> comprising a known gas is received at a monochromator <NUM> of the optical spectrophotometer <NUM>, at <NUM>.

The monochromator <NUM> is caused or configured to progressively transmit light <NUM> at each of a plurality of wavelengths within or spanning a selected range of wavelengths of the light <NUM>, at <NUM>. The range of wavelengths is associated with a self-absorption wavelength feature <NUM>. In some embodiments, the monochromator is configured to transmit light at each of the plurality of wavelengths by progressively varying the orientation (rotation angle, θ) of at least one of the diffraction grating <NUM>. As explained above, the processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to cause the monochromator <NUM> to select each of the plurality of wavelengths to progressively vary the wavelength of transmitted light <NUM>.

The selected range of wavelengths is associated with a self-absorption feature <NUM> according to a known spectral profile of the flash lamp <NUM>, which is characteristic of the type of gas of the flash lamp <NUM>. For example, for a Xenon filled flash lamp, it is known that the wavelengths having corresponding self-absorption features may be any one or more of <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, for example. It will be appreciated however that for a Xenon filled flash lamp, other wavelengths may also have corresponding self-absorption features. The range of wavelengths may be selected such that it is substantially centred about a wavelength associated with the self-absorption feature from the known spectral profile of the flash lamp.

The selected range of wavelengths may be a relatively narrow range of wavelengths <NUM> selected from a relatively wider range of wavelengths <NUM> of the light <NUM> received from the flash lamp <NUM>. For example, for a Xenon filled flash lamp, the relatively narrow range of wavelengths <NUM> may comprise <NUM> to around <NUM>. In some embodiments, at least some of the plurality of wavelengths of the selected range are spaced apart at a wavelength in the range of about <NUM> to about <NUM> to allow for detection of the one or more absorption features at a sufficiently high resolution, and for example, in some embodiments, to assist with the determination of the full width half maximum (FWHM) of the self-absorption feature. In some embodiments, the plurality of wavelengths spanning the range of wavelengths vary by an interval in the range of about <NUM> to about <NUM>.

A spectrum or partial spectrum of the flash lamp, such as spectrum <NUM> for example, is scanned or determined, at <NUM>. For example, the spectrum may be determined by determining a power or intensity value corresponding to each of the plurality of wavelengths of the selected range of wavelengths. In some embodiments, the processor <NUM> of the computing device <NUM> may be configured to execute instructions to obtain data indicative of the power or intensity of light <NUM> detected at the detector <NUM> for each of the plurality of selected wavelengths. The determined spectrum may be indicative of the power or intensity of the detected light across (spanning) the relatively narrow range of wavelengths <NUM>. The data may be recorded on a storage device or in a storage medium <NUM> of the computing device <NUM>.

A wavelength calibration error value is determined by comparing the determined spectrum <NUM> with a segment of a predetermined reference spectrum (not shown) associated with the flash lamp <NUM>, at <NUM>.

Each of the <FIG> illustrate an example segment of the reference spectrum corresponding to a feature wavelength of the source spectrum plotted over nominal wavelengths of reference spectrum vs intensity, and a corresponding segment of measured light intensity values for the actual wavelengths diffracted by the second grating across an angular displacement centred about the nominal angular position.

The segment of the reference spectrum may span a range of wavelengths that includes or at least overlaps with the selected range of wavelengths. The predetermined reference spectrum or the segment of the predetermined reference spectrum may, for example, be stored on a non-volatile storage medium <NUM> of the computing device <NUM> or may be retrieved from remote storage. The predetermined reference spectrum or segment of the predetermined reference spectrum may have been determined by measuring intensity/power values for each of a plurality of wavelengths within a selected range and re-sampling/interpolating the measured values to correct the wavelength positions of the wavelength features (notches and peaks) using accurate published data, such as the documented emission/absorption lines recorded in the NIST Atomic Spectra Database. In some embodiments, the segment of the predetermined reference spectrum corresponds to a spectrum comprising the self-absorption feature for a Xenon filled short arc flash lamp. For example, the predetermined reference spectrum may include a self-absorption feature <NUM> at around <NUM> or <NUM> for Xenon gas (e.g. see <FIG>). A method <NUM> of determining the wavelength calibration error value according to some embodiments is depicted in the process flow diagram of <FIG>. The processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to perform the method <NUM>.

An initial offset value is determined, at <NUM>. The offset value may be indicative of an amount by which the determined wavelength is shifted to the left or right of the wavelength of the predetermined reference spectrum. The initial offset value may be a best guess at an amount by which the wavelength of the determined spectrum deviates from the predetermined reference spectrum. In some embodiments, an offset value range comprising an initial offset value and an end offset value, as well as an offset increment amount, is determined. By performing a spectrum matching process over a range from the initial offset value to the end offset value, it is expected that an offset value in the range can be determined at which the offset spectrum and the reference spectrum can be said to "match" (i.e. are most highly correlated).

The initial offset value is set to be just larger in magnitude than the worst case wavelength deviation expected. In some embodiments, the initial offset value may be determined by summing the worst case expected errors from the tolerances of the components used to construct the spectrophotometer, and the worst case expected errors from the spectrophotometer and alignment of those components, and calculate from that a worst case wavelength error for an uncalibrated spectrophotometer. For example, if an angular accuracy of the drive component <NUM>, the accuracy of the ruling of the diffraction grating <NUM>, and a mechanical tolerance stack-up over the spectrophotometer's operating temperature range combined indicate that an uncalibrated spectrophotometer could be up to <NUM> in error at <NUM>, then a suitable initial offset value may be -<NUM>, and a suitable end offset value may be +<NUM>, knowing that any spectrophotometer's actual wavelength calibration error must lie within this range.

An offset spectrum is determined from the determined scanned spectrum, at <NUM>. In some embodiments, the offset spectrum is determined by shifting the wavelengths of the determined scanned spectrum by the initial or current offset value. For example, as illustrated in <FIG>, the measured spectrum or the determined scanned spectrum <NUM> is offset from the reference spectrum <NUM> by an offset value. The relationship between the measured spectrum and the reference spectrum is further illustrated for feature wavelengths shown in <FIG>.

A correlation value indicative of correlation between the offset spectrum and the predetermined reference spectrum is determined, at <NUM>. In some embodiments, the correlation value is a measure of the linear correlation between the offset spectrum and the predetermined reference spectrum, such as a Pearson correlation coefficient. In some embodiments, offset values and associated correlation value are collated in a correlation list.

In one embodiment, a correlation value "K" between the offset spectrum and the reference spectrum is calculated using the Pearson correlation coefficient. The correlation value "K" is a value between -<NUM> and <NUM>, with a value <NUM> denoting perfect correlation. <FIG> is a graph of correlation values "K" for the range of expected offset values -<NUM> < λoffset<<NUM> corresponding to the feature wavelength spectrum segment shown in <FIG>. As illustrated in <FIG>, the correlation value at λoffset1 = -<NUM> is roughly - <NUM>.

Similarly, <FIG> illustrates correlation values "K" for the range of expected offset values corresponding to the feature wavelength spectrum segment shown in <FIG>; and <FIG> illustrates correlation values "K" for the range of expected offset values corresponding to the feature wavelength spectrum segment shown in <FIG>.

If a sufficient number of correlation values has not been determined, at <NUM>, for example, where a current offset value is less than the end offset value, the offset value is incremented by the offset increment amount at <NUM>, and the method reverts to <NUM> to determine further offset value and associated correlation value pairs.

If a sufficient number of correlation values has been determined, at <NUM>, for example, in that a current offset value is not less than the end offset value, a plurality of the offset value and correlation value pairs are fit to a representative curve, at <NUM>. For example, the representative curve may be a least squares quadratic curve based on the equation below and the least squares quadratic curve as shown in <FIG>: <MAT>.

In some embodiments, only a subset of the determined offset value and correlation value pairs are fit to the representative curve. In some embodiments, a maximum correlation value of the determined correlation values is identified and the subset of offset value and correlation value pairs is centred on the maximum correlation value. For example, the subset may comprise <NUM> offset value and correlation value pairs, two on either side of the offset value and maximum correlation value pair. In some embodiments, if the maximum correlation is less than a threshold amount, such as <NUM>, for example, the method <NUM> may terminate with an error. Similarly, in some embodiments, if either of the first or last two of the offset value and correlation value pairs includes the identified maximum correlation value, the method <NUM> may terminate with an error.

Parameter values for the representative curve are determined from the best fit of the offset value and correlation value pairs to the representative curve, at <NUM>. For example, the determined parameters may be A and B of the above quadratic equation.

Determine wavelength deviation value is determined from determined parameters of the equation for the representative curve, at <NUM>. For example, the wavelength deviation value may be determined from the following equation: <MAT>.

Referring again to <FIG>, the monochromator <NUM> is caused or configured to progressively transmit light <NUM> at each of a second or further plurality of wavelengths within or spanning a selected second or further range of wavelengths of the light <NUM>. The second or further range of wavelengths is associated with a further wavelength feature such as a second or further self-absorption feature (not shown) or an emission peak <NUM>, at <NUM>. In some embodiments, the monochromator <NUM> is configured to transmit light at each of the plurality of wavelengths by progressively varying the orientation (rotation angle, θ) of the diffraction grating <NUM>. As explained above, the processor <NUM> may be configured to execute instructions (program code) stored in the storage medium <NUM> to cause the monochromator <NUM> to select each of the plurality of wavelengths to progressively vary the wavelength of transmitted light <NUM>.

The selected second or further range of wavelengths is associated with a further wavelength feature, such as a second or further self-absorption feature or an emission peak <NUM>, according to a known spectral profile of the flash lamp <NUM>, which is characteristic of the type of gas of the flash lamp <NUM>. The selected second range of wavelengths may be a relatively narrow range of wavelengths selected from a relatively wider range of wavelengths of the light <NUM> received from the flash lamp <NUM>. The size of the selected range <NUM> may depend on the specific known wavelength feature of the predetermined spectrum <NUM>. For example, a broad wavelength feature may require a broad range of wavelengths <NUM>. In some embodiments, at least some of the second or further plurality of wavelengths of the selected range are spaced apart from neighbouring wavelengths at about <NUM> to about <NUM>. In some embodiments, the plurality of wavelengths spanning the range of wavelengths vary by an interval in the range of about <NUM> to about <NUM>.

A further spectrum of the flash lamp is scanned or determined, at <NUM>. In some embodiment, the further spectrum is determined by determining or measuring power or intensity value for each of the second or further plurality of wavelengths. For example, the processor <NUM> of the computing device <NUM> may be configured to execute instructions to obtain data indicative of the power or intensity of light <NUM> detected at the detector <NUM> for each of the second or further plurality of selected wavelengths. For example, the second or further spectrum may be indicative of the intensity of the detected light across (spanning) the relatively narrow second range of wavelengths. The data may be recorded on a storage device or in a storage medium <NUM> of the computing device <NUM>.

A further wavelength calibration error value is determined by comparing the determined further spectrum <NUM> with a relevant segment of a predetermined reference spectrum (not shown) associated with the flash lamp <NUM>, at <NUM>.

In some embodiments, if wavelength calibration errors for a sufficient number of wavelength features have not been determined, the method <NUM> reverts to <NUM>, to determine a wavelength calibration error for a further wavelength feature. If wavelength calibration errors for a sufficient number of wavelength features (wavelength and wavelength calibration error pairs) has been determined, the method <NUM> proceeds to <NUM>. A sufficient number of wavelength calibration errors may be a number greater than a threshold value. For example, the threshold value may depend on the design and/or requirements of the spectrophotometer.

For example, a number of calibration error values necessary used to fit a representative error curve may depend on the calibration accuracy required, the forms of errors that are being modeled and corrected for, and/or a size of other error sources that are not being modeled. In some embodiments, a primary error being corrected for may relate to an eccentricity of mechanical placement of an encoder pattern with respect to a grating rotation shaft of the spectrophotometer. Typically, a form of this type of error is sinusoidal, but over a range of angles used, it may be adequately modeled by a simple parabola. In such a case, three calibration error values may be sufficient to define the correction. However, there may also be a secondary source of error in cyclic interpolation errors on the encoders, which may be treated as a source of random error. Lamp flash noise and/or measurement noise may also be treated as a source of random error. In some embodiments, to mitigate unmodelled errors arising from cyclic interpolation errors, flash lamp noise and/or measurement noise, for example, contributing some excessive errors at wavelengths at a relatively long distance from three calibration error values, more than three calibration error values may be determined for fitting to the representative curve at <NUM>. For example, in some embodiments, a zero order peak and eleven further feature wavelengths are used, which may be selected at relatively rough uniform spacing to constrain the fitted parabola and limit errors arising from small random errors. In some embodiments, conventional software modeling and Monte-Carlo simulation processing may be employed to determine a suitable number of wavelength features (wavelength and wavelength calibration error pairs).

The wavelength and wavelength calibration error pairs determined at <NUM> and <NUM> (which may be collated as a wavelength calibration error list) are fit to a representative curve as shown in <FIG>. In some embodiments, the wavelength and wavelength calibration error pairs are fit to a least squares quadratic curve with zero offset, such as: <MAT>.

In one embodiment, an array (λ, δλ) is populated with a wavelength value λ for each feature wavelength and a corresponding wavelength deviation/difference/error δλ (correlation value) associated with the feature wavelength. A least squares quadratic curve with the (<NUM>, <NUM>) offset is fitted to the (λ, δλ) array as shown in <FIG>. As shown in <FIG>, the scatter plot illustrates the values of array (λ, δλ) and the line graph is the curve of best fit.

Parameter values, such as E and F, are determined from a best fit of the curve to the wavelength and wavelength calibration error pairs, at <NUM>. Curve fitting to determine the parameter values may comprise optimising the parameters values to minimise the error.

A wavelength calibration error value for any particular wavelength is determined from the representative equation using the determined parameter values, at <NUM>.

A rotation angle adjustment value for the diffraction grating <NUM> is determined based on the wavelength calibration error value, at <NUM>. For example, the rotation angle adjustment value may be determined using the grating equation discussed above. In some embodiments, the rotation angle adjustment value may be further dependent on the temperature within the spectrophotometer <NUM>. For example, the rotation angle adjustment value for the diffraction grating <NUM> may be further based on the temperature determined from the temperature sensor provided within the spectrophotometer <NUM>.

The rotation angle of the diffraction grating <NUM> of the monochromator is adjusted by the rotation angle adjustment value, at <NUM>, to thereby calibrate the spectrometer to cause the monochromator to transmit light at the calibrated wavelength. For example, the processor <NUM> of the computing device <NUM> may be configured to execute instructions to adjust the diffraction grating <NUM> of the monochromator by the rotation angle adjustment.

The implementation of the aforementioned calibration method in a double monochromator is described in <CIT> (the contents of which are incorporated herein by reference). As described with reference to Figures <NUM> to 13c of <CIT>, each of the monochromators is calibrated by first determining the respective zero order angles. Once the respective zero order angles are determined, each of the monochromators is calibrated using the aforementioned calibration method whilst the other monochromator is set to its zero order position.

In some embodiments, the calibration method can also be used to calibrate monochromators including two or more dispersion elements, wherein each dispersion element may include one or more diffraction gratings and/or one or more prisms in any suitable configuration.

Whilst the above example embodiments have been described with reference to spectrophotometers, a person skilled in the art would also understand that the calibration method can also be used to calibrate monochromators in different optical apparatus such as telescopes, colour measuring instruments and medical apparatus.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments.

Claim 1:
A method (<NUM>) of calibrating a spectrophotometer (<NUM>) comprising a flash lamp (<NUM>), the method (<NUM>) comprising:
receiving light (<NUM>) from the flash lamp (<NUM>) at a monochromator (<NUM>) of the spectrometer, wherein the flash lamp (<NUM>) is a short arc noble gas flash lamp with transverse or axially aligned electrodes;
configuring the monochromator (<NUM>) to progressively transmit the received light (<NUM>) at each of a plurality wavelengths of a selected range of wavelengths, wherein the range of wavelengths is associated with a wavelength feature according to a known spectral profile of the flash lamp (<NUM>), and wherein the wavelength feature is a self-absorption feature (<NUM>);
determining a spectrum of the flash lamp, wherein the spectrum comprises a corresponding power or intensity value for each of the plurality of wavelengths;
considering the self-absorption feature of the determined spectrum as an inverted peak;
evaluating a peak center of the inverted peak; and
calibrating the spectrophotometer based on the evaluated peak center.