U.S. patent application number 14/728818 was filed with the patent office on 2016-12-08 for methods for collection, dark correction, and reporting of spectra from array detector spectrometers.
The applicant listed for this patent is Kaiser Optical Systems Inc.. Invention is credited to Francis Esmonde-White, Joseph B. Slater, James M. Tedesco, Patrick Wiegand.
Application Number | 20160356646 14/728818 |
Document ID | / |
Family ID | 57452530 |
Filed Date | 2016-12-08 |
United States Patent
Application |
20160356646 |
Kind Code |
A1 |
Wiegand; Patrick ; et
al. |
December 8, 2016 |
METHODS FOR COLLECTION, DARK CORRECTION, AND REPORTING OF SPECTRA
FROM ARRAY DETECTOR SPECTROMETERS
Abstract
Methods and systems for spectrometer dark correction are
described which achieve more stable baselines, especially towards
the edges where intensity correction magnifies any non-zero results
of dark subtraction, and changes in dark current due to changes in
temperature of the camera window frame are typically more
pronounced. The resulting induced curvature of the baseline makes
quantitation difficult in these regions. Use of the invention may
provide metrics for the identification of system failure states
such as loss of camera vacuum seal, drift in the temperature
stabilization, and light leaks. In system aspects of the invention,
a processor receives signals from a light detector in the
spectrometer and executes software programs to calculate spectral
responses, sum or average results, and perform other operations
necessary to carry out the disclosed methods. In most preferred
embodiments, the light signals received from a sample are used for
Raman analysis.
Inventors: |
Wiegand; Patrick; (Pinckney,
MI) ; Tedesco; James M.; (Livonia, MI) ;
Slater; Joseph B.; (Dexter, MI) ; Esmonde-White;
Francis; (Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kaiser Optical Systems Inc. |
Ann Arbor |
MI |
US |
|
|
Family ID: |
57452530 |
Appl. No.: |
14/728818 |
Filed: |
June 2, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J 2003/2866 20130101;
G01J 3/0286 20130101; G01J 3/44 20130101; G01J 3/027 20130101; G01J
3/28 20130101 |
International
Class: |
G01J 3/02 20060101
G01J003/02; G01J 3/44 20060101 G01J003/44 |
Claims
1. A method of dark current correction in a spectrometer having a
detector adapted to receive a light spectrum from a sample, the
method comprising the steps of: (a) acquiring a dark exposure using
the detector; (b) acquiring a light exposure from a sample; (c)
subtracting the dark exposure from the light exposures, storing the
result as an accumulation; (d) repeating (a)-(c) for N
accumulations; and (e) summing or averaging the N accumulations to
generate a result.
2. The method of claim 1, wherein the spectra acquired in (a) and
(b) are cosmically corrected.
3. The method of claim 1, wherein the spectrometer is a Raman
spectrometer, and the light spectrum from the sample includes a
Raman spectrum.
4. The method of claim 1, including the steps of: (a) acquiring a
dark exposure using the detector; (b) acquiring a light exposure
from a sample; (c) subtracting the dark exposure from the light
exposures, storing the result in a buffer as an accumulation; (d)
repeating (a)-(c) for N accumulations; (e) summing or averaging the
N accumulations to generate a result; (f) deleting the oldest
element in the buffer; and (g) repeating steps (a) through (f)
until a desired number of spectra have been collected.
5. The method of claim 4, wherein the spectrometer is a Raman
spectrometer, and the light spectrum from the sample includes a
Raman spectrum.
6. A method of dark current correction in a spectrometer having a
detector with a illuminated area adapted to receive light from a
sample and a non-illuminated area, the method comprising the steps
of: (a) receiving and storing data representative of a true dark
(TD) spectrum using the illuminated area of the detector in the
absence of light from the sample, and receiving and storing data
representative of an unilluminated dark (UD) spectrum using the
non-illuminated area of the detector in the presence of light from
the sample; (b) deriving a mathematical relationship between TD and
UD; (c) simultaneously receiving and storing data representative of
a light spectrum from a sample using the illuminated area of the
detector, and receiving and a new unilluminated dark (new UD)
spectrum using the non-illuminated area of the detector; (d)
calculating a new TD as a function of new UD; (e) subtracting new
TD from the data representative of the light spectrum to generate a
result; and (f) repeating steps (c) through (e) until a desired
number of spectra have been collected.
7. The method of claim 6, wherein the spectrometer is a Raman
spectrometer, and the light spectrum from the sample includes a
Raman spectrum.
8. The method of claim 6, including the steps of: storing the
result of step (e) in a buffer; repeating steps (c) through (e)
until the buffer is full, then: (g) summing or averaging the
elements of the buffer; (h) generating a result; (i) deleting the
oldest element in the buffer; and (j) repeating steps (c) through
(i) until a desired number of spectra have been collected.
9. The method of claim 8, wherein the spectrometer is a Raman
spectrometer, and the light spectrum from the sample includes a
Raman spectrum.
10. A method of dark current correction in a spectrometer having a
detector adapted to receive light from a sample, the method
comprising the steps of: (a) receiving and storing data
representative of a dark spectrum acquired by the detector at one
or more detector states and detector parameters; (b) calculating
and storing a detector-specific model based on the detector states
and detector parameters; (c) receiving data representative of a
sample spectrum, and storing the data along with one or more
detector states and detector parameters; (d) calculating a dark
spectral response based on the model stored in (b); (e) subtracting
the calculated dark spectral response from the sample spectrum; (f)
repeating steps (c)-(e) to generate N accumulations, then: (h)
summing/averaging the N accumulations to produce a result.
11. The method of claim 10, wherein the spectrometer is a Raman
spectrometer, and the light spectrum from the sample includes a
Raman spectrum.
12. The method of claim 6, wherein associated with the detector
states or detector parameters includes temperature or exposure
time.
13. A method of dark current correction in a spectrometer having a
detector adapted to receive light from a sample, the method
comprising the steps of: (a) receiving and storing data
representative of a dark spectrum acquired by the detector at one
or more detector states and detector parameters; (b) calculating
and storing a detector-specific model based on the detector states
and detector parameters; (c) receiving data representative of a
sample spectrum, and storing the data along with one or more
detector states and detector parameters; (d) collecting an
unilluminated dark (UD) spectrum using a non-collection area of the
detector; (e) establishing SMUD as a function of UD; (f) receiving
data representative of a sample spectrum, and storing the data
along with one or more detector states and detector parameters; (g)
calculating a dark spectral response based on SMUD; (h) comparing
the modeled dark and SMUD; (i) correcting the dark spectral
response using the modeled dark and SMUD; (j) repeating steps
(f)-(i) to generate N accumulations, then: (k) summing/averaging
the N accumulations to produce a result.
14. The method of claim 13, wherein the spectrometer is a Raman
spectrometer, and the light spectrum from the sample includes a
Raman spectrum.
15. The method of claim 13, wherein associated with the detector
states or detector parameters includes temperature or exposure
time.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to spectrometers and, in
particular, to methods for collection, dark correction and
reporting from such instruments.
BACKGROUND OF THE INVENTION
[0002] Electronic light-recording devices such as charge-coupled
display (CCD) cameras, single element arrays, as found in InGaAs
cameras, and so forth, have a dark response (i.e., a signal in the
absence of light) which must be corrected. Normally this involves
taking an exposure cycle in the absence of light from the sample to
be measured and storing it as a "dark spectrum." Light from the
sample is then passed to the camera for an identical exposure cycle
to generate an "uncorrected sample spectrum." A "corrected" sample
spectrum is then computed by subtracting the dark spectrum from the
uncorrected sample spectrum. (Other forms of correction are then
computed to correct for the spectral responsivity of the detectors,
the spectral mapping of the array, interpolation, etc., but these
are separate subjects outside the scope of this disclosure.) As the
time between collection of the dark and the collection of light
becomes larger, the dark data may not match the true camera
response in the absence of light due to temperature fluctuation or
other reasons.
[0003] If the dark spectrum is updated prior to each light exposure
cycle, this essentially doubles the amount of time required for a
total data collection cycle. Further, when analyzing spectra that
contain both very weak and very strong spectral components of
interest, the exposure cycle time required for adequate SNR
(signal-to-noise)/quantitation on the very weak components, such as
in analysis of gas mixtures by Raman spectroscopy, can be very
long--on the order of several minutes. Stronger components in the
same mixture may be accurately quantitated in a matter of
seconds.
[0004] Previous attempts to solve the dark correction problem
either are inefficient in the amount of time required, or
inaccurate in matching the true dark response at the time of light
collection. Existing techniques either collect one dark spectrum
and apply it to all future spectra in an experiment or monitoring
process, or collect a new dark spectrum before each signal
spectrum.
Standard Practice 1
[0005] FIG. 1 illustrates current standard practice involving a
single stored dark spectrum. A collection cycle consists of N
accumulations of single exposures, each within the dynamic range of
the array detector, summed or averaged to achieve a target SNR for
the most difficult (typically the weakest) spectral feature in
application. The resulting sum or average will henceforth be
referred to as a spectrum A dark exposure is acquired with signal
light blocked, thus acquiring one dark exposure at 102. A second
dark exposure is acquired at 104 for cosmic event correction, and
this process is repeated by summing or averaging the cosmic
corrected exposures at 108 for N accumulations (106). I accordance
with this disclosure, including the embodiments described here,
"cosmic correction" should be taken to mean combining two (or more)
exposures in such a way as to eliminate pixel signal if one of the
exposures show evidence of cosmic ray spikes, while averaging the
pixels from the both exposures if neither has a cosmic-ray-induced
spike. Further, "N" is typically determined by the ratio of the
strongest feature in the spectrum to the weakest feature, such that
each of the N accumulations is sufficiently short to avoid detector
saturation at the strongest feature, and the total exposure time T
over N accumulations provides the required SNR for the weakest
component.
[0006] The resulting dark spectrum is saved at 110 and subtracted
at 112 from all subsequent signal collection spectra acquired in
the same manner, but with signal light illuminating the detectors.
The result is output at 114. This approach may comprise a standard
practice for sufficiently stable dark current, which can be the
case for very stable dark current, typically characterized by very
stable thermal environments for both detector and spectrograph
hardware. It can also be the case for applications with very strong
signals relative to dark current. The cycle time for data within a
run is the shortest possible because once the single dark spectrum
is acquired, signal data is being acquired at all times. Total data
reporting cycle time for the method of FIG. 1 is T, as dictated by
the weakest component of interest.
Standard Practice 2
[0007] FIG. 2 illustrates an alternative standard practice
involving interleaved dark spectra. A dark spectrum is acquired at
202, followed by the acquisition of signal spectra at 204. The dark
signal is subtracted from the light spectrum at 206. A new dark
spectrum is acquired over N accumulations as described above in
between each signal cycle of N accumulations. This allows the
instrument to correct for changes in dark current over the course
of a data run. However, it doubles the data cycle time relative to
Standard Practice 1, because half of the time is spent acquiring
dark spectra, not signal. Thus, total data reporting cycle time is
2T.
SUMMARY OF THE INVENTION
[0008] This invention is directed to a system and method of dark
current correction in a spectrometer having a detector adapted to
receive light from a sample. The overall goal is to provide for
efficient dark correction while keeping the total data collection
cycle to a minimum. The various embodiments also enable more rapid
reporting of data than that which would normally be dictated by
accurate quantitation of the weakest signal of interest.
[0009] The invention affords better matching of dark subtraction to
the true dark when light data is acquired. This results in more
stable baselines, especially towards the edges where intensity
correction magnifies any non-zero results of dark subtraction, and
changes in dark current due to changes in temperature of the camera
window frame are typically more pronounced. The resulting induced
curvature of the baseline makes quantitation difficult in these
regions.
[0010] One disclosed method allows dark data to be pre-calibrated
during extended periods of time to improve the accuracy and reduce
noise, then these calibrations can be used at any point in the
future without incurring an increased measurement time. Alternative
methods provide additional metrics for the identification of system
failure states such as loss of camera vacuum seal, drift in the
temperature stabilization, and light leaks.
[0011] In system aspects of the invention, a processor receives
signals from a light detector in the spectrometer and executes
software programs to calculate spectral responses, sum or average
results, and perform other operations necessary to carry out the
disclosed methods. In most preferred embodiments, the light signals
received from a sample are used for Raman analysis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a diagram that illustrates a current standard
practice involving a single stored dark spectrum;
[0013] FIG. 2 illustrates an alternative standard practice
involving interleaved dark spectra;
[0014] FIG. 3 illustrates an interleaved dark exposure method
according to the invention;
[0015] FIG. 4 illustrates a rolling collection method according to
the invention;
[0016] FIG. 5 illustrates a scaled dark collection method according
to the invention;
[0017] FIG. 6 illustrates an embodiment of the invention that
combines the Methods depicted in FIGS. 4 and 5;
[0018] FIG. 7 illustrates a calculated full spectrum dark
correction method according to the invention; and
[0019] FIG. 8 illustrates an embodiment of the invention that
combines the Methods depicted in FIGS. 5 and 7.
DETAILED DESCRIPTION OF THE INVENTION
Method 1--Interleaved Dark Exposure
[0020] In accordance with this embodiment of the invention,
diagrammed in FIG. 3, a collection cycle comprises dark exposure
302, light exposure 304, repeat dark for cosmic correction check
306, repeat light for cosmic correction check 308, and generate one
accumulation by subtracting the cosmic-corrected dark exposure from
the cosmic-corrected light exposure (310). These steps are repeated
N times through decision block 312 for each accumulation. At 314
the accumulations are summed or averaged to build up the target SNR
for the application.
[0021] This improvement doubles the fastest possible cycle time and
better matches the true dark for light collection periods to the
stored dark as compared to Standard Practice 2. This can be
significant in applications where dark current can drift
significantly within a long single data cycle of N accumulations.
Data reporting cycle time is 2T, equivalent to Standard Practice 2,
but provides more accurate tracking of dark current drift than
Standard Practice 2.
Method 2--Rolling Collection
[0022] In the embodiment of FIG. 4, the acquisition cycle includes
performing Interleaved Dark Exposures of FIG. 3 for each
accumulation, storing each of the N accumulations in a buffer. Two
dark exposures are collected and cosmically corrected at 402; two
light exposures are collected and cosmically corrected at 404; with
the difference 406 being stored in a buffer at 408. When desired
number of exposures N has occurred (at 410), a sum or average is
taken at 412 a first collect cycle spectrum result is returned at
414, and the oldest buffer element is deleted. Steps 402-408 are
repeated through 416, and the result of each corrected exposure is
added to the buffer as newest buffer element. Another collect cycle
spectrum is returned which incorporates all buffer elements
including the newest one.
[0023] This process is repeated as a rolling sum or average
spectrum delivery until a sufficient number of spectra has been
achieved via block 418, in which case the process quits at 420.
Although full reaction to a step change in signal level is similar
to Standard practice 2, data reporting to indicate the onset of a
signal change is actually faster than the cycle time of Standard
Practice 1, returning a new spectrum with the target SNR on every
accumulation, instead of every N accumulations. The data reporting
cycle time is now 2T/N (except for the first spectrum which would
be delivered after 2T).
Method 3--Dynamically Modeled Dark Collection
[0024] In accordance with the embodiment of FIG. 5, a dark is
collected at the beginning of an experiment at 502 using the entire
data collection cycle and stored as in Standard Practice 1. This
dark uses the same region of the camera as light collection, but
with no light entering the camera, and will be referred to as the
true dark (TD). Subsequently, a second dark is collected at 504
using regions of the camera not normally illuminated during signal
collection, such as in between signal fiber images on a
2-dimensional CCD array, or non-illuminated regions of a linear
array detector. This dark is collected with light entering the
camera and will be referred to as the unilluminated dark (UD).
[0025] At 506, a relationship is developed dynamically between the
TD and UD, indicated as TD=fn(UD). In some situations the functions
may simply be a multiplication by constant. A light collection
cycle is then started at 508. Simultaneously, another UD is
collected at 510 using detector regions not illuminated by signal
light. Using the previously developed relationship between the TD
and the UD, a new TD is calculated at 512 using the monitored UD
signal. The calculated TD is then subtracted from the signal
exposure at 514. The result at 516 should closely match the signal
corrected by true dark during light collection. No additional
exposure time is required.
[0026] Data reporting cycle time after initial dark collection is
T, which is equivalent to Standard Practice 1. Drifting dark
current is now corrected, although not as accurately as with the
Rolling Collection approach. If the dark current drift is
reasonably consistent across the detector array, this can provide
sufficiently accurate correction. A new relationship between the TD
and the UD is developed each time the experimental parameters (such
as time of exposure or detector temperature) change. No additional
inputs to the function relating TD and UD are necessary other than
the UD.
[0027] Note that in this method, the initial dark may be taken for
a subset of total accumulations to save start-up time, but this
would compromise SNR. Also, the UD does not have to be a contiguous
stripe across the camera but can in fact be any collection of
unilluminated pixels.
Method 4--Combination of Methods 2, 3
[0028] The approach of FIG. 6 essentially combines the improved
Methods 2 and 3 (FIGS. 4 and 5). The technique represents a rolling
collection of both signal and dynamically modeled TD correction,
reporting data on every signal accumulation without interleaved
dark collections. Blocks 602, 604 and 606 are equivalent to the
initialization cycle of FIG. 5, and blocks 608, 610, 612
representing the collection cycle. At 614 the dark is subtracted
from the signal and the result being stored in a buffer at 616. As
with the process of FIG. 4, when desired number of accumulations N
has occurred (at 618), a sum or average is taken at 620, a first
collect cycle spectrum result is returned at 622, and the oldest
buffer element is deleted at 624. Steps 608-624 are repeated
through 626, and the result of each accumulation is added to the
buffer as newest buffer element. Another collect cycle spectrum is
returned which incorporates all buffer elements including the
newest one. The data reporting cycle time is T/N--Twice the speed
of Rolling Collection Method 2.
Method 5--Statically Modeled Dark Correction
[0029] For cameras with a consistent dark current vs. detector
temperature characteristic, the complete dark spectrum response to
relevant parameters, such as integration time and detector array
temperature, can be measured over the entire array and stored once
in advance at select intervals within the expected operational
ranges. These parameters can then be measured during operation, and
the expected operational dark signal calculated via interpolation
of the stored data. This provides the advantage of low noise dark
current subtraction, with the operational simplicity of Standard
Practice 1, although a new static model would have to be developed
for each instrument at the time of manufacture or
refurbishment.
[0030] The technique is diagrammed in FIG. 7. At 702, dark response
is measured at various detector states. At 704, dark response is
measured in conjunction with various detector parameters such as
different temperatures, exposure time(s), and so forth. The
responses acquired at 702, 704 are stored at 706 as a specific
model for that particular detector.
[0031] The collection cycle begins at 710, wherein the signal
spectrum is collected along with the state and parametric
information derived at 702, 704. This allows the dark spectrum to
be calculated using the stored model at 712. The calculated dark is
subtracted from the signal at 714 and this is repeated N times via
716. The corrected signal exposures are summed or averaged at 718
and the result delivered at 720. Data reporting cycle time can be
either T or TIN, depending on the incorporation of the rolling
average method described in Method 4.
Method 6--Scale-Enhanced Statically Modeled Dark Correction
[0032] The embodiment of the invention shown in FIG. 8 represents a
combination of Methods 3 and 5. As in Method 5, a functional
relationship is developed at 806 between true dark (TD) and
relevant operational parameters (e.g., integration time, array
temperature). In addition, another functional relationship is
developed at 810 between unilluminated dark at 808 (where light is
entering the camera but not falling on the UD regions) and the
operational parameters used in the first functional relationship.
This will be referred to as statically modeled unilluminated dark
(SMUD). As in Method 5, these functional relationships would be
developed at the time of instrument manufacture or refurbishment
and used for all future correction of exposures where signal light
is illuminating the detector regions.
[0033] In this embodiment, however, the statically modeled dark
correction is supplemented with a scaled dark correction factor
determined from the difference between the actual UD that is
measured and the UD that is predicted from the statically modeled
unilluminated dark. This accounts for camera instability or other
operational variables not accounted for in the implementation of
Method 5. This process includes statistical measures to determine
when the UD region differs significantly from the calculated UD
value, in turn triggering the application of an additional scaled
dark correction to supplement the statically modeled dark function.
This approach can also provide additional benefits, such as
correcting for interchannel smearing in shutter-free applications
and handling unexpected light leakage inside the spectrograph.
[0034] Data reporting cycle time can be either T or T/N depending
on the incorporation of the rolling average method described in
Method 4.
Selection of N Based on External Control System Requirements.
[0035] As described above, the total number of accumulations N is
typically related to the ratio of the strongest signal to the
weakest signal in the spectrum in order to avoid detector
saturation on any single accumulation. Improved methods 2 and 4
shorten the data reporting cycle to 2T/N or TIN respectively.
However, some applications may need still faster reporting cycles
to support control system requirements. An example of such an
application would be optimizing the efficiency of a natural gas
turbine power generator based on the varying concentrations of
different hydrocarbon constituents in the gas being fed to the
generator. In improved methods 2 and 4, the required signal
exposure time T may be divided in to a larger number of
accumulations N in order to report at a speed consistent with the
control application. The number N will be limited at some point by
increasing relative significance of detector read noise and A/D
quantization noise, as understood to those of skill in the art.
Component Selective Response Time
[0036] Improved methods 2 and 4 above provide more rapid
indications of an onset changes in sample constituents than
standard practice. However, they still nominally require time 2T or
T, respectively, to fully respond to a step change in the sample.
Methods 2 and 4 may be further modified such that the stronger
spectral components are assigned buffer sizes that are smaller than
the N accumulations as described in Method 2. As described above, T
is dictated by the weakest component in the spectrum, whereas
stronger constituents can achieve a target SNR in a shorter total
exposure time. By customizing the buffer size to be smaller than N
as appropriate for stronger spectrum components,
detector-by-detector, the system can be made to fully respond to
changes in concentration on stronger components more rapidly.
Method Selection Based on Application
[0037] The selection of a method described herein depends on
timing, accuracy, setup/computational resource priorities and
application requirements. Interleaved dark collection above is the
most accurate way to track dark current, particularly for single
row cameras with high and significantly varying dark current such
as an InGaAs linear array camera, and also the most accurate way
for a 2D array camera such as a CCD, providing both increased data
reporting rate at target SNR, and most accurate correction for
varying dark current. The Standard Practice of a single dark
collection is a faster, providing twice the data/response rate in
return for a less rigorous estimated tracking of dark current. The
Statistically Modeled Dark correction is the fastest overall method
(including manufacturing time and end-user time), as it requires no
additional effort at time of manufacture. However, this method
provides still faster reporting of data from the viewpoint of the
customer, although the customer may have to pay a charge for
developing the model as extra work is required at time of
manufacture. Finally, several of the methods can benefit by
implementation as a rolling average, if demanded by a process
control system, without actually changing the amount of time for
the system to fully respond to a step change in the process
constituents. Finally, customization of the amount of averaging
based on process control requirements or component concentration
can also be employed.
* * * * *