U.S. patent application number 13/342317 was filed with the patent office on 2012-05-03 for spectrometric measurement system and method for compensating for veiling glare.
Invention is credited to Nico Correns, Felix KERSTAN, Joerg Margraf.
Application Number | 20120105847 13/342317 |
Document ID | / |
Family ID | 38024326 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120105847 |
Kind Code |
A1 |
KERSTAN; Felix ; et
al. |
May 3, 2012 |
SPECTROMETRIC MEASUREMENT SYSTEM AND METHOD FOR COMPENSATING FOR
VEILING GLARE
Abstract
The present solution is directed to a measuring system and a
method for determining spectrometric measurement results with high
accuracy. The spectrometric measuring system, comprises a radiation
source, an entrance slit, a dispersion element, and a detector with
detector elements arranged in a linear or matrix-shaped manner in
one or more planes. The detector has an even distribution of at
least two different wavelength-selective filters on its detector
elements. While detectors from photography and video applications
are used for this purpose, use of the invention is not limited to
the visible spectral region. Further, color filters on the pixels
may be omitted or modified in the manufacturing process. It is also
possible to use other types of detectors in which the
wavelength-selective filters and associated detectors are arranged
one behind each other in a plurality of planes in which complete
color information is available to each individual picture
point.
Inventors: |
KERSTAN; Felix; (Jena,
DE) ; Correns; Nico; (Weimar, DE) ; Margraf;
Joerg; (Paulinzella, DE) |
Family ID: |
38024326 |
Appl. No.: |
13/342317 |
Filed: |
January 3, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12225904 |
Oct 1, 2008 |
8111396 |
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PCT/EP2007/002128 |
Mar 12, 2007 |
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13342317 |
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Current U.S.
Class: |
356/326 |
Current CPC
Class: |
G01J 3/0262 20130101;
G01J 3/51 20130101; G01J 3/2803 20130101; G01J 3/36 20130101 |
Class at
Publication: |
356/326 |
International
Class: |
G01J 3/28 20060101
G01J003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 1, 2006 |
DE |
10 2006 015 269.7 |
Claims
1. A spectrometric measuring system for compensation for veiling
glare, comprising: at least one radiation source; at least one
entrance slit; a dispersion element; and a detector with detector
elements arranged in a linear or matrix-shaped manner; wherein the
detector has an even distribution of different wavelength-selective
filters on its detector elements; and wherein the detector has an
evenly distributed arrangement of at least two wavelength-selective
filters.
2. A spectrometric measuring system for compensation for veiling
glare, comprising: at least one radiation source; at least one
entrance slit; a dispersion element; and a detector with detector
elements arranged in a linear or matrix-shaped manner; wherein the
detector has an even distribution of different wavelength-selective
filters on its detector elements; and wherein a color camera sensor
is used as a detector.
3. The spectrometric measuring system according to claim 2; wherein
the color camera sensor is known from photography applications and
video applications.
Description
[0001] The present application is a continuation of U.S. patent
application Ser. No. 12/225,904 filed on Oct. 1, 2008, which claims
priority from PCT Patent Application No. PCT/EP2007/002128 filed on
Mar. 12, 2007, which claims priority from German Patent Application
No. DE 10 2006 015 269.7 filed on Apr. 1, 2006, the disclosures of
which are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to a measuring system and
to a method for determining spectrometric measurement results with
high accuracy.
[0004] 2. Description of Related Art
[0005] To detect the light at the output of multichannel
spectrometers, detectors which are sensitive within the entire
spectral range acquired by the spectrometer are generally used
according to the prior art. The multichannel detectors comprise a
plurality of detector elements, also called pixels, which are
arranged in arrays or matrices.
[0006] A determined partial region of the total spectral region
whose light output is to be measured is associated with each pixel.
Since the separation of light into its spectral components is never
accomplished completely, a broadband detector of the kind mentioned
above always acquires a certain proportion of light from a
different spectral region not associated with the pixel in the form
of false light or veiling glare, as it is called. This leads to
inaccurate measurement results.
[0007] In grating spectrometers, light from diffraction orders
other than veiling glare can also be acquired. Various steps to
minimize the proportion of veiling glare or mutual crosstalk of
spectral channels are known.
[0008] For example, U.S. Pat. No. 6,181,418 B1 describes a
concentric spectrometer which has a special surface referred to as
a "light trap" for reducing stray light. This light trap is
integrated in the design of the imaging optics and is formed as a
beveled surface. The light trap is a surface specifically designed
to eliminate or mitigate stray light generated from the entrance
slit. It is intended to prevent stray light, including light of
different diffraction orders, from being imaged on the detector.
The light trap is a beveled surface with nonreflecting, absorbent
or scattering characteristics. In order to eliminate the greatest
possible proportion of stray light, the beveled surface of the
imaging optics is rendered coarse and additionally is coated with
an optically absorbent material. In addition to this beveled
surface, the inner surface of the housing of the concentric
spectrometer is formed in a corresponding manner, the mechanical
properties of the material such as elasticity, strength and heat
resistance being crucial in the selection of material.
[0009] U.S. Pat. No. 6,700,664 B1 describes a device by means of
which light beams are selectively split through linear variable
filters (LVF) and transmitted to a photodetector array so as that
the spectral characteristics of the transmitted light can be
determined. Linear variable filters (LVF) are formed on a substrate
by optical thin-film layers, and the thickness of the individual
layers can vary. The LVF can be designed either as a bandpass
filter or as a high/low-cut filter. The width of the selectively
split light beams can be adapted to the detector so that they
approximately correspond to the pixel width. It is disadvantageous
in this solution that the LVF cannot be arranged on the surface of
the detector array because this is difficult to accomplish owing to
the sensitive surface of the detector array and the wiring of the
detector array. Therefore, the different LVF elements are set on a
carrier disk that is arranged at a distance of several millimeters
from the detector array. Micro-objectives which focus the optical
light beams on the pixels of the detector array are used to reduce
the influence of unwanted light. On one hand, this makes the
construction of the device more complicated; on the other hand, the
micro-objectives can in turn cause additional light scattering.
[0010] In contrast to the latter, in the monolithic miniature
spectrometer by Carl Zeiss Jena GmbH (Type MMS), order filters are
arranged directly on the detector elements.
[0011] The problem in all of the solutions mentioned above is that
it is always only possible to minimize the veiling glare, but an
independent acquisition and/or compensation is not possible.
Further disadvantages of the solutions mentioned above include
increased resources on hardware owing to additional structural
components in the spectrometer or additional optical layers on the
detector.
SUMMARY OF THE INVENTION
[0012] It is the object of the present invention to develop a
spectrometric measuring system and a method by which the
measurement results can be compensated for with respect to veiling
glare without requiring increased expenditure on apparatus.
[0013] According to the invention, this object is met through the
features of the independent claims. Preferred further developments
and constructions are indicated in the dependent claims.
[0014] The above-stated object is met according to the invention in
that a detector with pixels arranged in linear shape or matrix
shape and with an even distribution of different
wavelength-selective filters (color filters) on the pixels is used
for detecting the light at the spectrometer output. The detector
can be a color camera known from photography applications or video
applications, Such color cameras are very inexpensive because they
are manufactured in very large quantities and in some cases are
less expensive than corresponding black-and-white cameras which are
only manufactured for special applications.
[0015] Use of the invention is not limited to the visible spectral
region. If required, the color filters on the pixels may sometimes
be omitted or modified in the last step of manufacturing the color
camera in order to optimize them for the required spectral
region.
[0016] But it is also possible to use other types of detectors in
which the wavelength-selective filters and the associated detectors
are arranged one behind the other in a plurality of planes, e.g.,
in the so-called X3 image converter by the US firm Foveon, Inc. In
contrast to conventional image converters, complete color
information is available for every individual image point.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a color camera sensor with an even distribution
of four different wavelength-selective filters;
[0018] FIG. 2 shows a spectrometer arrangement with three entrance
slits which are oriented parallel to the grating lines;
[0019] FIG. 3 shows a spectrometer arrangement with three entrance
slits which are oriented parallel to the grating lines and are
offset relative to one another;
[0020] FIG. 4 illustrates the relative sensitivity of the color
filters k as a function of the (effective) wavelength .lamda..sub.1
associated with the pixel i; and
[0021] FIG. 5 shows a spectrometer arrangement having a control
unit, and using a prism instead of a diffraction grating.
DETAILED DESCRIPTION OF EMBODIMENTS
[0022] It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements
that are relevant for a clear understanding of the present
invention, while eliminating, for purposes of clarity, many other
elements which are conventional in this art. Those of ordinary
skill in the art will recognize that other elements are desirable
for implementing the present invention. However, because such
elements are well known in the art, and because they do not
facilitate a better understanding of the present invention, a
discussion of such elements is not provided herein.
[0023] The present invention will now be described in detail on the
basis of exemplary embodiments.
[0024] The spectrometric measuring system according to the
invention with compensation for veiling glare comprises at least
one radiation source, at least one entrance slit, a dispersion
element, and a detector with detector elements arranged in a linear
or matrix-shaped manner in one or more planes. The detector has an
even distribution of at least two different wavelength-selective
filters on its detector elements. In particular, color camera
sensors known from photography applications and video applications
can be used as detectors.
[0025] FIG. 1 shows a color camera sensor with an even distribution
of four different wavelength-selective filters which are arranged
in a quadratic pattern. The detector uses, for example, the colors
cyan (Cy), yellow (Ye), green (Gn) and magenta (Mg). A control unit
7 (see FIG. 5) is arranged downstream of the detector for
determining, evaluating or storing the signal values of the
different-colored detector elements.
[0026] A diffraction grating or dispersion prism is used in a known
manner as a dispersion element. The entrance slit or entrance slits
is/are oriented parallel to the grating lines or to the roof edge
of the dispersion prism so that the partial spectra imaged on the
detector have the same wavelength correlation (see FIG. 2).
[0027] In another constructional variant, the entrance slit or
entrance slits is/are oriented in an offset manner relative to a
parallel line to the grating lines or to the roof edge of the
dispersion prism so that the partial spectra of every entrance slit
formed on the detector can acquire different partial regions of a
total spectral region (see FIG. 3).
[0028] The control unit arranged downstream of the detector is
capable of determining spectral intensity values I.sub.i of the
detector elements with identical color filters transverse to the
dispersion direction as weighted sums, electively with or without
compensation for crosstalk.
[0029] In the method according to the invention for compensation of
veiling glare in a spectrometric measuring system, the light from
at least one radiation source is imaged by at least one entrance
slit and a dispersion element on a detector with detector elements
arranged in a linear or matrix-shaped manner in one or more planes.
A detector having an even distribution of different
wavelength-selective filters on the detector elements is used.
[0030] A diffraction grating or a dispersion prism is preferably
used as a dispersion element. The detector has an evenly
distributed arrangement of at least two wavelength-selective
filters and corresponds, for example, to the color camera sensors
known from photography applications and video applications. FIGS. 2
and 3 each show a variant of a spectrometer arrangement with
imaging gratings.
[0031] A control unit arranged downstream of the detector
undertakes the determination, evaluation or storage of the signal
values of the differently colored detector elements.
[0032] In a first variant, the entrance slit or entrance slits
is/are arranged parallel to the grating lines or to the roof edge
of the dispersion element.
[0033] In this connection, FIG. 2 shows a spectrometer arrangement
in which three entrance slits are oriented parallel to the grating
structure of the dispersion element so that the light 2 coming from
the three entrance slits 1, 1' and 1'' is imaged on the detector 5
by the diffraction grating 3 in the form of three partial spectra
4, 4' and 4''. Each partial spectrum 4, 4' and 4'' has the same
wavelength scale. When using different radiation sources for the
entrance slits 1, 1' and 1'', the partial spectra are associated
with the individual radiation sources. With the same radiation
source, the partial spectra can be added to reduce noise.
[0034] In a second variant, the entrance slits are oriented in an
offset manner relative to a parallel line to the grating lines or
to the roof edge of the dispersion prism 6 (see FIG. 5).
[0035] In this connection, FIG. 3 shows a spectrometer arrangement
with three entrance slits oriented so as to be offset relative to
one another parallel to the grating lines so that light 2 coming
from the three entrance slits 1, 1' and 1'' is imaged on the
detector 5 by the dispersion element 3 in the form of three partial
spectra with different partial regions 4, 4' and 4'' of a total
wavelength range. The partial spectra 4, 4' and 4'' imaged on the
detector 5 are acquired separately by the control unit 7 (see FIG.
5) and joined to form a spectrum comprising the entire wavelength
region.
[0036] A prism whose dispersion direction is oriented perpendicular
to the diffraction grating can be used in addition to the
diffraction grating. Since the diffraction grating is dimensioned
in such a way that a plurality of diffraction orders of the
spectral region to be imaged impinge on the detector, the
additional prism, in accordance with the solution described in DE 1
909 841 C2, serves to separate the diffraction orders.
[0037] For purposes of signal processing, the net signal values
S.sub.i,k of every detector element are determined by the control
unit as the difference of the light signal and dark signal, and the
sum of the spectral intensity values I.sub.i is determined for
detector elements with the identical color filter transverse to the
dispersion direction. The determination of the net signal values
S.sub.i,j is carried out under otherwise identical conditions for
every pixel of the detector, where i is the column number and j is
the row number. Both the dark current and electronic null signal
are eliminated by the determination of the net signal values
S.sub.i,k.
[0038] Since the individual imaged spectra have different color
filters (k=color filter number) in the dispersion direction
(horizontal, index i), the net signal values are initially summed
with the same centroid in the dispersion direction and the same
color filter (color filter number k) transverse to the dispersion
direction. A weighted summing is then carried out in such a way
that the same weighted signals are associated with the same
wavelength index and the same color index.
[0039] For a detector according to FIG. 1 with an even distribution
of four different wavelength-selective filters (cyan-Cy, yellow-Ye,
green-Gn, and magenta-Mg) arranged in a quadratic pattern, the
weighted net signal values S.sub.i,k are determined as follows:
Cy(k=1):
S.sub.i,1=3(s.sub.i,1+s.sub.i,3)+(s.sub.i+2,1+s.sub.i+2,3))
S.sub.i+1,1=(s.sub.i,1+S.sub.i,3)+3(s.sub.i+2,1+S.sub.i+2,3)
Ye(k=2):
S.sub.i,2=(s.sub.i-1,1+s.sub.i-1,3)+3(s.sub.i+i,1,1+s.sub.i+1,3- ))
S.sub.i+1,2=3(s.sub.1+1,1+s.sub.i+1,3)+(s.sub.i+3,1+s.sub.i+3,3))
Gn(k=3): S.sub.i,3=4(s.sub.i,2+s.sub.i+1,4)
S.sub.i+1,3+4(s.sub.i+1,4+s.sub.i+2,2)
Mg(k=4): S.sub.i,4=4(s.sub.i,4+s.sub.i+1,2)
S.sub.i+1,4=4(s.sub.i+1,2+s.sub.i+2,4)
[0040] The formulas for summing a total of four rows is shown by
way of example. For detectors with a greater quantity of columns
and rows, the formulas must be amended in a corresponding manner.
The quantity of net signal values S.sub.i,k per color in the
dispersion direction is doubled through this method step.
[0041] In the variant of the detector with wavelength-selective
pixels in a respective plane (index k), only the net signal values
of every column are summed.
S t , k = j S i , j , k ##EQU00001##
[0042] Subsequently, the spectral intensity values I.sub.i of the
detector elements with the identical color filter transverse to the
dispersion direction are determined by the control unit as a
weighted sum:
I i = k = 1 n G i , k S i , k , ##EQU00002##
where I.sub.i is the spectral intensity value in column i,
G.sub.i,k, is the weight factor of the color filter k in column i,
S.sub.i,k is the net signal value of the color filter k in column
i, k is the number of the color filter, and n is the quantity of
color filters.
[0043] In another embodiment, the spectral intensity values I.sub.i
of the detector elements with the same color filter transverse to
the dispersion direction are determined by the control unit as a
weighted sum without compensating for crosstalk by taking into
account the following weight factors:
G i , k = R k ( .lamda. i ) l = 1 n R l 2 ( .lamda. i ) ,
##EQU00003##
where R.sub.k(.lamda..sub.i) is the relative spectral sensitivity
of the color filter k, .lamda..sub.i is the effective wavelength, i
is the column number of the detector, k is the number of the color
filter, and n is the quantity of color filters.
[0044] In this connection, FIG. 4 shows the relative sensitivity of
the color filters k as a function of the (effective) wavelength
.lamda..sub.i associated with the pixel i.
[0045] The signal with the least sensitivity in the effective
wavelength .lamda..sub.1 is suitable for compensating for
crosstalk. The corresponding weight factor must then become
negative. The magnitude of the negative compensation value must be
optimized for the given spectrometer and application based on
different samples which are sensitive to stray light.
[0046] The solution according to the invention for compensation of
veiling glare in a spectrometric measuring system makes it possible
to optimize the spectrometer for a maximum signal-to-noise ratio or
minimum mutual crosstalk without changing the apparatus
construction, depending on the application, merely by changing the
weight factors G.
[0047] By using a plurality of entrance slits, either the spectra
of a plurality of light sources or a plurality of portions of the
spectrum of a light source can be imaged on the detector. In both
cases, every entrance slit generates a spectrum trace on the
detector. The calculation of the spectral intensity values is
carried out in the manner described separately for every trace. The
summing in the column direction is then limited to the region of
every trace. Summing limits can be adapted in case of imperfect
orientation of the grating lines to the detector or in case of
wavelength-dependent stigmatism depending on the column number i.
This renders the results unsusceptible to manufacturing tolerances
and imaging errors.
[0048] It is particularly advantageous to use color camera
detectors known from photography applications and video
applications. Such color cameras are very inexpensive because they
are manufactured in very large quantities and in some cases are
less expensive than corresponding black-and-white cameras which are
only manufactured for special applications.
[0049] While this invention has been described in conjunction with
the specific embodiments outlined above, it is evident that many
alternatives, modifications, and variations will be apparent to
those skilled in the art. Accordingly, the preferred embodiments of
the invention as set forth above are intended to be illustrative,
not limiting. Various changes may be made without departing from
the spirit and scope of the inventions as defined in the following
claims.
* * * * *