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Description  |
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FIELD OF THE INVENTION
The present invention relates to a grating spectrometer and, more
particularly, to a flat field grating spectrometer of improved optical
design which provides better spectral resolution and extended spectral
range.
BACKGROUND OF THE INVENTION
Flat field grating spectrometers find wide application in many analytical
instruments, such as spectrophotometers and colorimeters, used to practice
spectroscopy in the ultraviolet, visible, near-infrared, and mid-infrared
regions of the electromagnetic spectrum. A flat field, i.e., spectral
image plane, is desirable when the detection means is flat, such as with
diode array detectors, image intensifier tubes, or photographic plates.
With development of aberration-corrected, concave holographic gratings, it
became possible to design flat field spectrometer optical systems using
the holographic grating as the only optical element between the entrance
slit and the detector.
However to obtain such a flat field concave holographic grating, the usable
image plane is tilted and displaced far from its preferred Rowland circle
location. As a result, the linear dispersion varies a large amount from
one end of the field to the other. This is a particular disadvantage for
diode array detectors, which usually have equally spaced elements that
then produce unequal spectral resolution across the field. The axis of the
cone of energy incident at the detector is far from normal to the
detector, complicating the design of order sorting filters.
Diode-array, concave holographic grating spectrometers have been made in
which the entrance slit lies below the center of the spectral image plane,
which is displaced above the plane containing the normal to the grating by
the same distance as the entrance slit is below this plane. This design
has two significant disadvantages. First, the spectral focus lies on a
curved image surface, which limits the spectral range over which
acceptable resolution can be obtained, and the out-of-plane design results
in larger aberrations than those of an in-plane design. This factor limits
the spectral resolution obtainable, and, to a considerable extent, the
throughput since the entrance slit height must be restricted to obtain
reasonable resolution.
Other prior designs place the entrance slit and detector in-plane, with the
slit located beyond the end of the spectral image plane. The holographic
grating parameters are adjusted to provide minimal aberrations, including
astigmatism, over a flat spectral field. These designs have good spectral
resolution over only a limited spectral range.
To overcome problems of spectral resolution, prior spectrometers have often
reduced the entendu (optical throughput) (cm.sup.2 -ster) by reducing the
numerical aperture of the grating, the area of the entrance slit and
detector element, or both. The resultant loss of energy reduces the
signal-to-noise ratio obtainable in measuring spectroscopic data.
Prior flat field concave holographic grating instruments have used a single
order of the grating, further limiting the usable spectral range. In many
applications it is desirable to obtain simultaneous measurements extending
over two spectral regions, e.g., the ultraviolet and visible or the
visible and the near-infrared, which span a spectral range exceeding a two
to one ratio of maximum to minimum wavelengths. In many cases, different
detector arrays may be required for different spectral regions. Combining
two different detector materials in a single array results in a gap
between the two subarrays.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
improved flat field concave grating spectrometer which enables spectral
measurements with better spectral resolution over a greater spectral range
than hitherto possible, while maximizing the energy throughput.
It is another object of the invention to provide a method and apparatus for
efficient, simultaneous use of two detector arrays which measure the
spectral energy in two different spectral regions.
It is yet another object of the invention to provide a method and apparatus
for efficient, simultaneous use of two different grating orders enabling
use of an additional spectral region without increased optical aberrations
and with improved spectral resolution in the higher order spectral image.
SUMMARY OF THE INVENTION
In accordance with the present invention, a flat field grating spectrometer
having increased spectral resolution and usable spectral range comprises
an entrance slit or port for selection of incident spectral energy to be
measured and a concave holographic grating for receiving the spectral
energy from said slit or port and for dispersing and imaging the incident
spectral energy. Also included is a field flattening lens for flattening
the field or spectral image surface of the energy dispersed and imaged by
the grating. Detector means are included in the form of a planar array of
detecting elements for detecting and providing signals representative of
the spectral energy distribution in the flattened spectral image plane as
a function of position and, therefore, as a function of wavelength.
The invention also encompasses use of a beam splitter which divides the
energy between two spectral image planes by transmitting part of the
energy to the original spectral image plane while reflecting part of the
energy to a second image plane. In this arrangement, a second field
flattening lens may be included and detector means is included for
detecting and providing signals representative of the spectral energy
distribution in said second flattened spectral image plane simultaneously
in time with the detection of the energy distribution in said first
spectral image plane by said first detector means.
Alternatively, the beam splitter may be an electro-optical device which
reflects or transmits energy in response to the application of electrical
or magnetic fields, thereby time sharing the two detector means. The
effective energy ratio between the two detector means may then be
controlled by the duty cycle of the electro-optical mirror.
For a better understanding of the present invention, reference is made to
the following description and accompanying drawings while the scope of the
invention will be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 represents a first embodiment of the flat field grating spectrometer
in accordance with the invention;
FIG. 2 illustrates a second embodiment of the invention with a second
detector placed at an angle to the axis of the energy dispersed and imaged
by the holographic grating; and
FIG. 3 is a schematic block diagram of the electronic circuitry for use in
the first and second embodiments of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a flat field grating spectrometer in accordance with
the present invention is shown. The spectrometer, in one embodiment, has a
total spectral range of approximately 520-2200 nm.
The spectrometer includes an entrance slit or port 11 for selection of
spectral energy to be measured and is typically 0.25.times.6.0 mm in size.
The energy passes from the slit or port to a concave holographic grating
12 which is mounted at an angle (in one embodiment 0.368 radians) to a
principal axis of a field flattening lens to be discussed further below.
The concave holographic grating 12 receives the spectral energy from the
slit or port and disperses and images this spectral energy. In a typical
embodiment, the concave grating radius of curvature is 99.6 mm, the
distance from the grating vertex to the entrance slit is 97.74 mm, the
distance along the principal axis of the field flattening lens to the slit
image is 108.0 mm and the grating has 244 grooves per mm. The energy
imaged by the concave grating is directed to a field flattening lens 14,
typically a plano-concave cylindrical lens made of SF10 glass with a
concave radius of curvature of 32.75 mm, a 3 mm center thickness, and
located with a center of curvature 105.75 mm from the grating vertex. The
field flattening lens 14 redirects the spectral energy which has been
dispersed and imaged by the grating onto a spectral image plane, typically
110.0 mm from the grating vertex.
After the energy has been redirected by the lens 14 so as to form a flat
spectral image, it is directed through an order sorting filter 22 which
transmits energy in the desired diffraction order and absorbs or reflects
energy in other orders to which the detector means is responsive. This
order sorting filter may comprise several regions with different
wavelength characteristics as may be required to obtain the required order
sorting function. After the energy is transmitted through the order
sorting filter, it is directed to a planar arranged detector array 15 at
the spectral image plane. The output of the detector array 15 (typically
an array of photodiodes similar to the Advanced Optoelectronics ADC 801976
element silicon diode array) is directed to electronic processing
equipment 16 comprising a preamplifier and low pass electronic filter
associated with each detector element in the detector array and a
multiplexer for sequentially sampling the output of each
preamplifier-filter so as to time division multiplex the amplified and
filtered output of all the array elements into a combined signal for
further processing. In a preferred arrangement of the present invention,
the preamplifiers, electronic filters, and multiplexers are combined in an
electronic module mounted immediately in back of the detector array 15 to
minimize the distance between the detector array elements and the
preamplifier. Additional electronic processing, such as programmable gain
and offset circuits, may be included in electronic processing equipment
16. In a preferred embodiment, the combined signal is then directed to an
analog to digital converter (typically a Data Translation DT2823 unit)
whereby it is converted to digital form for further processing and
analysis by means of a digital computer, typically a Dell Model 310.
The arrangement described above enables the spectrometer to provide useful
information of different parts of the spectral regions so as to provide
increased spectral resolution and a more usable spectral range.
In FIG. 2 another embodiment of the present invention is depicted. There, a
beam splitter 18 is shown as it would be incorporated in a spectrometer
such as that of FIG. 1 in the optical beam directed from the concave
grating toward the field flattening lens 14. Such beam splitter 18 is not
limited to use with a field flattening lens. A portion of the energy is
passed through the beam splitter to the lens 14 while a portion is
reflected and directed at an angle (preferably orthogonal) to the axis of
the field flattening lens. This reflected energy is directed to a second
field flattening lens 19, through a second order sorting filter 20, and
then to a second detector array 13. A second electronic processing
equipment 17, similar to 16, is mounted behind the second detector array.
In a preferred embodiment, the second detector array is formed of germanium
photodiodes responsive from 1030 nm to 1800 nm. Indium gallium arsenide
photodiodes may also be used for the second detector array and the first
detector array is formed of silicon photodiodes responsive from 515 to
1100 nm. In a preferred arrangement the beam splitter 18 is a dichroic
mirror which reflects substantially all the incident energy in a first
spectral region while transmitting substantially all of the incident
energy in a second spectral region. This dichroic mirror maximizes the
efficiency for both spectral regions. Alternatively, a neutral beam
splitter may be used which transmits a first fraction of the energy and
reflects a second fraction of the energy, such fractions being
substantially independent of the wavelength of the energy. The energy
ratio of the neutral beam splitter may be chosen to provide substantially
equal signals from the two arrays. In a preferred arrangement, the ratio
of the energy reflected toward a germanium diode array to the energy
transmitted to a silicon diode array is 86% to 14%, which ratio is
selected to make the signals from the germanium array approximately equal
to those from the silicon array, considering the energy distribution of a
tungsten lamp source, the efficiency of the grating, the responsivity of
the detectors, and the appropriate current to voltage transformation ratio
of the preamplifiers for each array.
In one form of the invention, the beam splitter includes an optical element
which is switched from reflective to transmissive by means of an
electrical or magnetic field thereby being responsive to a signal to
switch the optical beam between the first and second detector. When the
beam is switched more rapidly than the response time of the
pre-amplifier-filter, the effect is equivalent to dividing the energy with
a neutral beam splitter; however, the ratio can be varied by changing the
duty cycle of the switching.
In general, the principal axes of the grating and the field flattening lens
are arranged so they lie in a plane and form an angle less than
45.degree., with the principal axis of the field flattening lens passing
through or near the center of the grating. The entrance slit is located
near, but not on, the principal axis of the grating, typically displaced
0.038 radians, so that the zero order image of the slit is also close to
the principal axis of the grating and does not impinge on the walls of the
housing. The holographic grating parmeters are constrained during the
process of design to obtain the desired spectral resolution and spectral
range.
In a preferred arrangement of the circuitry of the present invention, shown
in FIG. 3, portions 25' of the electronics preamplifier 17 are the same as
the preamplifier portions 25 of electronics 16. It is a hybrid circuit
assembly containing 19 dual operational amplifiers (typically Analog
Devices type AD648), each with a feedback resistor and capacitor,
operating in the current to voltage transformation mode to provide 38
channels. The feedback resistor and parallel capacitor establishes the
time constant or high frequency rolloff of the preamplifier response,
typically at 1.6 milliseconds time constant which yields a 100 Hertz high
frequency rolloff. Two preamplifier modules (25 or 25') are used with each
76 element detector array, or one with a 38 element array. Electronics 16
also includes an 80 channel multiplexer hybrid circuit 26 (typically using
five ADG 526 analog multiplexers) which time multiplexes the 76 outputs of
two preamplifiers (plus 4 reference or test signals) in a single output
channel. The multiplexer 26 has 80 low pass input filters consists of a
resistor and capacitor to signal to further limit the signal bandwidth,
typically with a time constant of 4 ms. The multiplexing is digitally
controlled from an external source (30). A high speed operational
amplifier (e.g. an AD 744) voltage follower buffers the signal. When two
76 element arrays are used, there are two electronic modules, each
consisting of a multiplexer and two preamplifier hybrids. Optionally, a
programmable gain and offset amplifier 27 may be incorporated to adjust
the zero reference and magnitude of the signal. Similar units (multiplexer
26' and amplifier 27') are provided in the second channel. The output
signal is supplied to an A/D converter 28 whose output is provided to
computer and display 29 under control of digital control 30.
As described above, the spectrometer of the present invention provides
increased spectral resolution by the addition of the field flattening lens
so that the entrance slit is accurately focused on the flat diode array.
Without the lens, the aberration corrected holographic grating parameters
must be compromised to obtain a relatively flat field. In general, such a
design has a flat field over only a limited spectral range. If this
limited spectral range in satisfactory, the design does not receive use of
a filed flattening lens. By use of a field flattening lens, the geometry
can be arranged so that the line normal to the plane of the array at its
center passes through or near the vertex of the grating. Unlike prior flat
field designs, this geometry provides substantially constant dispersion,
i.e. constant spectral resolution, with equally spaced detector elements.
A more than 2:1 range of wavelengths can be covered by this approach. The
geometry also allows use of a beam splitter to provide a second image
plane, which would not generally be feasible with prior flat field designs
due to the large angle of incidence of light at the detector array.
Another detector array, located at this second image plane, can measure in
another order of the grating than the first, even if the orders are
overlapped. In this case, separate order sorting filters are employed, one
in front of each detector array after the beam splitter.
While the foregoing description and drawings represent the preferred
embodiments of the present invention, it will be obvious to those skilled
in the art that various changes and modifications may be made therein
without departing from the true spirit and scope of the present invention.
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