Imaging spectrometer

Details Imaging spectrometer Imaging spectrometer

1999-11-27

2001-09-11

Evans, F. L. (Department: 2877)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

Reexamination Certificate

active

06288781

ABSTRACT:

The invention relates to design and construction of prism spectrometers, and more particularly to imaging spectrometers.
Spectrometers are instruments used to measure the spectral content of light and other electromagnetic radiation. The spectral data is used to provide information on the sources of the radiation. There are many types of spectrometer, using different methods to distinguish the spectral components of radiation. In this invention, we are concerned particularly with the classical method in which the radiation is dispersed by a refracting element, usually called a prism.
A typical prism spectrometer is shown schematically in FIG.
1
. In this diagram, radiation from the source
100
is thrown onto an entrance slit
101
. The radiation passing through the slit, represented by the lines
102
and
103
, is then collimated by a collimator
104
, passed through a refracting prism
105
, and re-imaged by a re-imager
106
onto a detection system
107
. The collimator
104
and re-imager
106
may be lenses as indicated schematically in
FIG. 1
, or they may include curved mirrors. The prism produces different deflections of the radiation, according to wavelength, in the section orthogonal to the slit. The slit is therefore re-imaged as a spectrum.
The spectral content of the radiation can be read by using a detection system in the form of an array of detector elements,
107
, as indicated in FIG.
1
. Alternatively, the spectral content can be measured by using a single detector element, if the configuration includes some scanning capability to move the spectrum image with respect to the detector. We will, however, describe spectrometers in which a fixed optical system is used with a fixed detector array to provide simultaneous measurement of a complete spectral range.
An imaging spectrometer system is shown schematically in FIG.
2
. In this diagram, a radiation source
200
is imaged by a source-imager
201
onto a slit
202
. The beam is again collimated by a collimator
203
, dispersed by a refracting prism
204
, and re-imaged by a re-imager
205
onto a detection system
206
. However, in this case, care is taken to form a well-resolved image of the source onto the slit, and the collimator and re-imaging optics are designed to form a well-resolved image of the slit, in each component wavelength, onto the detection system. The detection system becomes an area-array of elements
207
, in which each point in the source image is reimaged as a line spectrum onto a detector-array column
208
, and each detector-array row
209
is associated with a different resolved waveband of radiation. When the detector array signals are read-out, they provide data on the spectral content of all spatially-resolved source points that are imaged onto the slit and re-imaged onto the detector array. The system thus provides simultaneous spectral information on a line of spatially-resolved points in the source area. Typically, this line is then scanned, by relative movement of the source and the spectrometer, to build a detailed data-base on the spectral content of an area of the source.
Imaging spectrometers are used to record the spectral content of source areas that show significant spatial variations of spectral radiance. For example, an imaging spectrometer may be used to make a detailed record of the colours of all points in a picture. An important application at present is in Earth observation. Imaging spectrometers are mounted on aircraft and flown on Earth-orbiting, satellites, to measure the spectral content of selected scenes on Earth surface. This data has a wide range of present and possible applications, for example:—vegetation can be identified, and its health analysed, from the spectral content of the image, minerals can be identified for possible mining exploitation, pollution of water can be assessed in open oceans, coastal zones and inland water areas.
Prism spectrometers give relatively low spectral resolution—with resolved wavebands typically in the range from 1 nm to some tens of nanometers, in the visible and near infrared spectral regions. This spectral resolution is not adequate for many scientific applications, for which different types of spectrometer are used. However, it is adequate for many applications, including general colour measurement, and for Earth observation. Prism spectrometers are often preferred in these applications because prisms allow wide spectral ranges to be covered efficiently and simply in a single instrument. For example, a prism spectrometer can be designed to cover the whole spectral range from 400 nm to 2500 nm. This range is of particular interest for Earth-observation from aircraft and satellites.
Designs for prism spectrometers in which lenses are used both for collimation and re-imaging, are indicated in FIG.
2
. All-mirror systems can also be used for collimation and re-imaging, as shown for example in FIG.
3
. In
FIGS. 2 and 3
, like features are indicated by like numerals. The collimator becomes a system of two mirrors,
301
and
302
. The re-imager is a system of two mirrors,
303
and
304
. Catadioptric systems (using a combination of mirrors and refracting elements) are also known. Performance of an imaging spectrometer is limited by the image quality provided by the collimators and re-imagers. Spatial resolution is limited by the resolution provided along the slit image in each wavelength, and spectral resolution is limited by the resolution of the optics across the slit image at each wavelength. Interpretation of the spectral data can also be complicated by distortion of the slit images in each wavelength, if the distortion produces non-straight images of the entrance slit on the straight rows of the detector array.
Imaging spectrometers that use only refracting lenses, as indicated in
FIG. 2
, are limited in performance particularly by the axial chromatic aberrations of the lenses. Axial chromatic aberration produces differences of focus, as a function of wavelength, which prevent good resolution at all the wavelengths in the spectrometer range, unless the range is restricted. It is possible to achieve correction for chromatic aberration over increased wavelength ranges by use of two or more refracting materials, so that good resolution is often achievable, for example, over the complete visible range. However, correction over much wider spectral ranges, that are often required in imaging spectrometers, becomes difficult or impossible without excessive optical complexity.
All-mirror systems are often favoured for spectrometers covering very wide spectral ranges, because they have no chromatic aberrations. Very simple mirror systems are used in spectrometers with single-point or linear array detection systems. However, more complex mirror systems are needed to give adequate resolution in imaging spectrometers using area-array detectors. It is possible to design a collimator that is well-corrected for all optical aberrations, using two mirrors, as indicated in
FIG. 3
, in which the mirror
301
following the slit is concave, and the mirror preceding the prism
302
is convex. A similar two mirror design can be used for the imager, as indicated in FIG.
3
. However, in this design form, all the curved mirrors must have aspherical surfaces so that the components tend to be expensive and difficult to align. The two-mirror system also tends to be large, since the separation of the two mirrors is approximately double the focal length of the combination.
More compact two-mirror systems can be well-corrected only for small field angles, making them less suitable for imaging spectrometers, and they also in general require at least one aspherical component. Three-mirror systems can be relatively compact and also well-corrected for use in imaging spectrometers, but they generally require at least two aspherical components. Systems of four or more mirrors can use only spherical mirrors, but they tend to be large, and the increased system complexity is undesirable.
U.S. Pat. No. 5,127,728 Warren uses “aplanatic” curved refracti

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