Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2001-10-05
2004-11-02
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S326000
Reexamination Certificate
active
06813019
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for the spectrochemical analysis of a sample in which a solid-state array detector is used to detect radiation of spectrochemical interest. In particular, the method and apparatus relate to utilisation of the solid state array detector.
Throughout this specification, the terms “light” and “radiation” have been used interchangeably.
BACKGROUND
It is known that chemical analysis of samples can be accomplished by a variety of spectroscopy-based techniques. For example, the amount of various chemical elements in a sample can be ascertained by optical emission spectrometry or by atomic absorption spectrophotometry. The concentration of various chemical species in a sample can be ascertained by ultraviolet—visible absorption spectrometry or infrared absorption spectrophotometry, or by ultraviolet-visible fluorescence spectrophotometry. These are only a few examples of spectroscopy-based chemical analysis techniques.
Apparatus for spectroscopy-based chemical analysis typically operates by measuring the intensity of light either as a function of wavelength or at one or more specific wavelengths. This may be done with a monochromator and a single detector collecting intensity data for each wavelength of interest in a serial fashion, but it is also possible to collect light intensity data for more than one wavelength simultaneously. Because of the greater time efficiency offered by simultaneous measurement, this approach is increasingly favoured for practical applications.
Modern simultaneous spectroscopic measurement apparatus typically includes an optical polychromator together with a solid state electronic detector device incorporating an array of optical sensor elements. The detector can be, for example, a charge-transfer device such as a charge-injection device (CID) or a charge-coupled device (CCD). A polychromator that is able to disperse the light in two dimensions (for example an echelle polychromator) can be employed, in which case a 2-dimensional array of optical sensor elements can be used with advantage as a detector. Alternatively a polychromator that provides dispersion in one dimension only (such as a single-grating-based polychromator) can be utilised, and a linear array detector used. The 2-dimensional approach offers better wavelength resolution for a given wavelength range and so is favoured for chemical analysis applications, particularly for elemental analysis by optical emission spectrometry. Compared to linear detectors, such two-dimensional arrays are more compact. The need for several detectors to cover the focal plane of the spectrometer can be avoided by the use of an appropriate two-dimensional array detector.
Elemental analysis typically involves operation at optical wavelengths extending from the visible to the far ultraviolet, which places limitations on the types of detectors that can be used. Primarily, the detector must be efficiently responsive to radiation across this range of wavelengths. Solid-state detectors of various types are known to be suitable for this application, for example charge transfer devices, both CIDs and CCDs, are known to be useful. Such devices are described, for example, in the book ‘Charge Transfer Devices in Spectroscopy’, J. V. Sweedler, K. L. Ratslaff and M. B. Denton, eds., VCH Publishers, Inc., New York, 1994. ISBN 1-56081-060-2. CCDs are discussed in ‘Scientific Charge-Coupled Devices’, J. R. Janesick, SPIE Press , Bellingham, Wash., 2001, ISBN0-8194-3698-4.
A specific example of such a detector is the CCD detector disclosed by Zander et al. in U.S. Pat. No. 5,596,407. This has a number of optically sensitive sites, generally referred to as pixels, that are distributed in a precise geometric arrangement over the surface of the detector to map accurately the optical image from the polychromator. Each optically sensitive site or pixel is capable of converting the energy of incoming light to free electrons, which are stored at the optically active site. The number of electrons, and thus the total charge, accumulated within each pixel will depend on the light intensity incident on that pixel and the time for which the pixel is exposed to said light, said time being usually referred to as the integration time.
Measuring the optical intensity therefore involves determining the amount of charge built up over a known integration period. In order to do this it is necessary first to collect the charge and then to transfer the charge accumulated at each pixel to appropriate readout electronics. Two principal ways of carrying out this process are available. The first, used in the detector disclosed by Zander et al. in U.S. Pat. No. 5,596,407, duplicates each optically active pixel with an optically inactive pixel. The first step in the readout process is a parallel transfer operation that transfers the charge from each row of active pixels to the corresponding row of inactive pixels so that these inactive pixels are used as the shift register nodes. The charge is then stepped through one optically inactive pixel to the next to readout electronics at the end of the row. The second approach uses the optically active pixels themselves as shift register nodes, so that with each move operation the charge on every pixel moves to the next pixel along, with the charge of the first pixel moving to the readout circuit.
Both approaches have their attendant advantages and disadvantages. The second approach has the advantage that most of the surface area of the CCD can be covered by active pixels, thus maximising the light sensitivity of the whole device. It also avoids the need for any secondary structure. That is, this approach provides more efficient utilisation of available light in spectroscopic applications. It also permits the use of relatively inexpensive, off-the-shelf detectors, or of custom-designed detectors that can be fabricated relatively inexpensively using the same technology as that used for the off-the-shelf detectors.
The disadvantage of the second approach is that the pixels continue to accumulate electrons generated by any incoming light during the readout process. As a consequence, as the charge from one pixel moves through other pixels on its way to the readout circuitry, it accumulates additional charge, the amount of which depends on the light intensity incident at each of those other pixels and the speed of charge transfer. This has the effect of smearing the resultant image data, which is unacceptable in a spectroscopy application.
The smearing problem does not occur with the first approach, since the readout occurs through optically inactive pixels. However this advantage is at the expense of the need for a secondary structure and a reduced overall light sensitivity, due to the loss of that proportion of the detector's surface area that is taken up by the inactive pixels. Additionally, detectors of this type have to be custom-designed and custom-built, and are consequently expensive.
In spectrochemical applications it is common for a sample to emit extremely intense radiation at certain wavelengths and to emit extremely feeble radiation at other wavelengths, depending on the amount of specific chemical elements present in that sample. To extract the required chemical information it is often necessary to measure both extremely feeble and extremely intense radiation from the same sample. This presents problems in that if the detector is exposed to radiation for a sufficient time to generate accurately measurable charge from extremely feeble radiation, those parts of the detector that are exposed to extremely intense radiation will have accumulated excessive charge. Charge accumulation is excessive when it exceeds the capacity of the device to store it. Not only is such excessive charge useless for measurement of the intensity of the radiation that generated it, but it can also spill over into adjacent regions of the detector and impede or prevent the correct functioning of those regions. Such a process is known as ‘blooming’. Conversel
Bricker Dower C.
Campbell Stewart R.
Hammer Michael R.
Layton Peter G.
Masters Martin K.
Berkowitz Edward H.
Davis Willie
Fishman Bella
Font Frank G.
Varian Australia PTY LTD
LandOfFree
Method and apparatus for spectrochemical analysis does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for spectrochemical analysis, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for spectrochemical analysis will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3280247