Hemispherical detector

Details Hemispherical detector Hemispherical detector

2000-10-30

2003-03-18

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S339070, C250S379000, C250S339120, C250S341800, C250S343000

Reexamination Certificate

active

06534768

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of spectroscopic detectors. Specifically, the present invention relates to a hemispherical detector for use with a transmittance or reflectance spectrometer which comprises a plurality of photodetectors.
BACKGROUND OF THE INVENTION
Infrared spectroscopy is a technique which is based upon the vibrations of the atoms of a molecule. In accordance with infrared spectroscopy, an infrared spectrum is generated by transmitting radiation through a sample and determining what portion of the incident radiation is absorbed by the sample at a particular energy. Near infrared radiation is radiation having a wavelength between about 700 nm and about 2500 nm.
In general spectrometers (e.g., a spectrophotometer) can be divided into two classes: transmittance spectrometers and reflectance spectrometers. In a transmittance spectrometer, light having a desired narrow band of wavelengths is directed onto a sample, and a detector detects the light which was transmitted through the sample. In contrast, in a reflectance spectrometer, light having a narrow band of wavelengths is directed onto a sample and one or more detectors detect the light which was reflected off of the sample. Depending upon its design, a spectrometer may, or may not, be used as both a transmittance and a reflectance spectrometer.
A variety of different types of spectrometers are known in the art such as grating spectrometers, FT (Fourier transformation) spectrometers, Hadamard transformation spectrometers, AOTF (Acousto Optic Tunable Filter) spectrometers, multiple discrete wavelength source spectrometers, filter-type spectrometers, scanning dispersive spectrometers, and double-beam spectrometers.
Filter-type spectrometers, for example, utilize a light source (such as a conventional light bulb) to illuminate a rotating opaque disk, wherein the disk includes a number of narrow bandpass optical filters. The disk is then rotated so that each of the narrow bandpass filters passes between the light source and the sample. An encoder indicates which optical filter is presently under the light source. The filters filter the light from the light source so that only a narrow selected wavelength range passes through the filter to the sample. Optical detectors are positioned to detect light which either is reflected by the sample (to obtain a reflectance spectra) or is transmitted through the sample (to generate a transmittance spectra). The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
Multiple discrete wavelength source spectrometers use infrared emitting diodes (IREDs) as sources of near-infrared radiation. A plurality (for example, eight) of IREDs are arranged over a sample work surface to be illuminated for quantitative analysis. Near-infrared radiation emitted from each IRED impinges upon an accompanying optical filter. Each optical filter is a narrow bandpass filter which passes NIR radiation at a different wavelength. NIR radiation passing through the sample is detected by a detector (such as a silicon photodetector). The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis. IRED reflectance spectroscopy is also possible.
Acousto Optic Tunable Filter spectrometers utilize an RF signal to generate acoustic waves in a TeO
2
crystal. A light source transmits a beam of light through the crystal, and the interaction between the crystal and the RF signal splits the beam of light into three beams: a center beam of unaltered white light and two beams of monochromatic and orthogonally polarized light. A sample is placed in the path of one of the monochromatic beams and detectors are positioned to detect light which either is reflected by the sample (to obtain a reflectance spectra) or is transmitted through the sample (to generate a transmittance spectra). The wavelength of the light source is incremented across a wavelength band of interest by varying the RF frequency. The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
In grating monochrometer spectrometers, a light source transmits a beam of light through an entrance slit and onto a grating element (the dispersive element) to disperse the light beam into a plurality of beams of different wavelengths (i.e., a dispersed spectrum). The dispersed light is then reflected back through an exit slit on to a detector. By selectively altering the path of the dispersed spectrum relative to the exit slit, the wavelength of the light directed to the detector can be varied. The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis. The width of the entrance and exit slits can be varied to compensate for any variation of the source energy with wavenumber. This approach lends itself to reflectance spectrometry.
Dual-beam spectrometers split radiation from a source into two beams, half passing into a sample-cell compartment and the other half into a reference area. The reference beam then passes through an attenuator and on to a chopper, which is a motor-driven disk that alternatively reflects the reference or transmits the beam to a detector. After dispersion by a prism or grating, the sample-cell beam passes to the sample and a detector detects the transmittance that passes through the sample or reflectance that reflects from the sample. If the two beams are identical in power, the detectors transmit similar electrical signals to a null detector. The null detector in turn produces an unfluctuating direct current. However, if the two beams differ in power, the detectors transmit differing electrical signals to the null detector. In this case, the null detector produces a fluctuating electrical current, which is used to generate the spectral data. For example, the fluctuating current can be used to drive a synchronous motor in one direction or the other depending upon the phase of the current; with the synchronous motor mechanically linked to a pen drive of a recorder, which the synchronous motor causes to move to generate the spectral data. This approach lends itself to both transmittance and reflectance spectrometry.
Detectors used in spectroscopy generally fall into two classes, photographic detectors, in which radiation impinges upon an unexposed photographic film, and electronic detectors, in which the radiation impinges upon a detector and is converted into an electrical signal. Electronic detectors provide the advantage of increased speed and accuracy, as well as the ability to convert the spectral data into an electronic format, which can be displayed, processed, and/or stored. Examples of electronic detectors include photomultiplier tubes and photodetectors. Photomultiplier tubes are quite sensitive, but are relatively large and expensive. Photodetectors provide the advantage of reduced size and cost. These detectors include IR detectors, pin diode detectors, charge coupled device detectors, and charge injection device detectors.
Conventionally, spectroscopic detectors are configured either as a single detector, flat detector, or a plurality of discrete detectors arranged in common plane (e.g. a flat array). In either case, these “flat” detector arrangements inherently detect only a 3% portion of the transmitted or reflected spectral data for 1 cm
2
detectors at a 2 cm distance from the source detector.
As described in Bums & Ciurczak, HANDBOOK OF NEAR-INFRARED ANALYSIS, pp 42-43 (1992), detectors for measuring diffuse reflectance are known which include either two or four opposing detectors arranged at a 45 degree angle from the sample. In general, PbS detectors are used for measurements in the 1100-2500-nm region, whereas PbS “sandwiched” with silicon photodiodes are most often used for visible-near-infrared applications (typically 400-2600 nm).
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