Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0272—Handheld
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0289—Field-of-view determination; Aiming or pointing of a spectrometer; Adjusting alignment; Encoding angular position; Size of measurement area; Position tracking
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0291—Housings; Spectrometer accessories; Spatial arrangement of elements, e.g. folded path arrangements
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
  • G01J3/108—Arrangements of light sources specially adapted for spectrometry or colorimetry for measurement in the infrared range
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/12—Generating the spectrum; Monochromators
  • G01J3/1256—Generating the spectrum; Monochromators using acousto-optic tunable filter
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
  • G01N21/3563—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing solids; Preparation of samples therefor

Definitions

• the present invention relates to devices for analyzing a material according to its optical reflectance or transmission spectrum.
• Plastics and many other materials can be identified by their infrared (IR) reflectance or transmission spectrum. Each type -- nylon, polyethylene, etc. -- has its own IR characteristic spectrum. If a generally constant-intensity IR beam incident on a plastic is scanned through a range of wavelengths, and the intensity of the reflected or transmitted light is measured as a function of the wavelength, then the measured spectrum can be used to identify the type of plastic.
• IR infrared
• mixtures of plastics or other materials can be quantitatively analyzed.
• the reflectance or transmission spectrum of a sample can show that it is, for example, 50% nylon and 50% polyethylene.
• the proportion of octane in a sample of gasoline can be measured, or the amount of fat in chocolate bar.
• IR spectrometers Some use diffraction grating or FTIR technology; these are bulky, delicate, and slow. They are not suited to rapid identification of plastics, use in various locations such as in the field, or for handheld use.
• AOTF acousto-optical tunable filter
• the acousto-optic tunable filter is based on a birefringent crystal, such as a crystal of Te0 2 (tellurium dioxide) which acts as an electronically tunable narrowband filter, in which diffraction results from an acoustic pressure wave in the crystal.
• the compression or pressure inside the crystal varies as the wave passes, causing a periodic variation in the refractive index.
• crystal compression varies, so does the birefringence of a beam of unpolarized visible light or IR that passes through the crystal in a direction normal to its entry and exit faces.
• the crystal acts as an optical filter passing that light or infrared having a wavelength proportional to the acoustic wavelength.
• the birefringent crystal acts as a frequency-selective narrowband optical filter, and sound of any acoustic wavelength can be passed through the crystal, any desired visible or IR wavelength can be selected at will, just by varying the frequency of an acoustic driver.
• the acoustic driver is a second crystal of the piezo-electric type (quartz or lithium niobate, LiNbo) , which is an acoustic transducer.
• a piezo crystal changes its size when subjected to an RF field.
• Birefringent Te0 2 bonded to piezo-electric LiNo in which the LiNo is subjected to a sinusoidally-varying AC voltage applied across the face parallel to the birefringent crystal, will act as a swept-frequency optical filter.
• the AC voltage impressed across the piezo crystal is at high radio-frequencies (RF) of 20-100 MHz, the acoustic wavelength corresponds to infrared (IR) light wavelengths.
• RF radio-frequencies
• the impressed voltage may be obtained from digital synthesizer, controlled by a software algorithm which determines the frequencies generated, and which can sequentially scan or hop in a random access fashion.
• Broad-spectrum white light from a halogen lamp, for example
• Typical IR wavelengths selected by the AOTF filter are from 1-3 microns (near infrared) or from 2-5 microns (mid-infrared) .
• the tuned infrared beam can then be either reflected from, or transmitted through, a sample to determine the spectrum and identification of the sample.
• the swept-frequency beam of light is made to shine onto a surface of the undetermined material, which will reflect different proportions of the light falling onto it at each of the various frequencies.
• a photodetector can be used to pick up the reflected light and turn it into an electrical signal.
• Electronic circuits can then plot the pattern of the material's reflectance of IR or light frequency, and use that pattern to identify the material by matching the pattern with known patterns corresponding to various materials.
• IR spectrometers can measure the proportion of a compound in a sample, by calibrating the circuitry to recognize samples having various percentages of compounds. The percentage can also be calculated according to Beer's law.
• the AOTF spectrometer Compared to other spectrometer instruments such as diffraction gratings and the FTIR, the AOTF spectrometer has the advantages of no moving parts, high speed wavelength tuning, and small size.
• previous AOTF spectrometers have consisted of a fairly bulky and heavy electronics and optical modules, so that the instrument cannot be portable and handheld or low cost.
• fiber optics it has been necessary to use fiber optics to pass the light between the instrument and the sample.
• the IR coming out of the AOTF is passed into an optical fiber or bundle of fibers, which traps the light and passes it along the bundle, but with substantial loss, especially at longer IR frequencies.
• Optical fibers can pass visible light with little attenuation, but IR is strongly absorbed.
• the end of the fiber or bundle can then be placed near the surface of the material, and reflected light picked up by other fibers of the bundle to be conveyed back to a photo-detector.
• optical-fiber arrangement allows the light-emitting end to be placed easily at any point, it has several drawbacks in addition to the aforementioned inefficiency in transporting IR.
• optical fibers are expensive.
• they are fragile.
• the coupling between a light source, such as the lamp/AOTF, and the fiber is very inefficient . Only a small fraction of the available light can be conveyed into and along the fibers.
• optical fibers are quite small the light they convey scatters widely from their ends, and is dispersed, so still more ' light is lost even if the end of the fiber bundle is placed almost against the surface of the material under test.
• the high losses of optical fibers require a very bright lamp in front of the AOTF, of 50 to 100 W. These high wattage lamps typically require cooling fans. The high- wattage lamps and fans in turn require a larger power supply, and the larger power supply may require heat dissipating fin plates or fans of its own, which then draw still more power and increase the size and bulk still further.
• the frame or housing must be larger and heavier to support the additional parts. Expense is increased and portability decreased.
• Prior-art AOTF spectrometers are locked into a "catch-22" size and weight restriction.
• the bulk of the lamp/AOTF units prevents them from being held up to a sample, so fiber optics are used to convey the IR from the housing to the sample; but since fibers waste energy, the housing must be large and heavy.
• Previous workers in the field have not achieved a portable AOTF spectrometer because they have not realized the root of the size/weight problem.
• AOTF spectrometers Another, related drawback of conventional AOTF spectrometers is that RF power is delivered to the piezo crystal via a coaxial cable from a power amplifier located in a separate housing. This arrangement wastes electrical energy, both by attenuation in the cable and also because of losses due to impedance-matching the RF amplifier to the cable, and then the cable to the crystal. (The cable typically has an impedance of around 50 ohms, but the piezo crystal impedance is much lower and varies with frequency. ) Moreover, previous AOTF spectrometers have applied more RF power to the AOTF crystal than was needed.
• the power supply must include circuitry to transform 120-volt AC, and a power line cord and plug must be provided. The unit then is still more difficult to transport and use.
• the present invention has an object, among others, to overcome deficiencies in the prior art such as noted above. Another object is to provide a spectrometer as a handheld, portable unit.
• a further object is to achieve such a portable unit by optical alignment of all optical elements.
• Yet another object is to achieve the first object by placing elements adjacent one another and eliminating cables and optical fibers.
• Still another object is to reduce the unit cost of an AOTF spectrometer.
• a yet further object is to contain all elements within a housing and provide a window in the housing acting as a spacing element for the aligned optical elements.
• the invention thus relates to a portable, hand-held AOTF spectrometer which is lightweight and small enough to be carried into the field for analyzing samples.
• One application includes identification of plastic waste for recycling purposes.
• the device can be brought in contact with the plastic waste and a measurement of the spectrum can be obtained in under one second for identifying the plastic.
• the hand-held AOTF spectrometer consists of an enclosure approximately the size of a video camera, and containing a small optical bench (with light source, AOTF crystal, and reflectance detector) and a small circuit board containing all electronics (including a frequency synthesizer, A/D converter, detector preamplifier, noise reduction circuit and computer interface) .
• a small RF amplifier is located in close proximity to the AOTF crystal, for better impedance matching and lower power consumption; by "close proximity", what is meant is no more than about 5 or 6 cm, and preferably directly adjacent one another as a single unit.
• the reflectance detector consist of several flat lead sulfide detectors, for example, which are facing the sample of interest in order to detect the diffusely reflected light.
• the computer can either be external or internal to the device. Battery operation is offered as an option. Since no fibers are used, the signal is larger and the device less expensive.
• Reducing the RF power to about 1 watt allows the use of a smaller RF power amplifier with less power consumption, and makes it possible to mount it next to the crystal 122, and that mounting position eliminates the need for a coaxial cable which further decreases the power needed.
• This reduction in drive power reduces the optical signal from the AOTF, since the AOTF typically needs between 1 and 3 watts to reach its maximum efficiency.
• the applicants have discovered a fact that was unknown to previous workers in the field, namely that when the drive power is reduced, the noise level is also reduced due to a reduction in electro-magnetic interference (EMI) with the lower wattage amplifier (especially when the detector preamp is located close to the RF amp) . Therefore, the signal to noise ratio remains approximately the same with the smaller amplifier as it was with prior-art large amplifiers, and operation of the device is not hindered. This unexpectedly high signal to noise ratio is what allows the RF amplifier to be made smaller, which in turn permits a handheld spectrometer.
• EMI electro-magnetic interference
• the invention places a pyramid-type detector, having a generally hemispherical shape, immediately adjacent to the area on the sample where the modulated IR beam impinges.
• the light collection efficiency is improved and multiple detector elements (cells) can be deployed inside the pyramid. This is not possible with fiber optics, where the light collection fibers are led to a single detector element.
• the invention saves power by using a frequency doubler. Previous spectrometers have used up to a 100-MHz frequency generator for driving the AOTF piezo crystal. The invention uses a 50 MHz or smaller generator which draws much less power, and then passes the 50 MHz signal through a frequency doubler.
• the doubler creates harmonics, which in turn cause the AOTF to pass optical harmonics, which are in the visible light range; this is the reason why previous workers did not use the doubler.
• a window is used which only passes IR, and blocks visible rays. Since the light makes two passes through the window in reaching the detector, the visible light is strongly attenuated and does not interfere with operation.
• the preferred frequency generator is a digitally-controlled synthesizer, which is faster, easier to control, and has much better stability and resolution than a voltage-controlled oscillator.
• the computer software can analyze the spectrum to determine the type and quantity of the sample, using previously stored calibration algorithms.
• the computer can then display the identification of the plastic, for example, PVC or polystyrene, for recycling purposes or for quality control purposes.
• Fig. 1 is a partially schematic elevational view of the invention.
• Fig. 2 is a perspective, phantom view of the invention.
• Fig. 3 is a graphical view of a light reflectance spectrum for polystyrene.
• Fig. 4 is a plan view of a realized embodiment of the invention.
• Fig. 5 is an elevational view of the embodiment of Fig. 4.
• light means all electro-magnetic waves that can be produced, detected, or controlled by optical means, and includes infrared (IR) , visible light, or ultraviolet (UV) unless otherwise specified;
• IR infrared
• UV ultraviolet
• pyramid detector means any detector with one or more light-to-electricity converting transducers deployed adjacent to a hole;
• Fig. 1 shows the interior of the handheld AOTF spectrometer according to the present invention, used to identify an unknown material M.
• the spectrometer is housed in a molded plastic enclosure or housing 10 which includes a window 15, handle 19 with trigger 17, and battery compartment for batteries B inside the handle 19.
• the window 15 is preferably one which is opaque or at least partially opaque to visible light, but transparent to IR.
• the housing 10 is approximately the size and shape of a handheld video camera, and can include a handle.
• the trigger 17 is used to power the unit to or circuits of the spectrometer for making a measurement.
• a proximity sensor 18, such as a small sonar device or a momentary-contact switch for pressing against the material M, may be used in place of the trigger 17.
• a computer 202 for data analysis and display can either be built into the device as shown or connected remotely using a serial or parallel port 232.
• a computer accessed through the port 232 can also augment the on-board computer 202.
• the device can either be plugged into the wall by a power cord (not shown) , but is preferably operated by batteries B.
• the optical module consists of several optical components mounted on the bench 100, which is preferably a solid plate, e.g., formed of aluminum, approximately eight inches long by five inches wide secured inside the plastic housing.
• the optical components include the following elements, provided in a linear relationship: a light source or lamp 110 (a tungsten-halogen lamp, for example) ; an AOTF crystal and case 120; a focusing lens 130; and a reflectance detector 140.
• the AOTF crystal preferably including Te0 2 (tellurium dioxide) , is about one inch long and one-half inch wide.
• the AOTF 120 includes a piezo-electric transducer, preferably of LiNo, bonded to one face of the bi-refringent Te0 2 crystal.
• a small RF power amplifier 124 is mounted in close proximity to the crystal 122; it produces about 1 Watt of RF power in the frequency range from 20 to 100 MHz.
• the lamp 110 is contained within a parabolic mirror 112 in order to collimate the beam.
• This beam then passes through the AOTF crystal 120, and emerges as a tuned, narrow-band infrared beam approximately 8 by 8 mm in size.
• This beam passes through the lens 130 , which focuses the beam through the window 15, onto the sample M to be analyzed.
• a reflectance detector 140 At the end of the optical bench is mounted a reflectance detector 140.
• This detector may include up to four or even more lead sulfide (PbS) or lead selenide (PbSe) flat detector elements or transducers 145, each about 10 by 10 mm in size and facing the sample through the window 15.
• the detectors are arranged on the inner surface of a 45 degree pyramid or cone.
• Fig. 4 shows a realized embodiment of the invention, with a carrying strap 19', and some other elements shown in Fig. 1.
• Fig. 5 is an end view of a realized embodiment of Fig. 4, showing the hole 143 and detector elements 145 as seen from outside the housing 10.
• the housing includes the window 15, which in the preferred embodiment is a transparent element having broad-band IR transmission but little visible transmission; it appears black.
• the optical properties of the window 15, like those of the other optical elements, are compensated for automatically when the device is calibrated using a pure white ceramic material.
• the window 15 is set at the end of an optional housing extension 13 ; the extension 13 may be placed directly against the sample M surface to obtain the preferred spacing of the surface from the optical bench 100 and its components, so as to maximize the light efficiency.
• the small printed circuit board 200 mounted above the optical bench 100 contains all of the system electronics 204, including: a digitally-controlled frequency synthesizer (used to generate the RF frequencies to tune the AOTF) , a detector preamplifier and bias voltage, an A/D converter, and computer interface (e.g., RS-232) .
• a digitally-controlled frequency synthesizer used to generate the RF frequencies to tune the AOTF
• a detector preamplifier and bias voltage e.g., RS-232
• computer interface e.g., RS-232
• amplitude modulator (and de-modulator) circuit which modulates the RF signal at about 5 Khz for improved signal to noise ratio.
• the frequency synthesizer is preferably a lower-frequency generator (e.g., up to 50 MHz) driving a doubler; this arrangement uses less power. Having the circuit board in close proximity to the detector and optics has several advantages: smaller size, lower noise, and less expense since fewer cables and connector
• a microcomputer circuit 202 it is also possible to include, besides a microcomputer circuit 202, a keypad 206 (the back side is shown on the inside of the housing 100 in Fig. 1) ; and a display 208 on the housing, for displaying alphanumeric messages such as "polystyrene: 25%".
• a keypad 206 the back side is shown on the inside of the housing 100 in Fig. 1
• a display 208 on the housing for displaying alphanumeric messages such as "polystyrene: 25%”.
• the arrangement of the AOTF 120 and a built-in reflectance detector 140 on one single small optical bench leads to many advantages, as discussed below.
• the electronic circuitry is placed in a "sleep" mode during the "off" cycle of the AOTF amplitude modulation.
• the clock generator (oscillator) for frequency synthesis is made temperature dependent with the value of the AOTF temperature dependency, but with an opposite sign.
• the RF synthesizer output frequency linearly depends on the clock generator frequency.
• Both the clock and the AOTF are placed into the same small sealed case 120, close to each other, resulting in each having the same temperature variation. This compensates the AOTF thermal wavelength drift .
• the cost of parts is reduced.
• the assembly time is reduced. This enables the device to be produced in large volume at much lower cost compared to previous large and complicated AOTF spectrometers.
• the cost of in instrument according to the present invention is projected to be less than $15,000, or about three times cheaper than currently-available AOTF spectrometers.
• the straight-line optical alignment of the lamp 110, AOTF 120, lens 130, and the hole 143 of the detector 140, and their fixed optimal spacing from the window 15 of the housing 10, provide the maximum optical efficiency with less light loss than in the prior-art designs.
• the lens 130 is able to focus the maximum amount of the available light onto the sample in an area that is optimal for pickup by the detectors 145.
• the ultraviolet and visible reflectance spectrum may also be obtained with the device.
• Applications include, for example, determining the color of paint, vegetation, and minerals in the field.
• the transmission spectrum of a sample may be obtained using the device by placing a mirror in back of the sample to reflect the transmitted beam from the AOTF back again through the sample and onto the detector.
• Applications include, for example, identification of clear plastic bottles or liquids of various types. (We have used this method to identify discarded soda bottles.)
• Fig. 4 An operational system, shown in Fig. 4, was built and used to identify plastics for recycling purposes. Unknown plastic samples were test scanned with the instrument, which had been previously calibrated with a set of known samples and with a ceramic reference sample. The near-infrared diffuse reflectance spectrum was measured in under one second, the spectrum was normalized using the reference spectrum, and a software algorithm identified the type of plastic. Since the detector could be brought up directly to the sample, it was not necessary to use fiber optics, which resulted in a larger signal. Types of plastic that were test identified included PVC, polystyrene, polypropylene, PET, etc. The small size and portability of the device enabled it to be carried into the field.

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• 1996
  • 1996-08-30 EP EP96929884A patent/EP0847524A4/de not_active Withdrawn
  • 1996-08-30 JP JP51064197A patent/JP3181596B2/ja not_active Expired - Fee Related
  • 1996-08-30 WO PCT/US1996/014128 patent/WO1997008537A1/en active Application Filing

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