Classifications

• • 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
  • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
  • G01N21/43—Refractivity; Phase-affecting properties, e.g. optical path length by measuring critical angle
  • G01N21/431—Dip refractometers, e.g. using optical fibres
• • 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
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N21/49—Scattering, i.e. diffuse reflection within a body or fluid
  • G01N21/53—Scattering, i.e. diffuse reflection within a body or fluid within a flowing fluid, e.g. smoke

Definitions

• This invention is concerned with novel applications of optical fibre probes.
• An optical fibre probe as described herein, consists of an elongate, substantially parallel bundle of optical fibres grouped and branched as input and output fibres. Such a probe has been referred to as a bifurcated optical fibre bundle.
• the fibres terminate at a polished probe tip which constitutes a window across which light may be emitted from the input fibres and reflected or scattered light collected by output fibres.
• the first cited article by Bailly-Salins, describes the use of an optical fibre probe for measuring the velocity of the back surface of a vibrating work surface, specifically an irradiated target. Both ordered and random arrays of the fibres are disclosed; the monitored total output of the return branch is held to be a function of the separation of the probe tip and the plane of work surface. At the most appropriate separations, the response of the instrument is substantially linear with respect to separation.
• the second cited article proposes the measurement of luminescence in optically dense or turbid media.
• United States patent 3906241 to Thompson describes a technique for monitoring Raman scattered radiation.
• the probe contains three fibre optic elements, each comprising single or multiple fibres: a beam of radiation is transmitted across the probe tip from one element to another under the third and the detector includes a filter, photodiode and amplifier.
• United States patent 4040743 to Villaume et al employs a four channel probe to determine the brightness and consistency, or fibre density, of pulp slurry. Back scattered, orthogonally reflected and transmitted energy is detected and first compared with the incident energy. By calculating certain ratios, inter dependent variables can be reduced to one measured variable .
• United States patent 4152075 to Rellstab et al proposes a pair of probes, each including a diode/amplifier detector for comparing an experimental fluid with a standard. Each probe tip is fitted with a fixed reflector, since transmitted light is of interest.
• optical fibres probes may advantageously be employed in nephelometers for the determination of the turbidity or light scattering power of substantially opaque, highly turbid liquids.
• nephelometer In the most widely used classical form of nephelometer, a beam of incident light is passed through the test liquid so that some light is scattered by particles suspended in the liquid. The light which is not scattered, or is scattered through a very small angle only, continues onto a transmission photocell detector. The ratio of the detected to the incident intensity, taking into account the path length through the liquid, is considered to be a reliable measure of the turbidity of the liquid, especially for medium scattering power. Alternatively, especially in the case of weak scattering, the scattered light may be directly measured generally orthogonally to the incident light. However, either approach relies upon one or more extended optical paths through the liquid and accordingly is only suitable for determining the turbidity of relatively clear or translucent liquids.
• a method for determining the turbidity or scattering power of a liquid comprising inserting the tip of an optical fibre probe into the liquid, which probe includes two or more optical fibres terminating at said tip and being grouped and branched into input and output fibres, applying known light to the input fibres for transmission to and emission from said tip, and monitoring the reflected and/or scattered light returned along the output fibres, whereby to obtain a measure of the turbidity or scattering power of the liquid.
• This may entail direct or indirect comparison of the incident and reflected light.
• the method may further include modulating the applied light, and utilising for said monitoring means sensitive to such modulation.
• Light may suitably be applied to the input fibres by a light-emitting device such as a light-emitting diode (LED).
• the reflected light may be monitored by way of a photosensor such as a photo-diode or PIN diode and a comparison made between an amplified output signal of this diode, representative of the reflected light, and the activation signal applied to the LED, which latter signal is arranged to be representative of the applied light .
• the applied light is modulated by activating the light-emitting device by means of an oscillator: said comparison can then be effected by a phase sensitive detector.
• the incident light may traverse a filter and the detector may include a frequency selective precision rectifier.
• the method utilises a probe in which the fibres are grouped in a predetermined orderly array rather than in an arrangement in which the input and output fibres are randomly interspersed as, inter alia the characteristics of a probe with an orderly array of fibres are more readily and reliably calculable.
• a refractometer probe comprising an optical fibre probe which includes two or more optical fibres terminating at a tip and being grouped and branched into input and output fibres, and means for forming or for defining a reference volume in intimate contact with said tip of the optical fibre probe whereby light passing from said tip must traverse the reference volume prior to impinging a test medium.
• the reference volume may be a wafer of glass or plastics material of known refractive index which substantially does not absorb the light employed.
• the reference volume may be secured to the tip of the optical fibre probe by means of a transparent adhesive, and is advantageously of a thickness of the order of several times the diameter of the individual optical fibres.
• said means may comprise a mount for the test medium, which mount defines an open space of determinable, perhaps variables width immediately in front of said tip, between the tip and the test medium.
• Figure 1 is a schematic illustration of an optical instrument for measuring the turbidity of a liquid in accordance with the invention
• Figure 2 is an optical ray diagram for the simplest optical fibre probe having single input and output fibres
• Figures 3, 4 and 5 are schematic sectioned illustrations of various forms of refractometer probe according to the invention.
• the optical instrument 9 depicted in Figures 1 and 2 includes an elongate optical fibre probe 10 which includes a multiplicity, say 128 in toto, of substantially parallel optical fibres terminating at a tip 12.
• the fibres are grouped and branched into equal numbers of input and output fibres : these are represented for convenience of illustration in Figure 1 as discrete input and output fibre bundles 14, 15 but it is generally preferred that the individual input fibres and output fibres can be intermingled and arranged in an orderly array.
• Tip 12 is finely polished and constitutes a window across which light is emitted from input fibres 14a and across which light returned to the tip by, for example, scattering- particles in a liquid 8 is collected by output fibres 15a.
• Light is applied to fibre bundle 14 by electrically actuable means comprising a light emitting device such as a light-emitting diode (LED) 16 connected by lines 17 for activation by an oscillator 18.
• LED 16 thus emits a modulated wave of light intensity.
• the LED is associated with suitable focussing means (not shown) for directing substantially all of its emitted light into the fibres of bundles 14, so that the signal on lines 17 is representative of the light amplitude applied to the fibres.
• Photodetector means including, for example a photodiode 20 which may suitably be a PIN diode, coupled to a pre-amplifier 22.
• the output signal of pre-amplifier 22 is representative of the amplitude and phase of the modulation of the light output from fibre bundle 15.
• This signal is fed on line 23 to a phase sensitive detector 24.
• Detector 24 which may be clocked from lines 17, correlates the two signals and outputs to a meter 26 a further signal indicative of the intensity of the characteristic chopped light.
• the amplitude modulation of the input light assists in discriminating the primary signals; the ultimate interest is the d.c. or averaged level of the detected signal.
• the reading on meter 26 can be rendered indicative of particular parameters of interest at the pole tip.
• the signal on line 23 is fed to a frequency selective precision rectifier, which outputs a d.c. reading proportional to the intensity of light received at diode 20 for constant incident light.
• the intensity of the incident light is controlled, e.g. to be substantially temperature independent.
• Figure 2 is an optical ray diagram for the simplest probe configuration comprising a single input or emitter optical fibre 14b and a neighbouring output or detector optical fibre 15b. Light emitted may be assumed to be in a uniform cone of illumination 30 with semi-angle ⁇ , in which the intensity of the light is diminished as r -2 assuming no scattering, where r is displacement from the mouth of emitter fibre 14b.
• detector fibre 15b collects light uniformly effectively if the angle of incidence to its mouth is less than the same semi-angle ⁇ and does not transmit light back at all if the semi-angle is greater than ⁇ . This means that the only possibility of detection of reflected or scattered light is from particles lying within both the cone of illumination and the cone of detection. If the diameter of fibres 14b, 15b and their distance apart from both small compared with the distance r p of a scattering particle P from their now virtually co-incident mouths, then for practical purposes the cones of illumination and detection become co-incident.
• the method of the invention utilises only a very small portion of liquid immediately in front of the probe, which portion is sufficiently small to be effectively transparent to the illumination and reflection light cones.
• Other possible applications of the instrument per se include the determination of blood count, engine oil purity, surface lustre after or during painting, and water particulate pollution levels.
• the instrument could also be adapted as a smoke detector, a colour matcher or a device for checking solar reflectance of a surface.
• the response from a region very close to the probe tip where the cones of illumination and detection do not overlap, say within a distance of the order of the mean fibre diameter, is comparatively negligible. Accordingly, it may be preferred to mask this region by occupying it with an optically transmissive wafer to prevent attenuation of light by turbid liquid which does not contribute to the ultimate response.
• This wafer preferably extends to just short of the intersection of the cones of illumination and detection, so as not to introduce extraneous reflections from the outer interface of the wafer.
• meter 26 can afford a direct reading of the scattering power or reflectance of the solution/suspension 8, or more specifically of a specific turbidity parameter such as butterfat content of milk.
• the wavelength of the incident radiation may be selectively variable to permit approximate analysis by size of the particles in a liquid under test. This adaptation may be especially useful in respect to highly turbid liquids with particles of diameter of the order of or less than ly.
• FIG. 3 schematically depicts an optical fibre refractometer probe 10", having input and output optical bundles 44, 45 fibre, modified in accordance with the second aspect of the invention for use as part of a continuous reading refractometer.
• the probe, and the instrument as a whole, are substantially as described with reference to Figures 1 and 2 but for the additional incorporation of a reference volume formed by a thin solid wafer 46, typically a glass or plastics, substance, secured in intimate contact with the tip of the probe by means of a transparent adhesive or by being retained on a screw-on cap adaptor.
• the thickness of wafer 46 is of the order of several times the diameter of the individual optical fibres making up bundles 44, 45.
• the angle of reflectance from a scattering particle in the liquid about probe 10" can not exceed the semi-angle of emittance/detectance, which for plastic optical fibres is of the order of 30° emerging from a plane surface into air. For quartz fibres, this angle is further reduced.
• the maximum angle of reflectance is the order of 20 or less in solutions or suspensions such as are likely to be encountered in industry or laboratory.
• n. is the refractive index of the liquid and n 2 is the refractive index of the solid particle.
• the refractive index of the liquid can be determined by measurement of the output from the detector fibres and a knowledge of the input light and of the refractive index of the material of wafer 46.
• the instrument of Figures 1 and 2 but with the probe 10" substituted for probe 10 can be calibrated for application as a refractometer by use of liquids of known refractive index.
• a null can be given at the refractive index required for the test liquid, and deviations only are thus recorded.
• a glass may be chosen with index well away from the required index, so that deviations give an approximately linear response rather than a square law response.
• the second surface of the wafer 46 may be frosted, whereupon there may be an enhancement of response with respect to the stray light in. the liquid.
• Figure 4 depicts an alternative embodiment of refractometer probe having means 50 for mounting a plane-faced test solid 52 so as to define a reference volume 54 for a fluid of known refractive index.
• the probe can be inserted into a liquid and the refractive index of the solid determined.
• Figure 5 shows a still further refractometer probe in which a powdered test solid 52' is placed at a fixed distance from the tip of the probe, and thus from the fibre mouths, by being coated on a rectangular spacer tube 50'. Tube 50' also defines an open-ended reference volume 54' for fluid of known refractive index. In both cases, the fluid can be air.
• An important advantage of the described refractometer is the possibility of continuous indication, which ismost useful for process solutions, e.g. sugar in the food industry.

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• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
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• Immunology (AREA)
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• Investigating Or Analysing Materials By Optical Means (AREA)

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• 1982
  • 1982-03-31 EP EP82900919A patent/EP0074976A1/de not_active Withdrawn
  • 1982-03-31 WO PCT/AU1982/000046 patent/WO1982003460A1/en unknown

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