WO1982003460A1

WO1982003460A1 - Application of optical fibre probes

Info

Publication number: WO1982003460A1
Application number: PCT/AU1982/000046
Authority: WO
Inventors: Scient Ind Res Org Commw
Original assignee: Coogan Clive Keith
Priority date: 1981-03-31
Filing date: 1982-03-31
Publication date: 1982-10-14
Also published as: EP0074976A1

Abstract

A method for determining the turbidity or scattering power of a liquid utilises an optical fibre probe (10) which includes two or more optical fibres terminating at its tip and being grouped and branched (14, 15) into input and output fibres. The tip (12) of the probe (10) is inserted into the liquid (8) and known light (16) applied to the input fibres (14) for transmission to and emission from the tip (12). The reflected and/or scattered light returned along the output fibres (15) is then monitored (20, 22, 24) to obtain a measure of the turbidity or scattering power of the liquid. Also disclosed is a refractometer probe comprising a like optical fibre probe (10") fitted with means (46, 50, 50') for forming or for defining a reference volume (46, 54, 54') in front of the tip of the optical fibre probe. The arrangement is such that, in use of the probe, light passing from the tip is reflected to one or more output fibres (45) from at least one interface between the reference volume and a juxtaposed test medium (48, 52, 52').

Description

"APPLICATION OF OPTICAL FIBRE PROBES"

TECHNICAL FIELD

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. By exposing the remote end of the input fibres to a known light source and then employing suitable instrumentation to compare the input light and the reflected or scattered light detected in the output fibres, conclusions may be reached concerning the environment of the tip of the probe. BACKGROUND ART

Optical fibre probes per se and applications for them, are described, for example, in 46 Rev. Sci. Instrum. (1975) 879, in 51(3) Rev. Sci., Instrum. (1980) 377, and in United States patents 3906241, 4040743 and 4152075.

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, by Grojean and Sousa, 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 .

Finally, 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. In accordance with a first aspect of this invention, it has been appreciated that 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.

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. Moreover, windows to the sampled liquid become clouded and are often difficult to clean without undesirable disruption. Indeed the only practical method of measuring the turbidity of substantially opaque, highly turbid liquids is to sufficiently dilute the liquid . This inconvenience is necessarily tolerated, for example in the determination of the butterfat content of milk, but in some cases dilution disturbs the nature of the suspension of the solids in the liquid and is thus doubly undesirable. In general, conventional nephelometers are relatively expensive and are not well suited to prompt, on site measurement of liquid turbidities.

DISCLOSURE OF THE INVENTION

In accordance with a first aspect of the invention, there is provided 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 . According to .one approach, 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. Alternatively, the incident light may traverse a filter and the detector may include a frequency selective precision rectifier. Advantageously, 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.

According to a second aspect of the invention, there is provided 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. Alternatively, 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:-

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; and Figures 3, 4 and 5 are schematic sectioned illustrations of various forms of refractometer probe according to the invention.

MODES FOR CARRYING OUT 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.

Light returned along fibre bundle 15 is monitored by 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. On calibration for particular applications to take account, inter alia, of optical and electrical losses, and where appropriate of response non-linearities, the reading on meter 26 can be rendered indicative of particular parameters of interest at the pole tip.

In an alternative arrangement, 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. To ensure the latter constancy, 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 θ , 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. Analogously, it may be assumed that detector fibre 15b collects light uniformly effectively if the angle of incidence to its mouth is less than the same semi-angle θ and does not transmit light back at all if the semi-angle is greater than θ. 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.

A mathematical analysis of the response of the probe can be derived from this elementary pencil geometry. We first consider a volume element defined as that part of a spherical shell between radius r and r + dr which lies within the illumination/detection cone. This volume element will be given by Ω r² dr where Ω is the solid angle of the cone. Thus the scattering probability of essentially retro-scattering

(back along the path of the incident light) from particles within the volume element will be .proportion to Ω r².dr, assuming weak scattering only, so that the incident illumination is effectively not diminished by scattering. On the other hand the intensity of the light incident on a particle will be proportional to r ^-2 and. the light from the particle falling on the mouth of the detector fibre will also be diminished as r^-2. Thus the total scattering from particles within the volume element so defined will be proportional to Ω, r². dr r ^-4 or Ω. r ^-2. dr.

Extension of this elementary analysis to the real case entails more complicated mathematics which have due regard to the presence of multiple, spaced fibres of finite diameterand finite separation. However, it has been realized by employing this model, and by more detailed graphical analysis, that a region very close to the tip of probe 10 will substantially determine the amplitude of light scattered and reflected to the output fibres and thereby dominate the response of instrument 9, and therefore that the instrument is especially suitable as a nephelometer for turbidity or scattering power measurements on substantially opaque, highly turbid liquids, exemplified by determination of the butterfat content of milk. As already discussed, traditional turbidity measurement techniques rely upon a substantial optical path through the liquid; 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.

In practice, 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. It will be seen that with appropriate calibration of instrument 10 of Figures 1 and 2, 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. In an adaptation, 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.

In addition to the special suitability of the inventive method to substantially opaque liquids, it will be appreciated that the method can be readily performed with portable instrumentation in the field. The previous need for sampling and dilution of the liquid is dispensed with. The probe is easily inserted into and withdrawn from the liquid and it is possible to inspect the probe for adhered matter, clean and reinsert it without disruption to the liquid itself or to any ongoing process. The method involves direct detection of reflected and/or scattered rather than unscattered light, which approach is preferred for sensitivity of response. Figure 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.

Light emerging from an emitter optical fibre is reflected (somewhat) from the glue/fibre interface and the glue/wafer interface, but as both are only microns from the fibre, any reflected light passes back along the emitter fibre only and does not reach any of the detector fibres.

When the probe is inserted tip-first into a liquid 48, it will be seen that light passing from the tip of the probe must transverse wafer 46 prior to impinging the liquid. Light striking the second surface of the glass plate, which is the liquid/wafer interface, is reflected so that some enters the detector fibres.

It will be appreciated from consideration of the optics that 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. In general, 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.

Now, the reflectance at normal incidence from a particle (or a plane face element of a particle) of a non-absorbing material is given by the Fresnel formula

wherein n. is the refractive index of the liquid and n₂ is the refractive index of the solid particle.

It can be demonstrated that there is little change in the total reflectance for unpolarized light for an angle of incidence up to 40º.

It has already been noted that the maximum angle of incidence likely to be encountered is 20°. Hence the equation

is a good approximation within the angles of reflectance likely to be encountered for a non-absorbing wafer 46 and non-absorbing liquid. Thus 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.

In practice, for a given wafer material, say glass, 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. By choice of the right glass (or other suitable medium) to match a desired refractive index in a system, a null can be given at the refractive index required for the test liquid, and deviations only are thus recorded. Alternatively, 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.

In the embodiment of Figure 3 , it will be appreciat that the material of wafer 46 serves as a reference volume while the liquid 48 is of a test medium.

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.

Claims

CLAIDS

1. A method for determining the turbidity or scattering power of a liquid, comprising inserting the tip of an optical fibre prcbe 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.

2. A method according to claim 1 further including modulating the applied light, and utilising for said monitoring means sensitive to such modulation.

3. A method according to claim 2 wherein the applied light is modulated by activating a light emitting device by means of an oscillator, and wherein said monitoring then includes the use of a phase sensitive detector, or of a frequency selective precision rectifier, arranged to receive the reflected and/or scattered light returned along the output fibres.

4. A method according to claim 1, 2 or 3 utilising a probe in which said fibres are grouped in a predetermined orderly array whereby to facilitate determination of the characteristics of the probe.

5. 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.

6. A refractometer probe according to claim 5 wherein said means is such that the reference medium is of a thickness of the order of several times the diameter of the individual optical fibres.

7. A refractometer probe according to claim 5 or 6 wherein said means is adapted to mount a test medium and the probe may be incorporated in the aforesaid optical instrument.

8. A refractometer probe according to claim 5,6 or 7 wherein said means form a reference volume comprising a wafer of glass or plastics material of known refractive index which substantially does not absorb the light employed.

9. A refractometer probe according to any one of claims 5 to 8 wherein said means from a reference volume secured to the tip of the optical fibre probe by means of a transparent adhesive.

10. A refractometer probe according to claim 5,6 or 7 wherein said means comprises a mount for the test medium, which mcunt defines an open space of determinable width immediately in front of said tip between the tip and the test medium.