WO1996016326A1

WO1996016326A1 - Optical instrument

Info

Publication number: WO1996016326A1
Application number: PCT/GB1995/002727
Authority: WO
Inventors: Mark Johnson
Original assignee: United Utilities Plc
Priority date: 1994-11-23
Filing date: 1995-11-23
Publication date: 1996-05-30
Also published as: GB2310282A; GB2310282B; GB9710452D0; GB9423618D0; AU3878195A

Abstract

An optical measurement instrument for monitoring characteristics of a liquid such as water. An optical beam is generated and directed into the thread (7) of liquid such that the beam is guided along the length of the thread by total internal reflection. Components of the beam that are affected by variations in characteristics of the liquid defined by the thread are detected. These components may be derived from the thread by causing the thread to impinge against a window (8) behind which the detector (9) is located or by detecting back-scattered components of the beam. Relatively long interaction lengths between the beam and the liquid can be achieved.

Description

OPTICAL INSTRUMENT

The present invention relates to an instrument for monitoring characteristics of a liquid, and in particular to such an instrument which is capable of being used to monitor the characteristics of a liquid such as water.

It is known to use optical techniques to monitor the quality of water. The known techniques optimally require long interaction lengths, that is to say a long optical path through the liquid. For example, the weak optical absorptions of typical concentrations of pollutants in water mean that the total absorption of light in a cell of, for example, 10mm or 40mm length is relatively small. This limits the resolution of the instrument and hence the ability of the instrument to detect pollutants.

In principle light can be collimated and shone through arbitrarily large path lengths, but in practice this is not generally convenient. The optical paths are difficult to align, leading to optical instability. A better performance can be achieved using guided optical paths, for example by placing a water sample within a capillary tube, the refractive index of which is less than that of water. Unfortunately, materials with a sufficiently low refractive index are relatively rare and their characteristics make them unsuitable for the purpose. For example, some highly fluorinated polymers similar to PTFE can be used, as well as micro-porous materials having a refractive index which is an average of the material and the air enclosed within the materials. Some crystals such as lithium fluoride (LiF) also have a sufficiently low refractive index. Unfortunately all of these materials are difficult to form into capillaries with the required surface quality. Furthermore, even if capillary tubes with the appropriate characteristics could be made, the surfaces of the tubes would suffer from unacceptable fouling in practical circumstances. The fundamental problem with practical industrial measurement systems relying upon the transmission of an optical beam through a liquid is that of fouling.

It is an object of the present invention to obviate or mitigate the problems outlined above. According to the present invention there is provided an instrument for monitoring characteristics of a liquid, comprising means for forming an unsupported thread of the liquid, a source of radiation for producing a beam of radiation to which the liquid is transparent, means for directing the beam into the thread such that the beam is guided along the length of the thread by total internal reflection, and a detector for detecting components of the beam that are affected by variations in characteristics of the liquid defining the thread.

The present invention also provides a method for monitoring characteristics of a liquid, wherein an unsupported thread of the liquid is formed, a beam of radiation is produced to which the liquid is transparent, the beam is directed into the thread such that the beam is guided along the length of the thread by total internal reflection, and components of the beam that are affected by variations in characteristics of the liquid defining the thread are detected.

The thread may be formed as a freely falling jet issuing from, for example, an aperture in a container, or as a pressurised jet. The thread may be caused to impinge upon a transparent window, the detector being mounted directly behind the window or optically coupled to the window. Alternatively, the detector may be arranged to detect components of the beam that are back-scattered by the thread of liquid. The thread may be in the form of an upper thread of the liquid to be monitored within a sheath of liquid of lower refractive index.

The beam may be directed into the thread through a window in contact with the liquid, or alternatively may be directed into the thread through a free surface of the liquid.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which :-

Fig. 1 illustrates a first embodiment of the present invention;

Fig. 2 illustrates a modification of the first embodiment of the invention shown in Fig. 1; and

Fig. 3 illustrates a second embodiment of the present invention.

Referring to Fig. 1, a liquid such as water is introduced into a header tank 1 through an inlet 2. An upper surface of the header tank is provided with an optically transparent window 3 above which an optical source 4 is located. Coupling optics represented by a lens 5 produce a beam focused on an aperture 6 in the base of the header tank. Water within the header tank falls from the aperture 6 in the form of a continuous thread 7. The cross section of the thread 7 will not be stable, but nevertheless will be capable of transmitting the beam coupled into it along its length.

The thread 7 impinges upon a window 8 on the rear surface of which is mounted a detector 9. The detector 9 detects the beam transmitted along the length of the thread 7 and thus enables characteristics of the water making up the thread to be determined. The instrument described in Fig. 1 enables an optical beam to be guided along a path defined by the water which is of substantial length, for example 150mm. Furthermore, the described arrangement has an input numerical aperture of greater than 0.5. Thus, not only is the problem of surface fouling in the beam guiding arrangement avoided, but the optical signal may be efficiently coupled into the thread of water.

In the illustrated arrangement the water thread was simply formed by allowing water to exit through the aperture 6. For any such thread there is a length beyond which there is a high occurrence of break-up of the thread into droplets. Useful lengths of thread can, however, be readily produced. In the arrangement illustrated in Fig. 1, the thread is produced simply by relying upon gravity but it will be appreciated that the thread could be formed as a pressurised jet by appropriate pressurisation of the header tank, for example.

As an alternative to the detector arrangement shown in Fig. 1, the detector may be remote from the window 8. For example, as illustrated in Fig. 2 the window 8 could be replaced by a prism 10 and an optical system in the form of, for example, a lens 11, could be used to focus light emerging from the prism onto a detector. The detector could be of any appropriate form, for example a spectrometer or photometer.

In the arrangements described in Figs. 1 and 2, there are two optical surfaces which could be subject to fouling, that is the window formed in the header tank and the window adjacent the detector. It would be desirable to reduce the risk of fouling by, for example, removing the window 3 from the header tank and relying simply upon light being coupled into the thread 7 after being transmitted into the liquid through a free surface of that liquid. This would, nevertheless, leave the window 8 as a potential source of problems due to fouling. The arrangement illustrated in Fig. 3 avoids even this potential problem.

Referring to Fig. 3, in the illustrated arrangement an optical beam is focused by a lens 13 onto an aperture 14 defined in an open header tank 15 to which water is supplied through an inlet 16. The beam is generated from an optical source 17, the output of which is transmitted to the lens 13 through a beam splitter 18. A thread of water 19 is formed beneath the aperture 14 and light from the source is coupled into the thread. Rather than causing the thread to impinge upon a window to enable detection of light transmitted along its length, however, in the arrangement of Fig. 3 light back-scattered within the thread 19 is coupled via the lens 13 and the beam splitter 18 to a detector 20. Given that light is delivered to the thread 19 through a free surface of the liquid and returned to the detector through that same free surface, there is no optical surface in contact with the water and hence no problem with surface fouling.

When the instrument operates in back-scatter mode as shown in Fig. 3, the system may rely upon the Raman effect, that is an input excitation beam of high power causes the formation of a frequency- shifted beam due to scattering of the input beam off dissolved molecules with Raman-active mechanical vibrations. Such frequency- shifted photons are emitted isotropically, and hence can be detected after travelling back along the thread of water. Thus the beam¬ splitter arrangement enables the instrument to perform a Raman scattering measurement without any optical component being required in contact with the water.

The water-guided arrangement of embodiments of the present invention is optically very efficient. In the case of the embodiment of Fig. 3, the strength of the returned Raman signal is dependent on the collection efficiency of the optical system. In the case of guided optics as described, the collection efficiency increases in proportion to the square of the guided wave numerical aperture. The numerical aperture is given by the square root of the difference between the squares of the refractive index of the water and the refractive index of the surrounding air. If water was guided through a simple capillary, a numerical aperture of the order of 0.12 could be expected. In the case of water "guided" in air, however, given the large difference between the refractive indices of water and air, a numerical aperture of 0.87 can be expected. The water thread, therefore, has a captive efficiency of scattered photons greater than that of a glass fibre system by a factor of the order of 50.

Similar efficiency advantages occur in other scattering systems, including ordinary Rayleigh scattering which can be used for turbidity measurement, direct absorption spectroscopy and particle velocity measurement by photon-correlation techniques.

In the described embodiments of the invention, the liquid to be monitored is the sole component of the thread into which radiation is directed. If the liquid is heavily contaminated with particles the liquid/air interface will be irregular, affecting the optical properties of the thread, because of surface tension effects. A more regular surface for the thread of liquid to be monitored could be achieved by forming an inner thread of the liquid to be monitored inside a sheath of another liquid having a lower refractive index than that of the liquid to be monitored. The lower refractive index could be achieved by using a different liquid for the sheath, or by other means, for example by having a temperature differential between the inner thread and the sheath. In such an arrangement, light will be guided down the inner thread without optical contact with a liquid/air surface. Additionally, it would be easier to establish a stable long thread if the volume of liquid making up the thread is increased by adding a sheath to the inner thread.

In summary, the present invention provides the following advantages: a) a long optical path length can be achieved without critical alignment problems. b) fouling problems can be significantly reduced by avoiding contact between optical surfaces and the liquid to be analysed, particularly in back-reflection mode in which completely optical window-less operation can be achieved. c) optical systems can be arranged to provide high numerical aperture. d) optical syst ems can be arranged to ensure a very high efficiency capture of scattered light.

Claims

1. An instrument for monitoring characteristics of a liquid, comprising means for forming an unsupported thread of the liquid, a source of radiation for producing a beam of radiation to which the liquid is transparent, means for directing a beam into the thread such that the beam is guided along the length of the threat by total internal reflection, and a detector for detecting components of the beam that are affected by variations in characteristics of the liquid defining the thread.

2. An instrument according to claim 1 , wherein the thread forming means comprises an aperture in a container of the liquid, the thread being in the form of a freely falling jet issuing from the aperture.

3. An instrument according to claim 1, wherein the thread forming means comprises a source of the liquid under pressure from which the thread issues in the form of a pressurised jet.

4. An instrument according to claim 1, 2 or 3, wherein the detector is arranged to detect components of the beam transmitted through a window onto which the thread is directed.

5. An instrument according to claim 4, wherein the detector is located immediately behind the window.

6. An instrument according to claim 4, wherein the detector is located remote from the window, components of the beam transmitted through the window being coupled to the detector by an optical system.

7. An instrument according to claim 1, 2 or 3, wherein the detector is arranged to detect components of the beam that are back- scattered by the thread of liquid.

8. An instrument according to claim 7, wherein the source generates a beam of high power such that Raman scattering occurs, the resultant photons being detected by the detector.

9. An instrument according to any preceding claim, wherein the beam is directed into the thread through a window in contact with the liquid.

10. An instrument according to any one of claims 1 to 8, wherein the beam is directed into the thread through a free surface of the liquid.

11. An instrument according to any preceding claim, comprising means for forming an inner thread of the liquid to be monitored and an outer sheath of liquid extending around the inner thread, the refractive index of the sheath being less than that of the inner thread.

12. A method for monitoring characteristics of a liquid, wherein an unsupported thread of the liquid is formed, a beam of radiation is produced to which the liquid is transparent, the beam is directed into the thread such that the beam is guided along the length of the thread by total internal reflection, and components of the beam that are affected by variations in characteristics of the liquid defining the thread are detected.

13. An instrument for monitoring characteristics of a liquid, substantially as hereinbefore described with reference to the accompanying drawings.

14. A method for monitoring characteristics of a liquid, substantially as hereinbefore described with reference to the accompanying drawings.