Optics: measuring and testing – Of light reflection
Patent
1993-11-10
1996-04-16
Rosenberger, Richard A.
Optics: measuring and testing
Of light reflection
G01N 2155
Patent
active
055088090
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical sensing and, more particularly, to an optical sensor which utilises the phenomenon of surface plasmon resonance.
2. Discussion of the Background
Surface plasmons are the quanta of a surface electromagnetic wave propagating along the interface between a conducting material such as a metal and a dielectric. They represent the coupling of a longitudinal oscillation of the surface charge density with its associated electromagnetic fields. Surface plasmons can be excited by an electric field of light if the component of the wave vector of the light wave along the surface equals that of the same frequency surface wave. The effect is then called surface plasmon resonance (SPR) and may be seen as an absorption of the light. Kretschmann and Raether (Z. Naturforsch., (1968) 23a, 2135) have produced a comprehensive description of the phenomenon. The effect is strongly dependent on the dielectric properties at the metal surface and can therefore be used to sense materials at or deposited on the metal surface or to sense changes brought about in a previously existing overlayer by exposure to some other substance.
Some known forms of optical sensors utilise this phenomenon by scanning the angle of incidence or by using the so called convergent beam method. In the latter approach polarised monochromatic or narrow band light is first expanded and then focused on to a metal film coated onto the back surface of a glass prism or the front surface of a diffraction grating. The position of resonance is detected by using a linear diode array and the source of radiation or light is usually a light emitting diode which requires polarisation filters for use as an SPR source or a polarised gas laser such as a helium-neon laser.
SUMMARY OF THE INVENTION
Applicants are particularly, though not exclusively, interested in optical sensors for sensing the presence of monoethylene glycol vapour (MEG). The UK gas distribution system for distributing natural gas to premises includes many miles of pipework with lead/yarn joints. In order for such joints to remain gas tight, the packing material (jute or hemp fibres) must be maintained in a swollen state. Natural gas is a dry gas which tends to cause the packing material to dry out and shrink and possibly result in some gas leakage at the joints. To prevent this problem occurring the natural gas is conditioned with MEG which is introduced continuously in vapour form into the pipework by "fogging" units. The MEG serves as a yarn swellant. The maintenance of the correct level of MEG in the gas is important since too much results in droplets condensing on the pipe walls and in MEG being wasted, while too little results in inadequate swelling of the packing yarn thereby increasing the likelihood of some gas leakage at the joints. Thus monitoring of the concentration of the MEG in the gas at various test points throughout the system is important.
An object of the invention is to provide an optical sensor which utilises the phenomenon of surface plasmon resonance for sensing the presence of an analyte in a fluid.
According to the invention an optical sensor for sensing the presence of analyte in a fluid comprises a light source; a surface plasmon resonance-sensitive device for reflecting light originating from the light source and which device, on exposure to the analyte, responds so as to alter the intensity of the light which is reflected; a light detecting means for receiving light reflected from the device and for producing in response to the received reflected light an output signal indicative of the presence of, or representative of the concentration or a concentration range of, the analyte in the fluid; control means for controlling the portion of the reflected light which is permitted to reach the light detecting means; and indicating means for receiving a said output signal via the light detecting means and, in response thereto, indicating the presence, concentration or concentration range of
REFERENCES:
patent: 4886355 (1989-12-01), Keane
patent: 4889427 (1989-12-01), Van Veen et al.
patent: 4997278 (1991-03-01), Finlan
patent: 5245410 (1993-09-01), Villuendas Yuste et al.
Sensors And Actuators, vol. 4, 1983, pp. 299-304, B. Liedberg, et al. "Surface Plasmon Resonance for gas detection and biosensing", see abstract, see p. 301, line 1--p. 302, line 2; FIG. 2.
Jaggers Henry C.
Peacock Stanley J.
Shaw John S.
British Gas plc
Rosenberger Richard A.
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