GB1563374A - Device for providing an indication of the refractive index of a fluid - Google Patents

Description

(54) DEVICE FOR PROVIDING AN INDICATION OF THE REFRACTIVE INDEX OF A FLUID (71) We, THE MARCONI COMPANY LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex CM 1 IPL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a device for providing an indication of the refractive index of a fluid.

It is known that the specific gravity of a fluid is related to the refractive index thereof, and it is an object of this invention to provide an indication of the refractive index of a fluid (which may be, for example, a battery electrolyte), and so give an indication of the fluid's specific gravity.

According to this invention, a device for providing an indication of the refractive index of a fluid includes a transparent body having a refractive index greater than that refractive index of the fluid to be indicated and an outer surface formed from at least a portion of a prolate spheroid and thereby having two foci, a source of electromagnetic radiation arranged on one of said foci to illuminate at least part of the interior of said surface, and a detector arranged at the other of said foci to receive electromagnetic radiation totally-internallyreflected from said surface, whereby the intensity of the received radiation is indicative of the refractive index of a fluid into which the surface is, in operation, immersed.

Although the electro-magnetic radiation may be in the invisible part of the radiation spectrum, conveniently the radiation source is a visible-light source and the detector is a photo-detector.

Preferably, the surface is part of a hemispheroid, and the eccentricity of the surface which is illuminated is governed by the range of refractive indexes to be indicated.

Although the transparent body may be a transparent shell filled with liquid, preferably it is a transparent solid body formed from glass, an epoxy resin, or a plastics moulding-for example, of a poly(methyl methacrylate) like that sold under the name "Perspex" (Perspex is a Registered Trade Mark).

Conveniently, the light source is a lightemitting diode and the photo-detector is a photo-diode or photo-transistor, both the light source and the photo-detector being arranged on a substrate to face normally to the plane of the substrate. Preferably, the light source and the photo-detector are sealed to the body with an adhesive having the same refractive index as said body.

Preferably, the prolate spheroid is dimensioned in accordance with the equation

where: a is the semi-major axis dimension of the spheroid; b is the semi-minor axis dimension of the spheroid; n max is the maximum refractive index of the fluid be indicated; and nO is the refractive index of the body.

Advantageously, a radiation prevention means is provided between the source of electro-magnetic radiation and the detector, which means is dimensioned and arranged to prevent radiation from surfaces of the body other than the spheroidal surface reaching the detector. Normally, the light source is energised by either a d.c. or a.c.

power source, and the photo-detector is coupled to an ammeter.

The invention will now be described, though only by way of illustration, with reference both to the drawing accompanying the Provisional Specification (referred to herein as Figure 1, which Figure shows a section along the major axis of a device in accordance with this invention), and with reference to the accompanying drawing (which is referred to as Figure 2, and which illustrates a compensation circuit for reducing the effect of ambient light).

The device shown in Figure 1 of the drawing has a transparent solid "Perspex" injection moulding in the shape of a portion of a prolate hemispheroid 1 which is mounted on a printed circuit board 2, the end portions 3,4 of the hemispheroid 1 being necessitated by the injection moulding process. A light-emitting diode light source 5 is provided at one of the foci of the prolate hemispheroid, and a photo-diode 6 is provided at the other of the two foci of the prolate hemispheroid. Both the lightemitting diode and the photo-diode are mounted in apertures (not shown) in the flat surface (bottom surface, as viewed) of the hemispheroid, and are sealed to the hemispheroid by adhesive having the same refractive index as the "Perspex". The lightemitting diode is connected to a d.c. power source 7, and the photo-diode is connected to provide an output signal to an ammeter 8.

A light stop 9 is mounted on the printed circuit board 2 along the minor axis of the prolate hemispheroid 1, the top portion of the light stop having the same part-spherical contour as the hemispheroid beneath which it is mounted. The purpose of the light stop is to prevent radiation reflected from the end portions 3,4 from reaching the photodiode 6.

The prolate hemispheroid 1 is a portion of a spheroid which is dimensioned in accordance with the equation given hereinbefore.

In operation, at least the curved surface of the prolate hemispheroid I is immersed in a fluid (not shown) whose refractive index is to be indicated, the functioning of the device being dependent upon total internal reflection of light within the hemispheroid 1. For total internal reflection it will be appreciated that the refractive index n of the fluid must be less than the refractive index nO of the Perspex of which the hemispheroid 1 is formed, and that the angle of the critical light rays, indicated by the arrow-headed lines 10, varies with change in the refractive index n of the fluid; the smaller the critical angle the lower the refractive index of the fluid n, and, consequently, the greater the intensity of light energising the photo-diode, so that the indication shown by the ammeter 8 is increased.When the refractive index of the fluid is a maximum (may), then the critical angle is a maximum, as indicated by the broken arrow-headed line 11, and the current monitored by the ammeter 8 is at its lowest.

The device shown in Figure 1 should be used in conditions where there is negligible background ambient illumination which could be picked up by the photo-detector 6.

However, it is possible to use the device even in the presence of significant background ambient illumination if suitable circuitry is used to enable the light from the light source 5 to be separated from the undesirable background illumination. A device which includes such circuitry is illustrated in Figure 2.

Referring to Figure 2, a prolate hemispheroid 1 is provided with a source 5 and a detector 6 in a manner similar to that shown in Figure 1.

The source 5 is not continuously driven in this case, but is pulsed on and off by means of a signal derived from clock pulses applied to a terminal 21. The clock pulses consist of a rectangular waveform having unity markto-space ratio. The terminal 21 is connected to a bistable 22 having Q andvoutputs. As is usual, the output of the bistable is the inverse of the Q output. The Q output is connected to a driver 23 and to the gate terminal of a first sample-and-hold circuit 24. The driver 23 is merely a power amplifier which provides sufficient energy to activate the source 5 as required. The light emitted by the source 5 is, after reflection within the prolate hemispheroid 1, detected by the detector 6.The output of the detector 6 is amplified by an amplifier 25, and routed to the input of the sampleand-hold circuit 24 and also to the input of a second sample-and-hold circuit 26. The gate terminal of the sample-and-hold circuit 26 receives the signal from the v output of the bistable 22. The outputs of the sample-and-hold circuits 24 and 26 are connected to the two inputs of a differential amplifier 27.

A clock pulse having a unity mark/space ratio is applied to terminal 21 so that the signal obtained from the Q output of the bistable 22 turns the source 5 on for 50% of the time. Whilst the source 5 is on, the light detected by the detector 6 results in a signal which is entered and stored in the sampleand-hold circuit 24 which, during this time, is enabled by the signal from the Q output.

When the bistable 22 changes state under the action of the clock signal, the signal held by the sample-and-hold circuit 24 is maintained unaltered, and instead the signal obtained from amplifier 25 is entered into the sample-and-hold circuit 26. During this time the source 5 is switched off, so that the only light received by detector 6 is that due to ambient background light. When the bistable 22 next changes state the input to sample-and-hold circuit 26 is closed under the action of the signal on its gate terminal, and the sample already held is maintained constant.

It will thus be apparent that sample-andhold circuit 24 stores a signal representative of the illumination originating from source 5 plus the ambient background illumination, whereas sample-and-hold circuit 26 stores a signal representative of only the ambient background illumination. The differential amplifier 27 consequently amplifies only the difference between these two signals, and provides, at an output terminal 28, a signal from which the effect of the background ambient illumination has been reduced or eliminated.

In each case, where the device is to be operated in a harsh, corrosive environment, the outer surface of the prolate hemispheroid may be protected with an inert transparent coating if required.

WHAT WE CLAIM IS: 1. A device for providing an indication of the refractive index of a fluid, including a transparent body having a refractive index greater than that refractive index of the fluid to be indicated and an outer surface formed from at least a portion of a prolate spheroid and thereby having two foci, a source of electro-magnetic radiation arranged on one of said foci to illuminate at least part of the interior of said surface, and a detector arranged at the other of said foci to receive electro-magnetic radiation totally-internally-reflected from said surface, whereby the intensity of the received radiation is indicative of the refractive index of a fluid into which the surface is, in operation, immersed.

2. A device as claimed in claim 1 and wherein the radiation source is a visiblelight source, and the detector is a photodetector.

3. A device as claimed in claim 1 or 2 and wherein the surface is part of a hemispheroid.

4. A device as claimed in any of the preceding claims and wherein the transparent body is a solid body.

5. A device as claimed in any of the preceding claims and wherein the light source is a light-emitting diode and the photo-detector is a photo-diode or phototransistor, both the light source and the photo-detector being arranged on a substrate to face normally to the plane of the subtrate.

6. A device as claimed in claim 5 and wherein the light source and the photodetector are sealed to the body with an adhesive having the same refractive index as said body.

7. A device as claimed in any of the preceding claims and wherein the prolate spheroid is dimensioned in accordance with the equation

where: a is the semi-major axis dimension of the spheroid; b is the semi-minor axis dimension of the spheroid; nmax is the maximum refractive index of the fluid to be indicated; and nO is the refractive index of the body.

8. A device as claimed in any of the preceding claims and wherein a radiation prevention means is provided between the source of electro-magnetic radiation and the detector, which means is dimensioned and arranged to prevent radiation from surfaces of the body other than the spheroidal surface reaching the detector.

9. A device as claimed in any of the preceding claims and wherein means are provided for compensating for the effect of ambient electro-magnetic radiation on the detector.

10. A device for providing an indication of the refractive index of a fluid substantially as illustrated in and described with reference to the Figure accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. and the sample already held is maintained constant. It will thus be apparent that sample-andhold circuit 24 stores a signal representative of the illumination originating from source 5 plus the ambient background illumination, whereas sample-and-hold circuit 26 stores a signal representative of only the ambient background illumination. The differential amplifier 27 consequently amplifies only the difference between these two signals, and provides, at an output terminal 28, a signal from which the effect of the background ambient illumination has been reduced or eliminated. In each case, where the device is to be operated in a harsh, corrosive environment, the outer surface of the prolate hemispheroid may be protected with an inert transparent coating if required. WHAT WE CLAIM IS:

1. A device for providing an indication of the refractive index of a fluid, including a transparent body having a refractive index greater than that refractive index of the fluid to be indicated and an outer surface formed from at least a portion of a prolate spheroid and thereby having two foci, a source of electro-magnetic radiation arranged on one of said foci to illuminate at least part of the interior of said surface, and a detector arranged at the other of said foci to receive electro-magnetic radiation totally-internally-reflected from said surface, whereby the intensity of the received radiation is indicative of the refractive index of a fluid into which the surface is, in operation, immersed.

2. A device as claimed in claim 1 and wherein the radiation source is a visiblelight source, and the detector is a photodetector.

3. A device as claimed in claim 1 or 2 and wherein the surface is part of a hemispheroid.

4. A device as claimed in any of the preceding claims and wherein the transparent body is a solid body.

5. A device as claimed in any of the preceding claims and wherein the light source is a light-emitting diode and the photo-detector is a photo-diode or phototransistor, both the light source and the photo-detector being arranged on a substrate to face normally to the plane of the subtrate.

6. A device as claimed in claim 5 and wherein the light source and the photodetector are sealed to the body with an adhesive having the same refractive index as said body.

7. A device as claimed in any of the preceding claims and wherein the prolate spheroid is dimensioned in accordance with the equation

where: a is the semi-major axis dimension of the spheroid; b is the semi-minor axis dimension of the spheroid; nmax is the maximum refractive index of the fluid to be indicated; and nO is the refractive index of the body.

8. A device as claimed in any of the preceding claims and wherein a radiation prevention means is provided between the source of electro-magnetic radiation and the detector, which means is dimensioned and arranged to prevent radiation from surfaces of the body other than the spheroidal surface reaching the detector.

9. A device as claimed in any of the preceding claims and wherein means are provided for compensating for the effect of ambient electro-magnetic radiation on the detector.

10. A device for providing an indication of the refractive index of a fluid substantially as illustrated in and described with reference to the Figure accompanying the Provisional Specification.