GB2261503A

GB2261503A - Monitoring a parameter of a magnetic or electromagnetic field

Info

Publication number: GB2261503A
Application number: GB9121282A
Authority: GB
Inventors: Stephen John Walpole
Original assignee: NOVACARE PRODUCTS Ltd
Current assignee: NOVACARE PRODUCTS Ltd
Priority date: 1991-10-08
Filing date: 1991-10-08
Publication date: 1993-05-19
Also published as: GB9121282D0

Abstract

A parameter of a magnetic or an electromagnetic field is monitored by placing a crystal 1 in the field, the crystal being irradiated with electromagnetic radiation, for example laser light 2. The crystal, which may be piezo-electric crystal (such as quartz), is of the kind in which an optical quality e.g. transmissivity of the crystal is affected by the parameter. That optical quality is monitored by photo-detector 3 and an output signal representative of that quality and therefore the parameter generated. The apparatus may be used for measuring the bio-electrical signals of the brain, when it is incorporated in a light-proof container 16 of a headset. <IMAGE>

Description

Title: Method of, and Apparatus for, Monitoring a Parameter of an Electromagnetic Field Field of the Invention The invention relates to a method of, and apparatus for, monitoring a parameter of an electromagnetic field, and is more specifically, but not exclusively, concerned with the monitoring of bioelectrical signals by monitoring the electromagnetic fields produced by such signals.

Background to the Invention It is known, for example from conventionar; electroencephalography (EEG), electrocardiography (ECG) techniques, to measure bioelectrical signals by placing electrodes against appropriate areas on the skin of the subject person or animal being monitored. These techniques do enable bioelectrical activity to be monitored but require that the electrodes make electrical contact with the skin.

To this end, the surface of the skin normally needs to be especially prepared to receive the electrodes which are subsequently fixed in position by means of, for example, self-adhesive strips of material.

Thus these techniques tend to be somewhat cumbersome, and may involve a degree of discomfort for the subject.

Devices have been developed which monitor bioelectrical signals without the need for direct electrical contact, and are known as Superconductive Quantum Interference Devices (SQUIDS). However these devices have components which must be maintained at relatively low temperatures (less than -220 C), require special shielding in order to avoid interference from external signals, and are consequently complex costly, and large.

Summary of the Invention According to the invention, in one aspect, there is provided a method of monitoring a parameter of an electromagnetic field, the method comprising the steps of: 1. placing in the field a crystal the optical quality of which varies in accordance; with the parameter; 2. irradiating the crystal with electromagnetic radiation so as to reveal any such changes in the optical quality of the crystal; 3. monitoring any changes in the optical quality of the crystal; 4. generating an output signal representative of those changes, and hence of the parameter.

The crystal is preferably a piezo electric type crystal which may with advantage be a quartz crystal.

Preferably, the optical changes are monitored by measuring the intensity of the electromagnetic radiation which has passed through the crystal.

The electromagnetic radiation may with advantage be visible light.

The applicant has discovered tha the proportion of incident light which is transmitted through a quartz crystal is related to the strength of the magnetic field (ie flux density) in which the crystal is situated.

Thus the method may be used to obtain an indication of the strength of a magnetic field, and the frequency of modulation of an electromagnetic field may be determined by monitoring the changes in transmitted light intensity over a period of time.

In another aspect, the invention provides'apparatus for carrying-out the method as herein above deescribed, the apparatus comprising a source of electromagnetic radiation; a piezo-electric type crystal positioned, in use, in the path of electromagnetic radiation from the source, the optical quality of the crystal being variable in a manner related to the parameter when the crystal is placed within the field; a photo detector for monitoring any such optical change and processing means connected to the output of the photo detector and so arranged as to generate a signal representative of the monitored parameter.

The invention also lies in apparatus for monitoring the electromagnetic fields produced by bioelectric signals, the apparatus comprising a source of electromagnetic radiation; a piezo-electric type crystal positioned, in use, in the path of electromagnetic radiation from the source and in the field, the optical quality of the crystal being variable in a manner related to the parameter; a photo detector for monitoring electromagnetic radiation which has interacted with the crystal; and processing means for processing the signal generated by the photo detector to give an indication of the value of the parameter.

The variable optical quality of the crystal may, for example, be the amount of incident light on the crystal which is transmitted therethrough.

In such a case, the crystal is preferably interposed between a source of electromagnetic radiation and a photo detector which monitors the intensity of electromagnetic radiation which has passed through the crystal.

The source of electromagnetic radiation in the described embodiment emits visible light. The source may, with advantage comprise a laser.

Description of the Drawings The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic block circuit diagram of apparatus according to the invention; Figure 2 is a schematic view of part of the apparatus shown in Figure 1; Figure 3 is a circuit diagram of part of the apparatus shown in Figure 1; Figure 4 to 6 show a headset for use in situations in which the apparatus is to be used to measure bio electrical signals from the human brain.

Detailed Description With reference to Figure 1, a piece of quartz crystal 1 is interposed between a laser 2 and a photodetector 3, the output of which is connected to an analogue to digital converter (ADC) 4, via an amplifier 5. The output of the ADC 4 is in turn connected to a computer 6.

With reference to Figure 2 the laser 2 i operable to emit red light which irradiates the crystal 1, and the photodetector 3, which comprises a photòdiaode, receives light from the laser 2 which has been transmitted through the crystal 1.

The laser 2, crystal 1 and photo detector 3 are all housed in a light proof casing (not shown) which prevents external light sources from causing interference in the output of the photo detector 3.

The output of the photo detector 3 is fed to the amplifier 5 shown in Figure 3. The circuitry of the amplifier 5 is self explanatory.

The output signal of the amplifier 5 is converted into a digital signal by the ADC 4 for subsequent analysis by the computer 6.

In use, the casing containing the crystal 1, laser 2 and photodetector 3 is positioned in the magnetic or electromagnetic field to be monitored. The intensity of light transmitted by the crystal 1, and hence the light detected by the detector 3, relative to that of the light incident on the crystal, is proportional to the magnitude of the field. Thus the computer 6 may process the signals produced by the ADC 4 to give an indication of field intensity (which may be recorded and/or stored).

Modulations of the field cause corresponding modulations of the intensity of transmitted light, and hence in the amplitude of the output signal of the photo detector 3.

In this case measurement of the frequency of the photodetector output signal therefore provides a measure of the frequency of modulation of the field.

As well as measuring the frequency of a mtdulating field, the apparatus may be used to measure the intensity of a static magnetic field. In order to do this, the apparatus needs to be calibrated by measuring the intensity of light transmitted through crystal 1 when the latter is substantially unaffected by any magnetic field to ascertain the photo detector 3 output which corresponds to "no field". The apparatus may then be placed in the magnetic field and the output of the photo detector 3 measured to give a second reading.

The strength (ie the flux density) of the magnetic field may then be ascertained by comparing the second reading with the "no field" reading. Clearly both readings will be affected by the earth's magnetic field. However, the earth's magnetic field will have substantially the same effect on both the "no field" and second readings and may therefore be compensated for in the subsequent analysis of those readings.

When the apparatus is to be used to measure the bioelectrical signals produced by the human brain, the head set shown in Figures 4 to 6 is employed.

The headset comprises a resiliently deformable bracket 10 adapted to fit around the head of the subject (and thus to act as a head band) which terminates in two pads, one of which is shown at 12, each of which, in use, engages a respective side of the subject's face to help retain the headset in position.

An arm 14 projects from the central region of the bracket 10 and carries a light-proof container 16 which houses the crystal 1, laser 2 and photodetector 3.

A pad 18 is attached to the under-side ofthe container 16 and enables the container 16 to rest on the top of the subject's head without causing undue discomfort to the subject.

With the headset 16 placed on the subject's head (20) as shown in Figures 5 and 6, the crystal 1 is positioned directly over the centre of the top of the head (C3O).

In addition to measuring bioelectrical signals, the apparatus may be applicable to other fields in which a magnetic or electromagnetic field needs to be measured, for example the location of subterranean pipes or cables.

Claims

1. A method of monitoring a parameter of a magnetic or an electromagnetic field, the method comprising the steps of: A. placing in the field a crystal the optical quality of which is affected by the parameter; B. irradiating the crystal with electromagnetic radiation so as to reveal any such changes in the optical quality of the crystal; C. monitoring the optical quality of the crystal; and D. generating an output signal representative of that quality, and hence of the parameter.

2. A method according to claim 1 in which the crystal is a piezo electric crystal.

3. A method according to claim 2 in which the crystal is a quartz crystal.

4. A method according to any of the preceding claims in which the optical quality of the crystal is monitored by measuring the intensity of the electromagnetic radiation which has passed through the crystal.

5. A method according to any of the preceding claims in which the electromagnetic radiation is visible light.

6. A method according to any of the preceding claims in which the crystal is irradiated with laser light.

7. Apparatus for monitoring a parameter of a magnetic or an electromagnetic field the apparatus comprising means for providing a source of electromagnetic radiation; a piezo-electric crystal positioned, in use, in the path of electromagnetic radiation from the source, the optical quality of the crystal being variable in a manner related to the parameter when the crystal is placed within the field, a photo detector for monitoring such a change and processing means connected to the output of the photo detector and so arranged as to generate a signal representative of the monitored parameter.

8. Apparatus according to claim 7, when adapted to monitor electromagnetic fields produced by bioelectric signals.

9. Apparatus according to claim 8 in which the crystal, the photo detector and the means for providing a source of electromagnetic radiation are mounted in a headset.

10. Apparatus according to any of claims 7-9 in which the variable optical quality of the crystal is the amount of incident light on the crystal which is transmitted therethrough.

11. Apparatus according to claim 10 in which the crystal is interposed between the means for providing a source of electromagnetic radiation and a photo detector which monitors the intensity of electromagnetic radiation which has passed through the crystal.

12. Apparatus according to any of claims 7-11 in which the source comprises a laser.

13. A method substantially as described herein with reference to the accompanying drawings.

14. Apparatus substantially as described herein with reference to, and as illustrated in, the accompanying drawings.