(19)

(11)

EP 0 197 112 A1

(12)

(43) Date of publication: 15.10.1986 Bulletin 1986/42 (21) Application number: 85905093.0 (22) Date of filing: 18.10.1985

(51) International Patent Classification (IPC):  G01V 1/ 18( . ) (86) International application number: PCT/GB1985/000482 (87) International publication number: WO 1986/002459 (24.04.1986 Gazette 1986/09)

(84) Designated Contracting States: AT BE CH DE FR GB IT LI LU NL SE (30) Priority: 18.10.1984 GB 19840026346 (71) Applicant: COVERT SECURITY SYSTEMS LIMITED Hitchin Hertfordshire (GB)	(72) Inventors: OWENS, Alwyn, Roberts Gwynedd (GB) HOOTON, Michael, Barry Bedford Bedfordshire (GB) (74) Representative: Thomson, Roger Bruce  W.P. THOMPSON & CO. Celcon House 289-293 High Holborn London WC1V 7HU London WC1V 7HU (GB)