Improvements in or relating to methods of and apparatus for detecting leaks in sealed vessels

945,827. Photo-electric detection of leaks in sealed vessels. GENERAL ELECTRIC CO. Ltd. Oct. 8,1962 [Oct. 6,1961], No. 36014/61. Heading G1A. The invention relates to a method of and apparatus for detecting leaks in vessels containing a gas at or pressure less than atmospheric, for example incandescent-filament lamps, the method comprising the steps of subjecting the gas to a high-frequency electric field to produce a glow discharge in the gas, filtering and modulating part of the resultant output radiations from the discharge to produce two reference beams of different radiation wavelengths, the intensities of the beams varying in opposite phase,and directing the beams on to a common photoresponsive device coupled to a detector circuit so that the circuit output signal varies in response to variations in the relative mean intensities of the two beams. In the embodiment shown, lamps 3 on a conveyer 1 are carried in turn to a testing position T where they are subjected to a high frequency electric field produced by an electrode 4 under the control of an oscillator 5. The resultant radiations are directed by a lens 9 through two filters 7 and 8 which pass a chosen narrow band of the spectrum peaking respectively at 386 mÁ and 405 mÁ, to a photo-cell 11. The resulting beams from the filters are alternately chopped in sequence by a rotating disc 12 having two sets of holes 14 and 15 arranged as shown so that the two beams incident on the photo-cell 11 are modulated in sinusoidal fashion but are 180 degrees out of phase with each other. A neutral density wedge filter 18 mounted in front of the filter 8 is initially adjusted so that the intensities of the light beam reaching the photo-cell 11 are equal when a satisfactory lamp is under test. If air is present in a lamp being tested an unbalance of the two beams will result and a corresponding A.C. signal will be generated by the photo-cell 11. The output signal from the photo-cell 11 is fed through an amplifier 21 and a filter 22 to a clipper 23 which converts a sine wave signal fed to it into a square wave which is in turn fed through a phase splitter 24 to a gate circuit 25, the gate circuit ensuring that only signals of the appropriate phase indicative of an unsatisfactory lamp are fed to a control circuit 26. The gate circuit 25 is controlled by positive and negative pulses derived from the output signals of a photo-cell 28 detecting the resulting chopped radiation of a lamp 29 shining through one set of hol