the general idea of the invention is illustrated in fig1 . the heart of the present leak detector is a cell 10 with an inner space 11 that is closed with a tracer gas inlet 12 . the tracer gas inlet 12 comprises a membrane 13 that is preferably permeable to the tracer gas , e . g . helium . preferably , it is a membrane selectively permeable only to helium . the membrane 13 is permeable in both directions . thus , a pressure develops in the inner space 11 of the cell 10 that is equal to the partial pressure of the tracer gas outside the cell . the cell 10 has been evacuated before , so that it cannot hold any other gas but the tracer gas . the cell 10 contains an excitation device ( not illustrated ) by which the tracer gas is taken to a higher state of excitation . the optical detection of the metastable tracer gas is effected with a measuring section 14 comprising a laser 15 and a photo detector 16 receiving the laser beam 17 . the wavelength of the laser beam 17 emitted by the laser 15 is set according to an absorption line of the tracer gas ( e . g . helium ). for instance , the wavelength of the laser beam is set to 1083 . 034 nm so as to achieve a higher energy level 2 3 p 2 starting from the metastable level 2 3 s 1 . with a laser frequency of 1083 . 025 nm , the energy level 2 3 p 1 would be achieved , and the energy level 2 3 p 0 would be reached with a wavelength of 1082 , 908 nm . when the tracer gas in the metastable state is illuminated by the wavelength mentioned , this wavelength is absorbed . reference may be made to de 198 53 049 c2 for details . the radiation of the laser beam 17 is modulated so that a region covering the basic wavelength is detected . absorption spectroscopy allows the detection of the absorption wavelengths . this principle is the same for all embodiments described hereinafter . the cell 10 could also be referred to as a spectrometer cell . preferably , it is made of glass . the laser 15 and the photo detector 16 may be arranged in the cell 10 or outside the same . in the embodiment of fig2 , the cell 10 is provided with the measuring section 14 . the cell is closed with a membrane 13 that is selectively permeable to the tracer gas ( helium ) or is at least preferably pervious to this gas . the membrane 13 has a porous supporting body 20 that is permeable to gas and a thin filter layer 21 of sio 2 or quartz of a few nm thickness . a heating device 22 , which is preferably external to the absorption section , serves to heat the filter layer . this heating device is arranged at a distance from the filter layer 21 so that tracer gas can pass through the filter layer . details on the structure of the membrane 13 are described in ep 0 831 964 b1 (= u . s . pat . no . 6 , 277 , 177 b1 ). the cell 10 comprises an excitation device 25 by which helium that has entered the cell is caused to assume a metastable state . in this instance , the excitation device comprises a cathode 26 which is part of a gas discharge section which produces a plasma from a buffer gas in the inner space 11 . the buffer gas is an inert gas , preferably a noble gas , except for helium which is used as the tracer gas . a ) be exposed either directly or indirectly to ambient air ( sniffer leak detection ), or b ) be connected to a test chamber 28 which contains an object under test filled with tracer gas ( integral leak test ), or c ) be connected with an evacuated object under test that is sprayed with tracer gas from the outside ( vacuum leak test ). the tracer gas will then pass the membrane 13 and reach the inner space 11 of the cell 10 . a test leak 30 can be provided in front of the inlet , from which tracer gas escapes in a defined volume flow . the test leak 30 is used to calibrate the leak detector so as to obtain quantitative information about the tracer gas concentration . the cell 10 is further provided with a buffer gas inlet 32 through which a buffer gas is fed that is ionized by the excitation device 25 . moreover , the cell 10 is connected with a vacuum pump device 34 via a connection 33 . the vacuum pump device draws the mixture of buffer gas and tracer gas from the cell 10 . the embodiment of fig3 differs from the preceding embodiment in that the cell 10 is not connected to a vacuum pump device . the cell 10 comprises the tracer gas inlet 12 connected with the test chamber 28 . the cell further includes the measuring section 14 as well as an excitation device 25 formed by electrodes . the cell 10 of fig3 is provided with a pump connection 35 connected with a suction chamber 36 . the pump chamber 36 has a buffer gas connection 37 and a pump connection 38 connected with a vacuum pump device ( not illustrated ). it is the function of the pump connection 35 to selectively conduct only tracer gas from the cell 10 into the suction pump 36 . it is configured in the same manner as the tracer gas inlet 12 . together with the pump chamber 36 , the pump connection 35 forms a pump or a one - way valve for removing the tracer gas from the cell 10 . the embodiment of fig4 is mostly similar to that of fig2 , however with the difference that a test chamber 28 ( fig2 ) is omitted . the tracer gas inlet 12 is exposed to ambient air either directly or by drawing in ambient air through a sniffer conduit and feeding it to the inlet 12 . when a tracer gas cloud 40 reaches the tracer gas inlet 12 , it is sucked in and the tracer gas gets into the cell 10 where it is caused to take a metastable state and is then detected . the cell further comprises a buffer gas connection 41 and a pump connection 38 that may be connected to a vacuum pump device as illustrated in fig2 . the embodiment of fig4 is suited for use as a sniffer leak detector . fig5 illustrates another embodiment of a leak detector comprising a cell 10 with a tracer gas inlet 12 , as well as a measuring section 14 of the kind described before . the cell 10 has no pump connection . when tracer gas is present in front of the tracer gas inlet 12 , a pressure develops inside the cell 10 that is equal to the partial pressure of the tracer gas ( helium ) in front of the membrane . if the tracer gas is removed from in front of the membrane , the tracer gas flows from the cell 10 through the tracer gas inlet 12 back into the atmosphere . the embodiment of fig5 also comprises an excitation device ( not illustrated ) that causes the helium in the cell 10 to assume a metastable state so that it is optically detectable . when the metastable helium contacts the wall of the cell or the tracer gas inlet or another device , it looses energy and is thus restored to its ground state . therefore , the helium escaping from the cell 10 is no longer in a metastable state . although the invention has been described and illustrated with reference to specific illustrative embodiments thereof , it is not intended that the invention be limited to those illustrative embodiments . those skilled in the art will recognize that variations and modifications can be made without departing from the true scope of the invention as defined by the claims that follow . it is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof .