as an illustration , a trial was carried out for controlling the atmosphere on an industrial prototype of the type shown in fig1 , under the conditions given below . in everything that follows , the volumes will be expressed as standard liters per m 2 of treated substrate ( and not , as is conventional , in m 3 / h ). this is very advantageous for being able to compare machines of different widths . the presence of entry / exit devices based on three components ( channel , injection slot and labyrinth ) as described above in relation to fig1 ; no treatment gas injection into the chamber ; and the central extraction system was stopped , as was the pressure regulation system . under such operating conditions , the trials consisted in measuring the oxygen concentration inside the chamber and at about 0 . 8 mm from the surface of the roll , by injecting about 1 . 4 sl / m 2 of nitrogen into each entry / exit device , with a 700 mm wide product running at speeds of between 50 and 250 m / min . the results of the measurements showed that the oxygen concentration was between 6000 and 8000 ppm depending on the speed used ( these results are shown in fig4 below ). by using higher nitrogen volumes ( 3 . 25 standard liters / m 2 in each entry / exit device ). it was possible to reduce this concentration to about 3000 ppm . the results clearly show that the use of these prior devices does not achieve a residual oxygen concentration low enough for many envisaged applications . in particular it may be seen that , even by having eliminated the reduced pressure inside the chamber , created by the central extraction , the performance of these systems is insufficient under the operating conditions tested ( especially the run speed ). one hypothesis that may be put forward is that this result can be explained by the absence of injection of the treatment gas mixture into the chamber , which helps to achieve a low oxygen concentration , the injection of the treatment mixture having been stopped for these trials ( very logically , since the intended application here is a uv crosslinking application ). the object of the invention is therefore to propose novel ultraviolet or electron - beam crosslinking equipment , the design of which allows the oxygen concentration within the chamber to be substantially reduced . the equipment according to the invention is based on the use of two devices , a chamber entry device and a chamber exit device ( see fig2 below ): the entry device consists of at least the following three components , seen in succession by the running product to be treated : a labyrinth system , a gas injection slot , and a channel ; the chamber exit device advantageously consists of at least the following three components , seen in succession by the running product to be treated : a channel , a gas injection slot , and a labyrinth system . as an illustration , the following geometric values in particular are considered satisfactory : height of the labyrinth grooves : 4 . 5 mm ; width of the labyrinth teeth : 2 mm ; width of the labyrinth grooves : 5 mm ; height of the channels : 3 mm ; and length of the channels : 38 mm . the height of the channel is advantageously between 3 and 5 mm . in this configuration ( arrangement and geometry of the components ), the chamber entry device may be considered to have two functions : owing to the pressure drop created by the inlet labyrinth , the injected nitrogen has a tendency to be directed toward the inside of the crosslinking chamber and to very considerably minimize the entry of air into this chamber . the same applies to the chamber exit device , which allows nitrogen to be directed toward the chamber and to limit a discharge of gas to the outside . in what has just been described above , it should be emphasized that the entry device plays a key role , whereas the exit device , if it were to be shut off or at the very least simplified in its structure for certain less demanding applications ( as will be seen below ), its presence is strongly recommended so as to work under optimum atmosphere conditions . the present invention therefore relates to an installation in which an operation of crosslinking a coating , such as an ink or varnish coating , is carried out by ultraviolet radiation or by an electron beam , in the presence of a gas mixture with a controlled residual oxygen content , the installation comprising a chamber having one or more uv lamps or a source of accelerated electrons , necessary for carrying out the crosslinking operation , which is characterized in that it includes an entry device adjacent the chamber and comprising at least the following three components , seen in succession by the running product to be treated : a labyrinth system , means for injecting an inert gas forming a gas knife , and a channel . moreover , the installation according to the invention may have one or more of the following features : the installation includes an exit device adjacent the chamber and consisting of at least the following three components , seen in succession by the running product to be treated : a channel (“ output channel ”), means for injecting an inert gas forming a gas knife , and a means for creating a pressure drop , such as a smooth profile , the distance between the smooth profile and the surface of the coating being less than the height of said channel ; the installation includes an exit device adjacent the chamber and consisting of at least the following three components , seen in succession by the running product to be treated : a channel , means for injecting an inert gas forming a gas knife , and a labyrinth system ; said entry device includes at least the following five components , seen in succession by the running product to be treated : a channel , a 1st gas injection slot , a labyrinth , a 2nd gas injection slot , followed by a second channel ; said means for injecting inert gas forming a gas knife comprise a plane - walled gas injection slot emerging inside the entry or exit device in question ; and the length / height ratio of at least one of said channels is at least 3 , preferably at least 6 . the concepts of “ labyrinth ” and “ channel ” according to the present invention refer to the “ labyrinth ” and “ channel ” concepts already used in the prior document wo 02 / 40738 discussed above , again in the name of the applicant . therefore , as clearly depicted in the figures below , the “ labyrinth ” concept relates to a system of open grooves facing the internal space of the entry or exit device in question and forming a labyrinth . fig3 below shows the result of trials carried out on equipment according to the invention comprising the entry / exit systems described within the context of fig2 , which trials consisted in measuring the oxygen content within the chamber , about 5 mm from the treated roll , for speeds of between 50 and 250 m / min and with nitrogen injected into each of the entry / exit devices at about 1 . 4 to 3 . 25 standard liters / m 2 ( the abbreviation sl / m 2 used in the figures must be understood as actually denoting standard liters / m 2 of substrate treated ). in fig3 , it should be noted that there are three curves : the curve of ♦ points for an overall ( entry + exit ) volume of about 2 . 8 standard liters / m 2 ; the curve of ▪ for an overall ( entry + exit ) volume of about 4 . 64 standard liters / m 2 ; and the curve of ▴ points for an overall ( entry + exit ) volume of about 6 . 5 standard liters / m 2 . the results of the measurements show that the oxygen content varies from about 34 to 380 ppm depending on the speed and nitrogen flow rate conditions employed . these trials demonstrate that an inert nitrogen atmosphere containing less than 50 ppm residual oxygen was obtained in the chamber of the equipment according to the present invention with a very acceptable gas consumption , since this was between 4 . 6 and 6 . 5 standard liters / m 2 . thus , fig4 shows the results , already mentioned above , such as those obtained with equipment of the prior art provided with entry and exit devices according to fig1 . in fig4 , it should be noted that there are three curves : the curve of ♦ points for an overall ( entry + exit ) volume of about 2 . 8 standard liters / m 2 ; the curve of ▪ points for an overall ( entry + exit ) volume of about 4 . 6 standard liters / m 2 ; and the curve of ▴ points for an overall ( entry + exit ) volume of about 6 . 5 standard liters / m 2 . as has already been mentioned above , these measurement results show that the oxygen concentration is between 6000 and 8000 ppm , depending on the speed used , for an overall volume of 2 . 8 standard liters / m 2 . the use of higher nitrogen volumes ( 3 . 25 standard liters m 2 in each entry / exit device , i . e . an overall volume of 6 . 5 standard liters / m 2 ) allows this concentration to be reduced to about 3000 ppm . fig5 shows a comparison of the results obtained in the case of fig3 with those obtained in the case of fig4 . plotted on the y axis is the oxygen content reduction ( in %) achieved thanks to the equipment according to the invention . the oxygen content reduction do 2 / o 2 expressed as a percent is defined by the following equation : d o 2 / o 2 ═(([ o 2 ] fig . 4 −[ o 2 ] fig . 3 )/[ o 2 ] fig . 4 )× 100 . it may therefore be seen that the reduction in residual oxygen content in the chamber is at least 94 % with the same speed and nitrogen volume parameters . it even reaches 98 to 99 % in the case of higher volumes . fig6 and 7 illustrate another configuration of the equipment according to the invention . in this configuration , the chamber entry device ( shown in fig6 ) has been modified — it consists here of five components , namely in succession : a channel , a first gas injection slot , a labyrinth , a second gas injection slot , followed by another channel . as regards the chamber exit device ( fig7 ), this is identical to that of fig2 , so as to consist of three successive components , namely a channel , a nitrogen injection slot , followed by a labyrinth . the orientation of the nitrogen injection slots to the roll is , in the case of the embodiment shown , about 90 ° for the first slot of the entry device and 45 ° in the case of the second slot of the entry device . the width of the slots is about 0 . 2 mm for the first slot and 0 . 4 mm for the second slot , respectively . the distance between the entry device and the roll is about 0 . 8 mm . the orientation of the nitrogen injection slot of the exit device is about 90 ° to the roll and its width about 0 . 3 mm . the distance between the exit device and the support roll is about 0 . 8 mm . the configuration illustrated by this embodiment makes the detachment of the air boundary layer located at the surface of the film to be even more effective ( compared with the configuration described previously in conjunction with fig2 ), and therefore provides greater insurance that the air conveyed to the surface of the film will not penetrate the treatment chamber . in fact , the entry device of fig6 may be considered as a combination of the entry devices of fig1 and fig2 : the first injection slot , owing to its position upstream of the labyrinth , tends to direct the gas toward the upstream and therefore to suppress the intake of air ; the second injection slot , owing to its position downstream of the labyrinth , tends to direct the gas toward the downstream and therefore to fill the chamber with gas . to measure the effectiveness of the latter embodiment , experiments on controlling the atmosphere in a chamber equipped with entry / exit devices such as those illustrated in conjunction with fig6 and 7 were carried out . the results are given in table 1 below . slots 1 and 2 correspond to those of the entry device , while slot 3 corresponds to that of the exit device . it should be noted in this table that both the flow rates in sm 3 / h ( as is conventional ) and volume in sl / m 2 of film treated have been indicated so as to be able to continue the comparison with the results presented earlier . the results show that , thanks to the equipment of fig6 and 7 , uv irradiation treatment may be carried out in an inert nitrogen atmosphere containing less than 40 ppm oxygen , whatever the speed , with a total volume of nitrogen between 4 . 2 and 5 . 8 sl / m 2 ( and therefore in general less than the volumes required within the context of the embodiment shown in fig2 ). in the foregoing , the invention was most particularly illustrated by examples using nitrogen , but it should be noted that it would be possible , without at any moment departing from the scope of the present invention , to use other gases or gas mixtures , and especially argon , co 2 , helium or mixtures thereof . it may even be indicated that it is preferable to use co 2 or mixtures containing co 2 , since it has been found that when co 2 ( as opposed to nitrogen ) is used : the volume of gas to be used for the same performance in terms of residual oxygen content in the chamber may be reduced ; for the same volume of gas , the residual oxygen content obtained in the chamber is reduced . such results are probably due to the density of co 2 , which is higher than that of nitrogen . it will be understood that many additional changes in the details , materials , steps and arrangement of parts , which have been herein described in order to explain the nature of the invention , may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims . thus , the present invention is not intended to be limited to the specific embodiments in the examples given above .