An apparatus including a process chamber accommodating a carrier for receiving a raw material powder. An irradiation device of the apparatus is configured to selectively irradiate electromagnetic or particle radiation onto the raw material powder on the carrier in order to produce a work piece by an additive layer construction method, wherein a transmission element allows the transmission of the electromagnetic or particle radiation into the process chamber. The apparatus further includes a gas inlet and a gas outlet for supplying and discharging gas to and from the process chamber which are configured to generate a protective gas stream for protecting the transmission element from being contaminated by impurities present in the process chamber. The gas inlet includes a gas permeable, porous component forming a gas inlet area.