Apparatus for additively manufacturing three-dimensional objects by means of successive layerwise selective irradiation and consolidation of layers of a build material which can be consolidated by means of an energy beam, the apparatus comprising: - a process chamber comprising a build plane in which layers of build material are successively layerwise selectively irradiated and consolidated by means of an energy beam during operation of the apparatus; - a gas stream generating device configured to generate a gas stream at least partly streaming through the process chamber during operation of the apparatus, the gas stream being capable of being charged with non-consolidated build material particles, particularly smoke or smoke residues generated during operation of the apparatus, while streaming through the process chamber; and - an optical determining device configured to optically determine at least one parameter suitable for characterizing the streaming properties of the gas stream streaming through the process chamber during operation of the apparatus at at least one defined location within the process chamber, wherein the optical determining device comprises at least one optical measuring unit configured to optically measure at least one measurement value within the process chamber, the measurement value being related to the gas stream, and at least one evaluating unit configured to evaluate measurement values measured by the optical measuring unit so as to determine the at least one parameter suitable for characterizing the streaming properties of the gas stream, wherein the at least one optical measuring unit is configured to measure measurement values at different locations within the process chamber, wherein the optical measuring unit is configured to measure a plurality of measurement values at different locations within the process chamber. - a process chamber comprising a build plane in which layers of build material are successively layerwise selectively irradiated and consolidated by means of an energy beam during operation of the apparatus; - a gas stream generating device configured to generate a gas stream at least partly streaming through the process chamber during operation of the apparatus, the gas stream being capable of being charged with non-consolidated build material particles, particularly smoke or smoke residues generated during operation of the apparatus, while streaming through the process chamber; and - an optical determining device configured to optically determine at least one parameter suitable for characterizing the streaming properties of the gas stream streaming through the process chamber during operation of the apparatus at at least one defined location within the process chamber, wherein the optical determining device comprises at least one optical measuring unit configured to optically measure at least one measurement value within the process chamber, the measurement value being related to the gas stream, and at least one evaluating unit configured to evaluate measurement values measured by the optical measuring unit so as to determine the at least one parameter suitable for characterizing the streaming properties of the gas stream, wherein the at least one optical measuring unit is configured to measure measurement values at different locations within the process chamber, wherein the optical measuring unit is configured to measure a plurality of measurement values at different locations within the process chamber.