This invention relates to retrievable measuring cell for optical measurements in gas, the cell being defined by a gas conducting pipe having an input end adapted to be connected to a gas flow input intruding gas into the cell and an output end adapted to be connected to a gas flow output. The pipe ends also being adapted to be coupled to optical components including an optical transmitter transmitting light into said cell and an optical receiver adapted to receive light having passed through said cell, the optical beam in said cell having a predetermined shape, the optical components including a light source, at least two minors and a light receiver being mounted in known positions on an external frame covered by the pipe ends. The cell has an elongated shape corresponding to the optical beam shape.