A concentration measuring device for determining a concentration of gas or particles in a measurement volume, comprising - at least one housing having an opening for communication with the measurement volume; - a light source for transmitting measurement light through the at least one housing into the measurement volume; - a light receiver for receiving the measurement light after its passage through the measurement volume, wherein the light receiver is arranged either in the same housing as the light transmitter or in a second housing having an opening for communication with the measurement volume; and - an evaluation unit which is designed to determine the concentration of gas or particles from the measurement light received at the light receiver, in particular from the absorption of the measurement light on the path from the light transmitter to the light receiver, characterized in that at least one body (18, 20) of solid material is arranged in the at least one housing such that the measurement light path largely passes through the at least one solid body (18, 20) within the at least one housing, with the portion of the measurement light path within the at least one housing not passing through the at least one solid body (18, 20) having a total length which is smaller than a length SL, where (i) SL = 2 · G · ML/K is true and G is the desired accuracy of the concentration determination on a measurement length ML in the measurement volume and the concentration of the gas to be measured or of the particles to be measured within the dead volumes occurring in the housing amounts to K; and (ii) the length SL is selected such that an accuracy of 0.1% is achieved with a measurement length ML of 1 m in the measurement volume and a concentration of the gas to be measured or of the particle concentration to be measured within the dead volumes occurring in the housing of 20%. - at least one housing having an opening for communication with the measurement volume; - a light source for transmitting measurement light through the at least one housing into the measurement volume; - a light receiver for receiving the measurement light after its passage through the measurement volume, wherein the light receiver is arranged either in the same housing as the light transmitter or in a second housing having an opening for communication with the measurement volume; and - an evaluation unit which is designed to determine the concentration of gas or particles from the measurement light received at the light receiver, in particular from the absorption of the measurement light on the path from the light transmitter to the light receiver, (i) SL = 2 · G · ML/K is true and G is the desired accuracy of the concentration determination on a measurement length ML in the measurement volume and the concentration of the gas to be measured or of the particles to be measured within the dead volumes occurring in the housing amounts to K; and (ii) the length SL is selected such that an accuracy of 0.1% is achieved with a measurement length ML of 1 m in the measurement volume and a concentration of the gas to be measured or of the particle concentration to be measured within the dead volumes occurring in the housing of 20%.