Apparatus for optical measurement of substance concentrations

An apparatus for measuring the concentration of a substance using optical radiation, said apparatus, comprising:

• a housing;
• at least one transmitter for emitting the optical radiation into a path, in use of the apparatus said path passing at least once through a layer of the substance, said layer having a thickness, said at least one transmitter being mounted on said housing or in said housing;
• at least one receiver for receiving at least a portion of the optical radiation which has passed at least once through the layer of the substance, said at least one receiver being mounted on said housing or in said housing;
• a deflection device disposed in said path between said transmitter and said receiver for deflecting the optical radiation toward said receiver, and
• an adjustment device for adjusting a distance between said deflection device and at least one of (i) said transmitter and (ii) said receiver, said distance being adjustable such that said thickness of the layer is adjusted by adjusting said distance and such that upon said distance being adjusted such that said deflection device is sufficiently close to said housing, a closed hollow space is formed between said housing and said deflection device.