Immersion probe having variable path length

An immersion probe for one or both of a quantitative and a qualitative analysis of a fluid (F), which includes a liquid or a gas, using light that is guided through the fluid (F) along a light path, the immersion probe comprising:

• a measuring head suitable for immersion in the fluid, having;
• an input light guide connected to a light source and configured to guide light to the fluid (F), and an output light guide configured to guide light from the fluid (F), the output light guide connected to a detector;
• an optical window and a reflector, a surface of the optical window is in contact with the fluid (F) and facing a surface of the reflector, the surface of the reflector also in contact with the fluid (F);
• and
• at least one light deflection device, which is arranged in the light path of the light exiting the input light guide and entering the output light guide and deflects the light coming from the input light guide through the optical window into the fluid (F) and deflects the light coming from the fluid (F) to the output light guide, the reflector being suitable for reflecting the light, which has passed through the fluid (F), back through the fluid (F) and the optical window, to a first light deflection device or a second light deflection device of the at least one light deflection device, wherein:
• the optical window and the reflector are arranged with respect to each other in a wedge shape, and the surface of the optical window and the surface of the reflector pointing toward an interior of the wedge shape and facing the fluid (F) to be analyzed, and
• one or both of the at least one light deflection device and the reflector, configured to move back and forth relative to the optical window substantially in a direction of an opening of the wedge shape.