Abstract: Photometric cuvette for optical analysis of through-flowing medium, made as a thin and narrow transparent tube requiring minimum sample amounts. Light, substantially parallel to the tube length, is led obliquely into the tube through its wall, is reflected and led obliquely out through the tube wall to a detector. The angle of incidence is selected so that after entry, the light is totally reflected against the outer tube wall where the wall is in contact with surrounding atmosphere, the light being supplied essentially uniformly around the tube.