Abstract: The invention concerns a light-optical detection device for determining the concentration of suspended particles in a gaseous medium within a light-tight housing. The latter essentially is formed by a short cylinder sheath having two front-surface coverings that are formed by a base plate and by an opposite cover plate. Cylinders extend into said housing for receiving, respectively, a light source and a photocell. The cylinders have axes which intersect at right angles at the center of the device. The axis of the cylinder containing the light source essentially runs in the direction towards the diagonally opposite corner of the interior of the device.