The D-R 290 operates using the double-pass method according to the auto-collimation principle. The light beam traverses the measuring path twice. The attenuation of the light beam by the dust content in the measuring section is measured and evaluated. The light source is a super-wide band diode (SWBD) which provides more stable measurements in comparison to conventional LEDs. The D-R 290 has an internal zero and reference point check as well as contamination measurement. The contamination is automatically compensated. A maintenance demand is immediately indicated.