A novel approach to measure both the particle and the NO2 concentration in the exhaust of diesel engines during the roadworthiness test in the periodical technical inspection is presented. It is based on a multi-wavelength extinction measurement and a mathematical inversion algorithm to obtain the concentrations from the extinction readings. Such individual concentration values can deliver valuable insight into the cause of engine or exhaust aftertreatment defects. Furthermore the extended opacimeter provides future-proofness, if nitrous gas emissions are incorporated in the roadworthiness regulations. In addition to a detailed description of the multi-wavelength approach this chapter provides an overview of particle and nitrous gas emissions by diesel engines, the related legislation, the extinction measurement using standard opacimeters, and the physical background for this optical measurement method. The applicability of the multi-wavelength method is derived mathematically and validated with first experimental results as well as with simulations.