The Advantest Q8221 employs a two-channel plug-in system. Various optical sensors and light sources are available as plug-in units. The two channels of the Q8221 can be used individually or simultaneously. Free combination of optical sensors and light sources enables diverse applications. The optical sensors for Q8221 assure high accuracy of ±2.5% at calibration point. In broad band wavelength region, they assure ±4.5% accuracy by compensating the sensitivity curve over wavelengths of each sensors. Further more, the linearity of ±0.5% is assured. Not only at the calibration point, these sensors also assure at the broad band wavelength region and the level to be measured.