An apparatus for photometrically testing several specimens each irradiated by a light source, the light altered by the specimens being detected by an optical device and analyzed, the apparatus including a light source, a plurality of sample holders configured adjacent to one another on a support, a detector that receives altered light from sample within the sample holders, the detector including a filter for eliminating interfering light, a sensor having a sensor face and a diffusing optical member located between the filter and the sensor, wherein light is diffused and shines on a greater portion of the sensor surface, and with a more homogeneous brightness as compared to when the light is not diffused.