The ISOPHOT data processing contains several corrections due to the properties of the detector, due to instrumental effects by electronics and optics, and due to external effects such as cosmic ray bombardments. In this chapter the effects are described that could lead to a better understanding of the data and of the photometric calibration. The descriptions start with the detectors and their properties in Section 4.2, the features generated by the instrument electronics in Section 4.3, and ionising radiation effects in Section 4.4, which have a dramatic impact on the detector output. Subsequently the optical (Section 4.5), spectral (Section 4.6), and straylight performance (Section 4.7) are discussed. Finally, we mention the instrumental polarisation (Section 4.8) and the influence of the background on the photometric data (Section 4.9).