The background levels in Table 1 correspond to 6 Jy, 0.44 Jy, and 1.30 Jy per aperture/pixel with P_60, C_100, and C_200, respectively. Thus, in case of a typical faint source measurement, the source is situated on top of a high background. To achieve the sensitivity limits listed in Table 1, ISOPHOT should detect a difference signal between the source and background positions with an accuracy of order 1 % or less of the total signal. Below we discuss the most important technical difficulties that must be overcome to achieve this accuracy.