Summary In this chapter the different factors required in a forward model, which predicts the image response to emission in a volume element in the ionosphere, have been described. In Chapter 3 the necessary requirement for accuracy was found to be approximately 1-3 %. For some factors in the forward model, sufficient accuracy requirements are obtained; for others some additional work is needed before the necessary requirements are met. To obtain a full forward model that meets the accuracy requirements seems to be the most important task to accomplish, i.e. there is an urgent need for an accurate and reliable absolute calibration of the ALIS cameras. Further it should be noted that the factors described in sections 5.4, 5.5, 5.6 and 5.8 preferably are done at once with a light source that gives isotrop illumination of the optics -- an integrating sphere (e.g. Urashima et al., 1999).