The Non-Ionizing Energy Loss (NIEL) – Hypothesis is assuming that radiation damage to solid-state detectors originating from displacement damage to the crystalline bulk of the sensors scales linear with NIEL. Within this concept every radiation field or monoenergetic particle source can be scaled to a 1 MeV neutron equivalent fluence by a hardness factor. The hardness factor is given by the ratio of NIEL deposited by the particle or particle field in question to the NIEL deposited by a 1 MeV neutron in the same sensor material. The NIEL can be calculated by considering all possible interactions an impinging particle can have with the detector material. Due to the multitude of possible interactions and their complexity, Monte Carlo based simulators for particle passage through matter, like Geant4 or FLUKA, are the most adequate tools for NIEL calculations. They can thus be used to produce hardness factors for any kind of radiation source. On the experimental side, hardness factors are usually obtained by comparing the amount of radiation damage observed after exposure to the radiation source in question as compared to the radiation damage observed after exposure to a radiation source with a well-known hardness factor. In this work we review the limits of the hardness factor concept based on NIEL for silicon detectors and we propose a revised NIEL concept that considers the spatial distribution of displacement damage in form of single point defects and clustered crystalline defects. First estimations on the generation ratio of point to clustered defects in silicon for various particles and particle energies are given. They are based on a combined Geant4, SRIM and OPTICS simulations approach. At the same time, we propose to consolidate the quantity and quality of the available experimental data on hardness factors for various radiation facilities. The key deliverable in this context is the production of a set of silicon dosimeters for hardness factor evaluations at various radiation facilities. First measurements on the silicon dosimeters, that have been produced in form of p-i-n diodes, are presented.