A handheld PDA-based system is being developed to help blind people in their daily tasks. The design combines in a continuous process Users' involvement and Engineers' effort. This interaction is made efficient thanks to a specialist able to communicate with both parties and to extract useful knowledge for them. The system can be viewed as a main loop including the User taking the snapshot, Text/Picture detection, Optical character recognition, Text-to-speech synthesis, Feedback to the User, until a useful output is reached. Each task is carried out by algorithms integrating both technical performance and user requirements.