2.3 System Theory
In a comparison with system theory, which sees the eye as a vision system and any surgery like an intended system change, we can easily find a first set of criteria for evaluating each approach. Although most of these criteria seem like essential requirements when just getting rid of glasses, they can change from essentials to achievable goals, if the person suffers strongly from ametropia.
In system theory, stability is the most vital criteria for any design, meaning that after certain stimulation the system returns to its equilibrium, at best the former working point. There exist many categories of stability. However, in our case this criteria will mean that after surgery and the healing period there will be no change in the achieved refraction. Neither in the long term, for instance, a hyperopic shift, nor daily changes may occur.
2.3.2 Deviation from Target
The second criteria for approaches in system control is the deviation from the target after the system is stable. In our case: How close does the patient get to emmetropia after the healing period?
2.3.3 Behaviour in Transitional Phase
The third criteria covers the transition period from the induced change to the newly reached stable position. The usual goals are a quick progression to the stable position, and only a small deviation from the target size until stability is reached. These are often two contrary goals. In our situation this means that the desired emmetropia is reached quickly after surgery and with limited overcorrection during this time of healing.
The last criteria is robustness which stands for unchanged well functioning in changed, unusual condition. Will the refraction change while montainclimbing or doing deep sea diving?