According to Eley et al. (1993) :
The emphasis of most lighting calculations is to predict illuminance levels in lux or foot-candles. In addition, it is possible to calculate more specific lighting quality measures, which take many other factors into consideration. Relative Visual Performance (RVP) and Equivalent Sphere Illumination (ESI) are measures of visibility and visual performance that measure a person's ability to perform visual tasks under specific lighting conditions. Visual Comfort Probability (VCP), on the other hand, addresses visual comfort and the presence or absence of discomfort glare. These visual quality measures have certain lighting design applications (as well as limits) and are features of many lighting analysis programs.