Eley et al. (1993) further point out that:
ESI, RVP and VCP values are all influenced by factors other than lighting system illuminance. Factors may include task type, age of the observer, angle of viewing the task, and lighting system characteristics. In order to use lighting quality metrics effectively, the designer must know all the factors listed below with absolute certainty. This will allow the designer to calculate these values accurately for specific types of tasks and task locations.
Different ESI and RVP values will result in the same room and lighting system for different tasks. Values will not only be higher or lower, but a wider range will be evident with veiling reflection-sensitive tasks. The size of the task must be known (such as the solid angle of the letter "e" on this page).
Significant differences in luminance between the task and surrounding room surfaces may have a profound effect on task visibility and/or visual performance. This is particularly relevant for work spaces containing VDT terminals.
Exact viewing direction and angle of the task must be known within a few degrees. The same task may have widely different ESI and RVP values depending on the angle from which it is viewed in the room.
The room under study must be described exactly and the photometric data of a specific luminaire must be used in the study. This can be a limitation if, for instance, the computer program is unable to include the effects of partitions or task lights.
Human visual systems deteriorate with age, and older eyes allow less light to reach the retina than do younger eyes. Therefore, in order to predict how visible a task will be, it is necessary to account for this reduction. RVP requires a user age input and applies an average light reduction for that age group.
Visual performance computations, when performed properly, are much more accurate indicators of the visibility of visual tasks than are simple illuminance measures. The IES has announced that in the future, visual performance will replace illuminance level computation as the standard for lighting design. Computation of visual performance metrics is fairly easy to perform, given the right computer program. However the application of ESI and RVP metrics is relatively limited unless the designer or engineer is experienced with the technology and its limitations, and is cautious with its application. Formal training is available and highly recommended for those desiring to employ visibility metric analysis.
ESI, RVP, and VCP are useful at comparing competing lighting systems and designs on a statistical basis when specific tasks and locations are unknown. As long as reasonable assumptions are made in regard to possible tasks, viewing directions, viewing angles, and task locations, the probability that one system will provide superior performance over another may be ascertained, assuming the analyst respects the statistical significance of the results.
