Reference lighting conditions are those produced in a photometric sphere, which gives a perfectly diffuse illuminance and in which interior luminances are equal from point to point. Sphere illumination produces very few veiling reflections, but it is not necessarily the optimum for good visibility. Rather it is a convenient and easily reproducible reference lighting condition. ESI can be measured directly or calculated using computer programs, but its general application to lighting design is not without problems. Any single value of ESI is a function of the photometric properties of the luminaire, lighting geometry, room reflectances, viewing position and reflectance properties of the task.
Direct measurement using relatively inexpensive instrumentation is now available.
The values obtained, however, apply only to a standard pencil task and it is not yet clear how the values for more common visual tasks will compare. The ESI concept can and does help designers to produce good quality lighting and reduce lighting energy consumption, encouraging them to design for visibility and avoid unnecessary high illuminance levels in non-working areas. Reducing Power Input and Lighting Quality One of the most important considerations in lighting system design is the specification of the required illuminance level. ESI designs to date have been more than adequate in lighting quality, and have in many cases been regarded as providing a more interesting, luminous environment than traditional monotonous, uniform illuminance schemes. The non-uniform lighting principle advocates lighting systems that deliver to a work station the quantity and quality of lighting suitable for the task to be performed, its location, operator characteristics, and other related factors.