The architecture of light: architectural lighting design concepts and techniques. A textbook of procedures and practices for the architect, interior designer and lighting designer pdfdrive com

IES ILLUMINANCE LEVEL CRITERIA
A more specific way to arrive at illuminance level targets for specific tasks and critical effects is to use the well-established system put forth by the Illuminating Engineering Society of North America (IESNA). The IESNA is a scientific body dedicated to the study of lighting and its effects on human visual performance.
The IESNA has committed significant resources to the study of illuminance
levels onto tasks.
Remember that Illuminance levels are descriptions of how much light we are putting onto an object. Illuminance levels don’t account for reflectance or how light will leave a surface. This limits their value in trying to describe how a surface, object or space will look.
Illuminance levels are exceptionally useful when determining appropriate
light levels for tasks. This works primarily because visual tasks involve objects
with known reflectances. If we know the reflectance of the materials involved in a task, it is safe to prescribe a quantity of light that should be cast onto the task to create the necessary contrast. Reading, for example, involves dark text on light-colored paper. As we cast more light onto a reading task, the light-colored paper reflects more light back at us, while the dark ink continues to reflect very little. In this manner, the contrast between the two increases. This is the fundamental basis for the long list of Illuminance values that the IESNA provides. The IESNA publishes a book that categorizes a variety of tasks articulated with a variety of materials. For each of these task and material combinations, the IESNA provides a recommended Illuminance level. IESNA literature can be ordered from www.iesna.org.
The IESNA recognizes that there are many factors that dictate what is appropriate lighting. Another significant aspect of the IESNA illuminance selection procedure is the assessment of the designed space and consideration for all of the other visual performance issues that must be addressed. This includes concerns like color rendition, evenness, and glare. It is helpful for a designer to develop his/her own list of lighting issues and concerns like the one presented in Figure 19.3. Rating the importance of each - on a scale of one to ten, say - can add even more value to such a list.
Figure 19.3 A list of lighting concerns that should be considered for every project
The IES selection procedure can certainly help a designer avoid under- lighting critical situations, but it is really intended as a procedural guide for visual tasks (the fifth layer of our 5-layer system).
For visual effect, aesthetics, mood, and spatial organization (The
remaining four layers from our 5 layer system), intuition and experience will serve us better than exact numbers. This is the reason we have gone to great lengths to voyage through a thought process that leads to understanding and intuition, before diving in to quantification. Illuminance values are a welcome level of information to add to our design intent, but they are only one part of a much bigger picture that has its foundation in visualization and graphic rendering of lighting concepts.
Remember to visualize lighting effect first and then draw and describe the lighting effect. Only if the designer feels the need to further articulate does he/she need to move on to specifying target Illuminance levels. We add these values to our light map to clarify our intent and make the selection and placement of lighting equipment easier. If targeting a specific task or accent light level does not serve to progress our design, it is not worth taking the time to identify.