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

Chapter 19 Understanding Illuminance Levels

Light is most commonly described by how much we detect to be leaving a surface and striking our eyes. “Brightness” is a generic term for this phenomenon. Oddly, in lighting design study, we more commonly describe the quantity of light striking a surface. Very often this is because we are discussing light quantities as they are needed to perform visual tasks. These visual tasks involve materials of various reflectances that create contrast. This quantification of light “on to” a surface is called Illuminance. And when we get scientific about quantitative lighting study, our discussions will revolve around measurements and expressions of illuminance. Therefore, we will commonly express light levels in terms of foot-candles (the unit of measuring Illuminance) incident “onto” a surface.
The study of specific quantities of light desired is a natural progression of the Light Map process. When we start addressing each individual lighted surface – especially accented surfaces (Layer 3), or task surfaces (Layer 5) – we will need to identify specific illuminance levels that we want to provide so that these accented surfaces stand out from their surrounding environment and so that the task surfaces receive enough light to perform the visual task at hand.
ILLUMINANCE LEVEL INTUITION
There is a wealth of published information about what specific illuminance levels are appropriate to provide onto surfaces to accommodate different types of visual tasks, and many people use these guidelines as the basis of their design. Adhering to prescribed illuminance levels for tasks and neglecting the lighting effect of a whole space can, however, lead to a very one- dimensional experience.
Because we are after a much more holistic approach to our design, we are
interested in an intuitive ability to visualize different Illuminance levels and what they represent. Though prescribed illuminance levels are really intended only to
be considered for a specific surface, a designer can put Illuminance levels to good use by utilizing them to describe the overall, ambient glow of a space where many surfaces are experiencing a similar illuminance level. This is technically an abuse of the intention of illuminance level prescription, but is such a useful way for designers to communicate lighting intention that we take the liberty. When one describes an entire space as exhibiting an “average” illuminance level, one must account for contrast, accent, and areas of higher and lower light levels within it.
Assembled here is a short list of illuminance levels (expressed in foot- candles) that can help a designer to visualize what these different light levels can mean to a design. As you consider each space type, close your eyes and visualize the space described if it were lighted fairly evenly. What you are visualizing on the surfaces of the space is likely the illuminance level listed. Remember this is not a list of recommendations, but rather a reference for visualizing.