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

LIGHTING FOR VISUAL INTEREST: THE “2X” RULE
As designers primarily interested in adding impact and emotion to our environments, much of what we light are accented surfaces and objects that must stand out from the surfaces around them. When we paint light onto objects with the intent of drawing attention to them, it is helpful to identify specific light level targets to make sure the impact of design stays intact.
Now that we have a basic understanding of what illuminance levels mean,
we will introduce a useful rule-of-thumb for designing accent and feature lighting. We call this rule the “2 times” contrast rule, and we use it frequently for creating accents and visual interest. The rule is based on fundamental vision science that tells us that an object must be twice as “bright” as a surface adjacent to it to appear noticeably “brighter”. When we light an object or surface with the intent to make it “pop” or serve as a focal element, we implement a simple version of this theory by illuminating the accented object with at least twice as much light as the surrounding environment. This simplified solution of addressing only the amount of light cast onto an object ignores the reflectance and color of the object, but it is a good starting point. There are more complex ramifications of this rule that have to do with object color, and reflectance, but for our design purposes, we can safely rely on the broad version of this rule.
Generally, the more light we cast onto a surface, the greater the accented effect.
The other end of the contrast rule comes from the desire to not create too high a level of contrast that might be translated as glare. To avoid uncomfortable
glare and excess contrast, we avoid lighting an object to be more than 5 times as bright as the surrounding environment.
So the “2 times” contrast rule really becomes the “2 to 5 times” contrast
rule. We say that for creating visual interest in designed spaces, we want to light our objects to be twice as bright as their surroundings, but not more than 5 times as bright. We accommodate this in a very simplified manner of illuminating objects to two to five times the illuminance of the surrounding environment.
The effects of this rule can be written onto the light map to further clarify
the lighting design intent. The process is as simple as identifying the objects and surfaces one wants to use as focal elements in each space. Because the designer has already assigned a desired ambient glow to these spaces, he/she has an illuminance value to use as the basis for the “2 to 5 times” contrast rule.
Mentally walk through the design, space by space, and identify objects and label
them with a target accent illuminance level.
An example might be a dining room where the design intention calls for a minimum ambient glow of the space to be around 10 foot-candles. Using the 2 to 5 times rule, we see that our accented objects should be illuminated to levels between 20 and 50 foot-candles. The “2 to 5 times” rule requires that we have first established an ambient illuminance level with which to work. This is why we went through the process of targeting overall ambient illuminance levels for the whole space. These two simple steps of defining an ambient glow for a space first and then using that as the basis for accents are quick and effective.
Figure 19.2 Establishing ambient illuminance level targets (in green) allows a designer to go back and define areas of heightened light levels (in pink).
How we will actually get the light there can be worked out later. For now, we are simply adding more information to our Light Map to make it increasingly easier for us to locate and select luminaires.