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

below.
Relying on Adaptation
Adaptation refers to the ability of the human vision system to perform well under different light levels. All of us have experienced the phenomena of waiting for our visual system to adjust from one lighting scenario to another. When we walk from a sunlit parking lot to a darkened movie theater, our eyes and brains work together to maximize the quantity of light entering our visual system.
When we transition from the dark theater back to the bright parking lot, our eyes
and brains work to limit the amount of light being translated. Although in both cases, this process takes time, eventually, our eyes and brains adjust so that we are able to function in both situations. The light levels that we are dealing with in the two extremes are drastically different. The variation between light levels under a high noon sky and a moonlit night are on the order of tens of thousands. It is truly a feat of physiology that we are capable of reading a book in both of these situations.
The design implications of this adaptation work tremendously in our
favor. Because our visual system is constantly at work adjust to the surrounding light levels, we can deduce that excess light introduced into a space loses effectiveness as our visual system works to adapt and “even-out” our experience. Thus, we can likely get away with far less light in many spaces as our visual system will change to make the most of what light is available.
These effects are most noticeable in evenly-lighted enclosed spaces. A
room filled with flat, evenly distributed light will appear very similar, no matter the actual light level, as the visual system adapts.
Adaptation also tells us that a space full of surfaces receiving different
amounts of light will always exhibit contrast, no matter what the actual light levels are.
Brightness through Contrast
Brightness is a common term used to describe the perceived lighted effect of the surfaces in our day to day environments. Brightness is not, however, an absolute property of a surface. Because the human visual system adapts to each lighted situation, brightness is a subjective judgment made by an individual in a specific lighting situation. It is valuable to understand that contrast between objects is what defines vision and drives our brightness judgments. Our eyes tell us where one object begins, and another ends because the objects reflect light differently. When we read a book, the dark ink reflects less light than the white paper, and
we can distinguish the shapes of the letters on the page, even though both materials are receiving the same quantity of light. As we apply more light to the pages of the book, the white paper reflects more light and appears brighter, while the dark text continues to reflect very little light. Another simple example is the corner of any room: Inevitably, we can tell where one wall meets the other because of the contrast between the two surfaces. If two intersecting walls exhibited the same brightness, our perception would be of one continuous surface. When we apply higher light levels to visual tasks or accented objects, it is an attempt to improve visibility by increasing the contrast between the objects or materials. Objects are not visible simply because they have light cast onto them; they are “visible” because they reflect light differently than the their surroundings.