I think it is important to introduce a second technical report, as I like to call it, to define another important material qualities: the reflectance. I wish readers will be patient, because I think that a full understanding of the material qualities allow us to conveniently use light. In future articles of Architect Daylight I will expose less technical issues.
As we saw in the previous technical report, the reflection from a surface depends on its roughness and directly affects the reflected light. However, the surfaces have another quality: the reflectance.
In short reflection defines how light is reflected and the reflectance indicates how much is reflected. If the reflection depends on the roughness of the surface, the reflectance depends on the brightness.
Thereby the darker surface reflects less light and vice versa. As an example we see the following figure with three spheres of different brightness: white, gray and black:
The following image shows the reflected light (equivalent to luminance, a concept that we will see in better time) emitted by the three spheres, in false color map. The blue color represents less reflected light and red more:
As you can see, the white sphere reflects much more light than the other two spheres. This is because it reflects almost all incident light and absorbs very little. On the black sphere, opposite happens. Whenever a beam of light strikes a surface, some are absorbed and partly reflected. Reflect more light, more brighter the surface:
And now, two statements that seem to come from opiate reasoning:
1. - The pure white does not exist. The TV ads were lying.
Every surface, no matter how brilliant, absorbs a minimal portion of light. On the other hand, there is no pure black (except in dark matter or black holes).
2. - The amount of reflected light is what determines the brightness of an object, not vice versa. That is, when a surface reflects a lot of light it is perceived like white, whereas if it absorbs a lot of light it is appears like black.
"Now I understand why I see the things I see"
In simple words: light defines how we see things and not the reverse.
I hope this article has been clearer than troubling, to encourage potential readers to continue reading on light in architecture.