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

“Light” is the name for a group of specific wavelengths of radiation that happen to be detected by our eyes.
Color is not a property of an object. Color is our brain’s translation of the radiation being reflected from an object to our eyes.
BASIC LIGHTING INTERACTION
TERMINOLOGY
To move forward and speak with clarity about the ways light interacts with our environment and our visual system, it is important to point out that in all cases of light, we are talking about the fundamental piece of light: The Lumen. Lighting science makes the task of talking about light a little bit tricky by lending different names to the phenomena of lumens of light interacting in different ways. The specifics of measuring lumens of light will be discussed later in chapter 18. For now, the critical knowledge is merely a bit of vocabulary.
Illuminance is an expression of the quantity of light falling onto or “striking” an object. The Illuminance onto a surface does not necessarily tell us what that surface will look like, as it does not define the amount of light that will reflect off of that surface. Knowing the illuminance level onto an object does, however, allow us to predict the contrast that will be created between materials of different reflectances.
Exitance is an expression of the total quantity of light leaving a surface. Exitance is easy to understand because it is simply a matter of counting up the number of lumens of light that leave a source or surface. This simplicity is also what limits the usefulness of exitance as a description. Exitance tells us how much light is leaving a source or surface, but it doesn’t tell us in what direction or where that light ends up.
Luminance is the phenomena of light leaving a surface at a specific density in a specific direction. Although luminance is difficult to measure, it is extremely useful because it describes light coming off of a surface just as a viewer would experience it. Describing the luminance levels of a particular situation allows us to visualize the lighted effect of the space.
The safest way to use these terms properly is to get used to the prepositions related to each. It is appropriate to talk about Illuminance “on to” a surface. We generally speak of the Exitance “off of” a surface. We generally talk about the luminance “coming from” a surface.