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

Figure 18.1 The two common ways we consider light interacting with simple surfaces.
Illuminance:
Illuminance is the measurement of lumens of light striking onto a surface.
Illuminance is measured and expressed in foot-candles (FC).
1 foot-candle is the equivalent of 1 lumen of light distributed evenly onto a 1 square foot surface.
Figure 18.2 Diagram of one foot-candle (density of light onto a simple surface).
Exitance:
Exitance is the measurement of lumens of light leaving a surface or source.
Exitance simply accounts for the total number of lumens leaving and gives no information about the density or direction in which the light is leaving. The exitance of a reflective surface is the illuminance onto that surface multiplied by the reflectance of the surface. If a surface is 50% reflective, the exitance off of the surface will equal half of the illuminance on to the surface. In the case of a light source, the exitance is the sheer number of lumens that the light source is creating and emitting.
Exitance is seldom used to describe light levels, but understanding it is useful for visualizing how lumens interact with surfaces and objects.
Luminance:
Luminance is the measurement of a specific density of lumens of light
Figure 18.3 Diagram of Candle-power (density of light leaving a light source
leaving a surface or source in a specific direction. It describes light how an observer experiences it and explains our instinctive undertanding the intensity of light on a surface diminishes as that surface gets further from the light source.
Luminance is measured and expressed in candelas per square meter (CD/sq.M).
In order to understand what luminance represents, it is
necessary to understand what a candela represents.
The candela is the unit of light density. It is a useful way to express how densely light leaves a source or surface. One candela is the equivalent of light leaving a source with a density of one lumen per “steradian,” of spherical area. A steradian is a contoured area made of a portion of a sphere. The area of a steradian is such that there are always 4Π (four pi, or 12.56) steradians in any sphere. Thus in the case of a sphere with a radius of 1 ft. (and thus a total spherical surface area of 4Π), each steradian is 1 sq.ft. in area. As a sphere grows larger, the size of a steradian carved from the sphere grows larger exponentially. Figure 18.3 shows two different spheres and one steradian of area carved out of each of them. Because light spreads out spherically as it leaves its origin, the same number of lumens passes through each steradian despite the area difference. But, an object of fixed size receives fewer and fewer lumens as the distance increases and light spreads out.
This candela output is very useful for describing directional light sources like accent luminaires that direct far more light forward than off to the
side. Knowing the candela value that comes from the center of an accent luminaire is useful for calculating the illuminance level that will occur at a point on a surface or object due to that specific luminaire.
The nature of the candela means that the luminance value exhibited by a light source or surface differs based on the angle from which the source or surface is being viewed. It is luminance that the eye detects and uses as the basis for decisions about source or surface brightness.
Although we have crafty names for all of these interactions and their units, the most important and commonly-encountered in lighting design is illuminance expressed in foot-candles. This expression of light onto objects is easy to measure and helps us to understand the levels of contrast we can create in an environment. Illuminance values onto a surface are independent of the color or reflectance of the surface, so an illuminance value alone does not indicate how an object will look. A black object and a white object, side by side on a table, may be experiencing the same Illuminance level from the luminaires above them. The drastic difference in appearance between the objects is a product of their reflectance. To successfully visualize a situation, we must be told the reflectance of the objects (values and colors), as well as the Illuminance level onto the objects. This gives us enough information to interpolate the exitance from the objects. If we also know the texture of the books and how they will direct reflected light, we can estimate their luminance, which is much more closely related to our description of object “brightness.” It is worth noting that all of these measurements of light do not dictate objectively how bright a surface will appear. Brightness is a judgment made by a viewer and is dependent on the adaptation of the viewer and the contrast of the environment.
In all three of these cases of lighting measurement, we are talking about
lumens interacting with objects, so if all else fails; it is always safe to describe light as lumens.