Luminance: Reflections on Reflections
The human visual system is truly remarkable. When I was a kid, I was taught that the human eye was like a camera. I have learned that the human eye is so much more awesome and powerful than any giga-mega-pixel camera anywhere.
We are only now beginning to understand that our eyes, much like our ears, have two functions. Our eyes take in light down two neural pathways – one is visual and one is non-visual. While we know a great deal about the visual path, the non-visual neural pathway is a mystery that is just beginning to reveal itself.
Imagine turning on a flashlight in a very clean and dark room. We see the light reflected from the wall but we don’t see the light between the wall and flashlight. The light that is incident on the wall is called illuminance and is measured in footcandles (fc). The light that is reflected from the wall is called luminance and is measured in candela per meter squared (cd/m2). Why we switch to metric units for this for another time. In simplistic terms, Illuminance measures the light we can’t see while luminance measures the light that we can see from a specific direction).
Illuminance is useful when trying to get a general feel for how much light is in a space or striking a surface. All of the light level criteria found in the IESNA Lighting Handbook is given in footcandles. Be aware, though, that it tells us very little about how the space will be perceived because it says nothing about the surface that the light is striking. Is the surface smooth or rough, light or dark, etc.? This can be particularly problematic in low light applications like roadway or path lighting. That said, illuminance as a metric is not going anywhere any time soon.
Our perception is influenced by the information we take in through our senses. Our visual system relies on light reflected from surfaces in our field of view. As I begin to think about the lighting design of an interior space, I think of it in terms of brightness. Knowing that the human eye is inherently drawn to the brightest thing in the space, how can I use brightness to guide the eye of the people in the space. From this perspective, lighting can be thought of as a “composition of brightnesses.” If the priorities of the space are orchestrated correctly, the viewer’s perception is guided exactly where it needs to go. This can be done in any application from a parking garage to a library to a retail store. It is most powerful when used in retail applications to separate the prospective buyer from their money.
Luminance and brightness are not interchangeable. ‘Bright’ can be used to describe the sun or a headlight, for example, but is general and not specific. Luminance is the more technical term that describes the luminous intensity per unit area in a specific direction.
Luminances occur in many different forms. Some which are excessively bright and uncomfortable are usually considered “glare.” Glare can be classified as a nuisance or, in extreme cases, disabling which renders the viewer temporarily blinded or with a lasting after-image on the retina.
If you’ve ever tried to read a glossy magazine or a tablet next to a window in the daytime, and couldn’t get rid of the reflections, you are experiencing “veiling luminance.” A veiling luminance obscures the view and makes it hard to see what is beyond the brightness.