In lighting design, dimensioning is one of the main tasks of a lighting designer. Here, one is guided by specifications from the standards and norms (e.g. EN 12464-1 (interior) or -2 (exterior)) or other national regulations in which a certain average illuminance (E [lx]) and uniformity (Uo) are defined as requirements.
However, planning a specific illuminance level is only a "crutch" and has limited to do with whether a surface or room appears "bright" or "dark".
The only photometric basic parameter we can see is luminance (L [cd/ m²]). In simple terms, luminance can also be called "perception of brightness". Humans only perceive luminance and no illuminance.
Luminance has something to do with the reflection factor of a surface in combination with the illuminance level. If the surface has a high reflection factor, it appears much brighter under the same illuminance than a surface with a low reflection factor.
There is a simple formular which shows the relationship between illuminance and luminance:
It seems trivial, but often this fact is disregarded and planning is reduced only on the value of illuminance.
For example, a dull dark grey wall that is intended to achieve a guiding effect should be brightly lit. You don't have to be an expert to realise that there is a conflict of objectives. Of course, technically it is possible to generate a few thousand lux on that wall, but due to the low degree of reflection (diffuse reflection), the wall remains quite dark in perception.
It would be much more efficient (and energy-saving by the way) to work with a lower illuminance level and a higher reflection factor (i.e. white or at least a light colour). Unfortunately, as a lighting designer you rarely have any influence on the colour scheme in the room. So, this leaves only the "crutch" of dimensioning via illuminance.
Both EN 12464-1 and EN 12464 -2 recommend a „well balanced“ luminance distribution. Too high luminance contrasts may lead to high eye strain. Luminance contrasts that are too low may lead to a very monotonous and boring atmosphere. Accordingly, the european standard also recommends values for surface reflectance.
Our experience, however, is that lighting designers often focus exclusively on meeting the required illuminance values and the uniformity of illuminance on a surface, since concrete normative values are given here that can be calculated and thus evaluated.
What is your experience regarding this issue?
Does luminance play a role in your lighting design and do you succeed in making your clients aware of this?