C1 LD94330C1_RTO4 - Illuminating Engineering Society

colorful experiences which then carry on for the rest of
the sunlit day.
1 cd/m 2 —Sunrise (minus 30 minutes). The sun has
risen to four degrees below the horizon, and the horizontal
illuminance is 13 lux. As the adaptation rises, the
low reflectance of the earth is augmented by sky brightness
so the luminance is equal to 13 times the
reflectance divided by pi, and the overall reflectance
becomes 0.24.
A careful review of the recommended level of illumination
for circulation, social conversation, and simple
tasks (such as reading 12-point type for short periods
of time) resulted in an adaptation of 1 cd/m 2 . Here we
come face to face with the superiority of adaptation as
the basis for lighting design. Adaptation does not yield
a solitary metric such as illuminance, but instead provides
a series of measurements that can be fulfilled at
that particular adaptation level.
Hardy and Perrin describe the adaptation of 1 cd/m 2
“as appropriate for interiors at night.” As stated earlier,
it is also a daytime level. The next adaptations, 10 and
100 cd/m 2 , occur in both interior and exterior scenes;
the final, 100 cd/m 2 , is likely to be found in exteriors
only.
10 cd/m 2 —Sunrise (minus 15 minutes). With an
average reflectance of 0.32 the illuminance will be
approximately 100 lux (10 fc). The eye can usually see
quite well over the two adaptation levels from 1 to 100
cd/m 2 . Because of this overlapping of the two adaptations—one
above and one below—the range of vision at
the 10 cd/m 2 level is quite extensive and is probably
the one most commonly found in interior designs.
As this was described in detail in last month’s issue,
we will move on to the next adaptation.
100 cd/m 2 —Sunrise (plus 30 minutes), Bright
Sunlight. If 1 cd/m 2 is the lowest of interior recommendations,
100 cd/m 2 is the highest. Both, of course,
are experienced in the natural scene as well. This level
should be reserved for the tasks that improve in visibility
and performance at the higher illumination. Many
tasks require directional light to create shadows, or
optical magnification may be the only solution for adequate
legibility. Sparkle for jewelry is achieved by using
extremely small directional sources.
High adaptations may work against the design purpose.
In viewing oil paintings on my wall, I found close
examination disturbing. At times, the underlying canvas
revealed its texture, and individual touches of contrasting
color appeared strangely out of place, whereas they
blend for the desired effect at three meters.
The experience enhanced my appreciation of the
artist’s ability to place the lights, darks, and colors in a
manner that conveys realistically the water, sky, and
the different ships portrayed. Artists can see and reproduce
patterns of light before them. This is visual communication
at its finest, an international language that
needs no translation.
Visual ability depends on the wise selection of brightness,
color, and distance, with an innate understanding
of the psychological factors that the selection will
evoke. The theater designer strives for strong emotional
responses; the everyday architectural designer wishes
to achieve a pleasant productive atmosphere supported
by clarity and purpose in establishing visual priorities.
1000 cd/m 2 —Blazing Sunshine. The brightest sunshine
occurs for only a few hours in the sunniest climates
and for only a few days of the year. At this level;
the designer’s problem reverses. He is concerned with
protecting the viewer from excessive brightness. One
seeks shade, reaches for the hat with a wide brim,
wears sunglasses, turns to face away from the direct
sun, or seeks other means to protect his eyes. Too
much sun is injurious to man’s most sensitive receiver
of information from the visual world.
C.P. Steinmetz warned, “The light which enters the
eye is converted into heat, and if its power is considerable
it may be harmful or even destructive, causing
inflammation or burns.”
In closing this proposal for a new approach to the
design of lighting, I would like to pay tribute to C.P.
Steinmetz, whose book Radiation, Light, and
Illumination inspired me to undertake my writing
career; and to Max Planck who firmly rejected the idea
that there could be an equation between physical and
mental processes. Too long neglected is the work of
Albert Munsell and his color system, which assesses
the perception of black, white, and the grays between.
And there were my contemporaries: Sol Golden, who
persuaded the IESNA to publish my Views on the Visual
Environment. And, Dick Corth, who wrote, “Vision is
not physics; it is biology,” thus emphasizing the difference
between light-as-energy and light-for-vision.
Adaptation-Reflectance is the method I’ve developed.
Reflectance, the unvarying characteristic of the
external scene, is the stimulus. Adaptation by the retina
for human exploration is the response. It minimizes
calculations, maximizes the mind’s ability to comprehend
the meanings of the visual scene.
Editor’s Note:
Louis Erhardt’s column “Views on the Visual
Environment” first appeared in the October 1981
issue of LD+A. This educational forum has
appeared regularly in these pages since then.
The March column is his last in the series.
The IESNA Presidential Award was presented
to him in 1984 and yet again in 2001, the IESNA
Medal in 1993. He has been a member of the Society
since 1938.
Howard Brandston observed upon awarding
him the Presidential Award:
“To Louis Erhardt, who has made the art, science,
and practice of illumination the true purpose
of our Society, we offer our heartfelt thanks
and deepest appreciation.”
20 LD+A/March 2003 www.iesna.org