C1 LD94330C1_RTO4 - Illuminating Engineering Society

“The extent to which our eyes
are capable of adapting themselves
to the dark is quite astonishing,
and it is also all the more
VIEWS ON THE
VISUAL
ENVIRONMENT
Louis
Erhardt
remarkable how many more stars
we can see after waiting for an
hour than when we first go outside.
One almost thinks the
whole sky is luminiferous.”—
M. Minnaert, Light & Color. Dover
Publications, New York 1954.
“One of the most useful operations
in any sensing system,
natural or artificial, is a running
normalization. In psychology it is
called adaptation; in engineering
it is automatic gain control.
The idea is to adjust the
sensitivity of the system to the
average level of input so that all
changes are made to lie in the
same limited dynamic range”—
Phillipe Brou, Thomas R.
Scisscia, Lynette Linden, and
Jerome Y. Lettvin, “The Colors of
Things,” Scientific American,
September 1986, p. 80.
Last month’s column dealt
exclusively with average adaptation
(10 cd/m 2 ) and average
reflectance (0.32). They are the
most prevalent. Other adaptations
and reflectances are defined by
their sensitivities. A brief summary
of their properties follows.
Adaptation—0.001 cd/m 2 —
Rod Vision. Deane Judd introduced
this adaptation as night vision.
“Rods are responsible for our ability
to see by moonlight, starlight, or
even when the stars are obscured
by clouds.”—Deane B. Judd, Color
in Business, Science and Industry
John Wiley & Sons, New York 1975.
We are faced with the complexities
of thresholds since not all frequencies
become visible at the
same intensity. A further problem
arises from the limits set for scotopic,
mesopic, and photopic adaptations,
which differ extensively
among authorities. The only consistent
number, 0.001 cd/m 2 , is the
transition point from rods, only, to
the first action of cones, at which
point the Purkinje phenomenon
occurs. Judd describes the change:
“The Purkinje shift makes an
object that looks red by day appear
black by starlight, whereas a daytime
blue object appears light gray.”
Low scotopic luminances, found
in the region outside the galaxy, are
in the neighborhood of
0.00000005, the calculated
absolute threshold being
0.00000002 cd/m 2 . Another low
value is an overcast horizon with no
moonlight, 0.00003 cd/m 2 . Visual
sensitivity is limited to brightness
alone.
Added complications arise when
the foveal and peripheral adaptions
occur simultaneously. The concepts
of dual adaptations were presented
by Ian Lewin in the March 1999
issue of this magazine in the article
“Road Scholar”:
“When we look directly at an
object we are using our cone receptors...
The rods are responsible for
human vision at low light levels and
are prevalent in the peripheral field
of view, away from the line of sight...
During practical driving at night.
both receptor types are active.
Objects viewed directly by the eye
are seen by the cones. Off-axis
objects are seen primarily by rods.”
(Peripheral rods at the outer retina
are extra-sensitive to motion.)
Colors of the stars seen at this
adaptation must be attributed to
cone vision even though the adaptation
implies rod vision alone
Lighting designers are rarely
charged with providing illumination
at this level, but they must understand
the challenge to those who
work under these conditions. These
include airline pilots who must read
instruments while traveling through
very dark surrounds, and ships’ captains
who must navigate in almost
total darkness. To be prepared for
these conditions, lighting designers
must be familiar with all levels of
adaptation.
0.01 cd/m 2 -Moonlight. Illuminance
from the full moon is about 0.2
lux. Reflectance from water and
foliage is about 0.16. Since luminance
equals illuminance times
reflectance divided by pi, the resulting
luminance is 0.01 cd/m 2 . Ralph
Evans observed, “Even at levels as
low as the light from the full moon,
detail vision is moderately good.”
Whereas the previous adaptation
(0.001 cd/m 2 ) had the single variable
Brightness, each log step
upward (at low levels) adds two
more Munsell values. Colors are
The idea is to
adjust the
sensitivity of the
system to the
average level
of input
so that all changes
are made
to lie in the
same limited
dynamic
range
separated from their backgrounds
as differences in Value, but remain
colorless. Moonlight also adds acuity
of large letters at the closer
viewing distance (30 cm). One can
usually make out newspaper headlines.
In LD+A issues from February to
September of 1992, I recounted my
personal observations at 3 meters
and at 30 centimeters—the latter
being the closer viewing distance.
Visual acuity, the ability to distinguish
fine detail, is the reciprocal of
the minimum angular size in minutes
of arc. The smallest discernable
separation between two lines
is one minute of arc. This angle has
a tangent 1/60 of a degree =
0.00029 = 1/3438, a height of one
at a distance of 3438 in any measure.
Size, shape and position are
18 LD+A/March 2003 www.iesna.org