Research matters - Illuminating Engineering Society

RESEARCH MATTERS Nothing New Under the Sun?
John D. Bullough,
Lighting Research
Center, Rensselaer
Polytechnic Institute
IS THERE NOTHING NEW
under the sun? Fifty years ago, the
IESNA published a paper in its journal,
then titled Illuminating Engineering,
that simply amazes me.It is a paper by
the eminent vision scientist W. S.
Stiles, and it bears the inconspicuous
title “Visual factors in lighting.” 1 The
paper was one of a series of papers
given at what appears to have been a
special session of the IESNA’s annual
technical conference, “A Symposium
on Light and Vision,” along with other
authors such as Weston, 2 Helson, 3
Blackwell 4 and Fry. 5
The paper by Stiles is remarkable
for several reasons: it succinctly summarizes
a series of issues pertaining
to color, spectral distribution, brightness
and glare, all areas of research I
personally find particularly interesting.
Further, I believe this information
is pertinent to many of today’s vigorous
discussions taking place at venues
like LIGHTFAIR International, the
IESNA Annual Conference and in the
letters pages of LD+A on these very
topics.
In this column, I hope to touch on
just a few of the “new” ideas in lighting
that Stiles discussed all those
years ago.
Luminance Or Brightness?
The photopic luminous efficiency
function, V(λ ), provides a universallyapplied
weighting for radiant power
making up the portion of the electro-
FIGURE 1
magnetic spectrum known as light.
V(λ ) was derived largely from flicker
photometry studies that seem to do
a decent job at capturing our spectral
sensitivity for visual performance.The
IESNA uses lighting quantities based
on V(λ ) in all of its lighting recommendations,
6 and we often use these
quantities as stand-ins for responses
The paper by Stiles
is remarkable for
several reasons:
it succinctly
summarizes a
series of issues
pertaining to color,
spectral distribution,
brightness and
glare
other than visual performance, such
as brightness perception, but as Stiles
points out, doing so can lead to serious
errors.
Brightness is a complex perception
that depends upon many factors,
including color saturation. Stiles
shows a graph in his paper (Figure
1) 7 that shows the relative brightness
of colored objects of various dominant
wavelengths and saturations.
Relative brightness-to-luminance (B/L) ratios as a function of wavelength
and saturation (λ ).
Higher saturation results in higher
brightness, as do dominant wavelengths
that are both longer and
shorter than about 580 to 590 nm
(“yellow” light). In fact, the spectral
power distribution of neodymiumcoated
incandescent lamps, with a
“notch” in spectral content near 580
nm, may increase brightness by taking
advantage of this phenomenon.
Similar brightness correction factors
have been published since 8,9 but it is
interesting to see that this phenomenon
has been known since V(λ ) was
relatively young!
Awash In LED Light?
Another very interesting graph
published in the paper by Stiles shows
spectral sensitivity for the brightness
perception of an object as it moves
from the line of sight to the far
periphery of the field of view, out to
70 deg off axis. 10 As the object moves
further from the line of sight, the
spectral sensitivity changes dramatically.
Close to the line of sight, brightness
seems to be determined by a
function that is not terribly different
from V(λ ), but at angles from 25 deg
to 70 deg off-axis, the spectral sensitivity
is completely different.
Figure 2 10 shows the large
increases in short-wavelength
(“blue”) sensitivity for off-axis
objects. These increases happen at
both high luminances (about 300 cd
per sq m) and relatively low luminances
(about five cd per sq m).
Interestingly, the resulting spectral
sensitivity is not all that dissimilar in
shape from the spectral power distribution
of a white light emitting diode
(LED). When LEDs reach the light
output required for interior lighting,
perhaps LED “wall washers” can be
effective tools for dramatically
increasing the brightness of a space,
filling our field of view with shortwavelength-rich,
white light?
A Glaring Case of
Foresight?
In previous “Research Matters”
columns 11,12 we’ve read about how
recently introduced high intensity discharge
(HID) headlamps have been
singled out, justly or unjustly, to
receive the wrath of the American
driving public. (European and
. . . . . . . . . . . . . . . . . . . . . June . . . 2004 . . . LD+A . . . 18 . . www.iesna.org
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