Visibility of Radiant Energy
This classic paper [1] from 1923 reports the results of
one of the most enduring projects ever undertaken at
NBS, research into the physical description of human
vision. The principal result of this work was the
âvisibility curve,â a quantified model of how well a
typical person can see the different wavelengths (colors)
of light. Today this model function, essentially un-
changed, underlies all physical measurements of photo-
metric quantities and their interpretation in photometric
units of measure.
It has been understood since the time of Isaac Newton
that white light is a combination of a spectrum of differ-
ent wavelengths, each seen as a pure color. Light is a
form of radiant energy, with a power that can be mea-
sured in watts, but the connection between this physical
description (or the âmechanicalâ description as it was
then known) and the visual result in the human eye was
not well established. This was the challenge undertaken
by K. S. Gibson and E. P. T. Tyndall: to carry out a
study of the visibility of radiant energy or, in quantita-
tive terms, the ratio of the luminous (perceived) power to
the radiant (physical) power at the different wavelengths
in the spectrum.
Gibson and Tyndall were neither the first nor the last
to study the visibility of light, but their work is perhaps
the most notable for its thoroughness, timeliness, and
impact. The first experiments on this subject were
undertaken by Fraunhofer in 1817, and the first energy
measurements were made by Langley in 1883 [2].
By 1905, Goldhammer had crystallized the idea of a
definite relationship between visibility and power at
each wavelength, and at the young NBS, Nutting intro-
duced the term âvisibility curveâ in 1908 [3]. The
Bureauâs forefront research continued through the sub-
sequent decade, leading to the major study of the
sensitivity of the eye across the spectrum by Coblentz
and Emerson in 1918 [4].
However, these and other data accumulated around
the world were not consistent. Different experimental
methods were a chief cause. In the equality-of-bright-
ness matching method, two lights were projected onto a
split-screen viewer, while with the flicker method, two
lights were alternately projected on a simple viewing
screen in rapid succession. In each case, the lights were
adjusted in a known way until an observer declared a
brightness match. The equality-of-brightness method
was the more precise of the two, but only so long as the
color quality of both lights was similar. When the colors
were very different, different observers would make
different matches. The flicker method did not give as
sharp results for similar lights, but the data quality was
not much affected by color differences.
Seeing the need to bring closure to the question,
Edward P. Hyde (who had left the NBS staff in 1908 to
go to the General Electric Nela Research Laboratories),
as president of the U. S. National Committee of the
International Commission on Illumination (the CIE),
requested the Bureau of Standards to make an additional
investigation using the so-called step-by-step method.
This form of equality-of-brightness matching, where
comparisons were made between a series of only
slightly different colors, held promise as a means of
obtaining more reliable data.
Gibson and Tyndall were neither the
first nor the last to study the visibility
of light, but their work is perhaps the
most notable for its thoroughness,
timeliness, and impact.
NBS undertook the challenge under the sponsorship
of General Electric. Director Burgess appointed a
special committee of experts to oversee the work, which
was conducted by Gibson and Tyndall. The University
of Nebraska loaned a Brace spectrometer to the Bureau,
to be incorporated into an elaborate apparatus that made
the best use of the Bureauâs primary standard lamps.
Special care was taken in all aspects of the experi-
ment; issues that were believed to affect the consis-
tency between previous experimentsâsuch as the
size and brightness of the viewing fieldsâreceived
particular attention.
The results included the brightness-matching data
from 52 observers, some of them in common with
previous studies. As hoped, the new equality-of-bright-
ness data were within the range of data obtained with
flicker methods (except in the outer regions of the
spectrum).
However, the strength of the paper was not so much
in the new experimental results as it was in its extensive
analysis and critical review of all existing data. Gibson
25
and Tyndall carefully compared their own results with
those of their predecessors and proposed a mean visibil-
ity curve based upon the accumulated data from more
than 200 different observers. They were guided in this
task by the prevailing theories of the day, which were
believed to dictate certain balance in the curve [5].
The result was a smash success, quickly winning
wide acclaim. In 1924, the 6th Session of the CIE
adopted the Gibson-Tyndall curve as a world standard.
In 1933, the ComiteÂŽ International des Poids et Mesures
(the supervisory body of the worldâs metric system)
followed suit.
The achievement of Gibson and Tyndall might have
remained an academic one were it not for the changing
needs in metrology and the advances of technology. As
surprising as it might seem today, until 1948 there was
no universal standard for the brightness of light. The
âstandard candle,â once made from whale oil, is a part
of popular lore, but in reality, different laboratories each
had their own favorite âstandard.â Some used gas
lamps, some used liquid-fueled lamps, and following
the trend towards electric lighting at the turn of the
century, some (including NBS) used electric lamps. It
was difficult to compare lighting devices to the
standards, and the standards to each other, because
different fuels and different lamp constructions would
produce lights of different color. The only available
instruments that could reliably report how bright a light
appeared, or how lights compared to standards, were
humans, and as we already know, equality-of-bright-
ness matching was unreliable when the colors were
significantly different.
Research in the 1930s, interrupted by World War II,
led to international agreement in 1948 to use a plat-
inum-point blackbody as the sole international standard
of the luminous intensity of light. When objects are hot,
they give off light. By âblackbody,â we mean that the
object does not reflect ambient lightâall the light we
see from it is thermally generated, an intrinsic function
of the objectâs temperature. The trick was to operate a
blackbody at a temperature that anyone could repro-
duceâin this case, the temperature of molten platinum
as it begins to freeze while cooling. Many felt that this
would provide the necessary world-wide stability and
uniformity. A unit of measure of luminous intensity was
then defined, now known as the candela, to relate the
new blackbody standard to a typical standard candle of
times past.
This development had an unintended consequence.
Unlike the previous lamp and flame standards, the
behavior of blackbodies are calculable from first
principles, using Planckâs radiation law. We had a light
source that we understood in detail. The other piece
of the puzzle was an understanding of how the eye
responded to this light, and this is where the work of
Gibson and Tyndall fit in. Suddenly, it became feasible
to design and build electrical devices to measure bright-
ness just as a human would, or at least the ideal human
modeled by the Gibson-Tyndall curve 25 years earlier.
The definition of the candela in 1948 had the effect of
eliminating the need to have someone actually looking
through a visual comparator, a process today called
âvisual photometry.â It began an era of âphysical photo-
metryâ in which luminous intensity could be evaluated
through more objective measurements, yielding better
precision and accuracy.
As time went on, the platinum-point standard fell into
disfavor. The devices were difficult to maintain, their
temperature was much lower than that of common
electric lamps, and the melting-point temperature itself
was too uncertain. This limited how well their emission
spectra could be calculated. Finally, in 1974, Bill Blevin
from the National Measurement Laboratory in Australia
and Bruce Steiner at NBS published the seminal paper
that said âenough is enough.â They formally proposed
that the SI base unit for photometry, the candela, be
redefined so as to provide an exact numerical relation-
ship between it and the SI unit of power, the watt [6].
They stated the case so well that, in 1979, the world
metrology community effected a redefinition of the
candela.
The 1979 redefinition puts even more reliance upon
the work of Gibson and Tyndall. It says, âthe candela is
the luminous intensity, in a given direction, of a source
that emits monochromatic radiation of frequency
540
â«»
10
12
hertz and that has a radiant intensity in that
direction of (1/683) watt per steradian.â That specific
frequency corresponds to a wavelength of 555 nm,
which is where the peak of the Gibson-Tyndall curve
lies. In order to determine the luminous intensity of light
at other wavelengths, one uses the Gibson-Tyndall curve
(more precisely, its modern, smoothed form, denoted as
V(
â
)) to find the corresponding number of watts. The
era of visual photometry is truly over. Today, essentially
all photometry is physical photometry, relying upon this
definition and the V(
â
) curve to characterize any real
light source or the performance of any tangible light
detector.
This success of the Bureau in the early 1920s led to
another important success towards the end of the
decade. Having solved the problem of modeling bright-
ness, Bureau staff next turned their attention to model-
ing color. This was a field that remained active well into
the 1960s, but in those early days, a young staff member
named Deanne Judd made his mark through another
compelling analysis of existing data, resulting again in
establishing the principles and methods that led to inter-
national consensus [7]. In 1931, the CIE adopted the
26
system for quantitative color nomenclature that has
continued to be used for 70 years. Judd, serving as the
U.S. Joint Representative to the CIE, was one of the
principal architects of that standard. His paper laid out
technical recommendations that were accepted a year
later, together with additional data developed by John
Guild of the National Physical Laboratory (NPL) in the
UK [8].
The system of colorimetry that Judd envisioned in
1930 has been a foundation for technologies not even
dreamed of thenâcolor photography (Kodachrome
was invented in 1935), color television, modern color
printing, and digital imaging. The tools of todayâs
electronic commerceâcolor scanners, color-calibrated
computer monitors, and all manner of color printersâ
all still rely on the 1931 CIE color system for âdevice
independentâ color specifications.
As beautiful as the Gibson and Tyndall work was, it
was not without warts. The most famous occurs in the
blue-violet portion of the spectrum, where they were
forced to choose between conflicting data. They wrote,
âThe I. E. S. [Illuminating Engineering Society] data in
the violet have been accepted by the authors for lack of
any good reason for changing them, but the relative as
well as absolute values are very uncertain and must be
considered as tentative only.â Their guess was wrong,
but it so quickly earned acceptance that it did not remain
âtentativeâ for very long. Years later, Judd attempted to
institute an âimprovedâ version of the visibility curve
[9], but the Gibson and Tyndall version had been so
thoroughly adopted that the revision never gained wide
usage.
The second problem is more subtle and beguiling.
The world of Gibson and Tyndall did not include the
narrow-band light sources so common today: the phos-
phors in fluorescent lamps and CRT displays, lasers and
LEDs, and the high-efficiency outdoor lighting that
turns nighttime into a murky orange. The modern
system of physical photometry based upon a simple
visibility curve is no longer enough, not because of
flaws in the curve, but because the human visual system
is much more complex than this simple model suggests.
Our vision responds nonlinearly to combinations of
narrow-band lights, and perceived brightnesses can
differ markedly from the predictions of their model. In
a sense, it is the same problem that was recognized in
the 1920s as the limitation of equality-of-brightness
matching. The data told a story which was not under-
stood then, nor of much technological importance.
Today, vision researchers are revisiting the issue in an
attempt to improve upon the standard model.
Nonetheless, to the extent that we continue to use
electronic instruments to observe our surroundings, and
to the extent that physical photometry remains the gold
standard around the world for the metrology of lighting,
the Gibson and Tyndall curve continues to play an
essential role in estimating our perception of light more
than 75 years after its introduction.
Kasson S. Gibson received his education at Cornell
and joined NBS in 1916. In addition to the work
described here, he made important contributions to the
design of optical filters for transforming radiation from
incandescent lamps to simulate natural daylight. He
headed the work on photometry and colorimetry at NBS
from 1933 to his retirement in 1955, publishing over 100
papers in spite of his administrative responsibilities.
Gibson served as president of the Optical Society of
America from 1939 to 1941 and was a Fellow of the
American Physical Society, Illuminating Engineering
Society, and American Association for the Advance-
ment of Science. He died in 1979 at the age of 89.
Edward P. T. Tyndall worked at NBS in 1917-1919
and later returned for shorter stays as a visiting
researcher. He spent most of his career as Professor of
Physics at the University of Iowa, where he did impor-
tant research on the optical and electrical properties of
metals. He distinguished himself as a teacher and super-
vised 74 masters and doctorate students. He also died in
1979 at age 86.
Prepared by Jonathan E. Hardis
.
Bibliography
[1] K. S. Gibson and E. P. T. Tyndall, The Visibility of Radiant
Energy,
Sci. Pap. Bur. Stand.
19
, 131-191 (1923).
[2] Y. Le Grand,
Light, Colour and Vision,
2nd ed., translation by
R. W. G. Hunt, J. W. T. Walsh, and F. R. W. Hunt, Chapman and
Hall Ltd., London (1968).
[3] P. G. Nutting, The Luminous Equivalent of Radiation,
Bull. Bur.
Stand.
5
, 261-308 (1908).
[4] W. W. Coblentz and W. B. Emerson, Relative Sensibility of the
Average Eye to Light of Different Colors and Some Practical
Applications to Radiation Problems,
Bull. Bur. Stand
14
, 167-236
(1918).
[5] P. K. Kaiser, Photopic and Mesopic Photometry: Yesterday, Today
and Tomorrow, in
Golden Jubilee of Colour in the CIE,
The
Society of Dyers and Colourists, Bradford, UK (1981).
[6] W. R. Blevin and B. Steiner, Redefinition of the Candela and the
Lumen,
Metrologia
11
, 97-104 (1975).
[7] D. B. Judd, Reduction of Data on Mixture of Color Stimuli,
Bur.
Stand. J. Res.
4
, 515-548 (1930).
[8] W. D. Wright, The Historical and Experimental Background to the
1931 CIE System of Colorimetry, in
Golden Jubilee of Colour in
the CIE,
The Society of Dyers and Colourists, Bradford,
UK (1981).
[9] D. B. Judd, Report of U. S. Secretariat Committee on Colorime-
try and Artificial Daylight,
CIE Proceedings
Vo1. 1, Part 7, p. 11
(Stockholm, 1951), Central Bureau of the CIE, Paris. See also G.
Wyszecki and W. S. Stiles,
Color Science: Concepts and Methods,
Quantitative Data and Formulae,
2nd ed., John Wiley & Sons,
New York (1982) p. 330.
27