Rejuvenating Seurat`s Palette Using Color and Imaging Science: A

Rejuvenating Seurat’s Palette Using Color
and Imaging Science: A Simulation
ROY S. BERNS
It has long been known that Seurat’s Sunday on
La Grande Jatte—1884 does not have the appearance that the artist originally intended. Like any
painting, the work has changed with the passage of
time—the oil medium Seurat used has darkened
and yellowed, and the coarse linen support he
employed has also darkened. As was first noted by
Félix Fénéon in 1892, there was also an unexpected and rapid deterioration of a number of
colors Seurat used in his “second campaign” of
painting. Inge Fiedler was the first to identify the
particular unstable pigment at fault—zinc yellow—
which is present in a number of paint mixtures.1
Seurat added yellow, green (ranging from green to
yellow-green), and orange dots that included the
zinc yellow pigment to indicate points of reflected
light; these later shifted to an ocherlike color, drab
olive, and reddish brown, respectively. These
changes have dramatically influenced the painting’s
luminosity (for this and other color-science terms,
essay explores the elaborate process—taking non-
can be performed nondestructively, that is, without
see Glossary, p. 225).
destructive spectral reflectance measurements,
the need to remove samples from the painting.
re-creating Seurat’s paint mixtures, capturing the
Spectral reflectance, measured with an instrument
physically on the actual canvas, we were able to
painting with digital photography, and image editing
called a spectrophotometer, indicates the amount
manipulate high-resolution digital images of the
with Adobe Photoshop—used to achieve this rein-
of light reflected or absorbed by an object at the
painting in order to reinstate the colors that Seurat
vigorated digital version of a portion of La Grande
different wavelengths of the visible spectrum, and
intended and recapture to some extent its initial
Jatte (see FIG. 1 ).
is a characteristic of the pigmentation of the object.
While these alterations cannot be corrected
In general, color is determined by absorption in
effect. We arrived at an approximation of the
original appearance of the painting following a two-
In Situ Analysis of La Grande Jatte
complementary wavelengths: yellow absorbs blue;
step process. First, we digitally “un-aged” the
The first step in realizing the simulation of the
green absorbs magenta (a combination of blue
painting as a whole, correcting for the natural
original colors of La Grande Jatte was measuring
and red); blue absorbs yellow (a combination of
yellowing and increased translucency of the paints.
the spectral reflectance of a number of different
green and red). We took approximately fifty meas-
Then we corrected for the deterioration of the
dots—focusing mainly on the discolored ones—of
urements; thirty of these spectral “fingerprints”
zinc yellow in its various paint mixtures. This
paint throughout the work.2 These measurements
are shown in
214
Berns
FIG. 2.
They have been categorized
FIG. 1
Portion of La Grande Jatte
before rejuvenation (opposite)
and after (below).
Rejuvenating Seurat’s Palette Using Color and Imaging Science
215
match is measured using a spectrophotometer, and a
whites, and yellows. The yellows, oranges, and
paint recipe is generated that, when mixed, applied
greens include the dots containing darkened zinc
to a surface, and allowed to dry, has a similar spec-
yellow. These spectra can be used to help identify
tral fingerprint and color. This ability to determine
the specific pigment types Seurat used and, as
the color that results from a mixture of pigments
described below, to define color. For example, the
dispersed in a medium enabled the digital rejuvena-
relatively high reflectance in the part of the spec-
tion of Seurat’s palette, as is described below.
trum near 730 nanometers (nm) of the blue curves
(see F I G .
2B)
indicates the use of cobalt blue. The
stepped shape of the curves of the orange spectra
(see F I G .
2D)
reveals that Seurat produced orange
An object does not truly have color until it is
illuminated and viewed by an observer. A colored
object’s reflected light is absorbed by our visual
system’s three color-receptors (cones). Following
with a mixture of red and yellow pigments, rather
complex physiological and cognitive processes, the
than using a single orange one.
hundreds of wavelengths of light are reduced to
These curves are directly related to the absorp-
three responses. This fundamental property of color
tion and scattering properties of the pigments,
vision, known as trichromacy,5 was first theorized
paint medium, and ground. For instance, a pigment
in the nineteenth century, and the color theorists
appears white if it reflects (scatters) most of the
who influenced Seurat were well aware of it.
light that hits it, that is, nearly 100 percent. The
white spectra in the case of La Grande Jatte
FIG. 2G)
are much lower than 100 percent,
Colors scientists at the beginning of the twentieth
V
80
B
G
Y
O
R
60
Percent Reflectance
as blues, greens, oranges, purples, pinks and reds,
40
20
0
380
430
480
530
580
630
680
730
Wavelength (nm)
A
to CIELAB coordinates.7 This color space (only one
century wished to find a way to quantify our percep-
among many such spaces that have been created) is
tion of color. They aimed at developing something as
based on opponent color-vision theory, first devel-
because of an increase in paint transparency, which
sensitive as the human eye but standardized, thus
oped by Ewald Hering in the 1870s, which postulates
decreases scattering. The spectra also absorb in
allowing for “colorimetric specification” derived from
that our color vision has opponent perceptions of
the violet wavelengths, a result of the yellowing of
numerical and instrumental methods. This research
black or white, red or green, and yellow or blue (see
the paint medium.
forms the basis of colorimetry. A three-dimensional
F I G . 4 ).8
color space for specifying and visualizing colors
nations to specific colors. For instance, a pure (100
(see
3
Since the 1940s, color technologists have been
6
FIG. 3)
was developed in 1976,
CIELAB allows us to assign numerical desig-
able to relate spectral reflectance of paint films or
called CIELAB (see
batches of paint to the pigmenting agents needed
onto which the properties of every color can be plot-
on the lightness scale (L*) and a value of 0 on the
(either singly or in mixtures) to replicate that exact
ted. In addition to two perpendicular horizontal axes
redness-greenness (a*) and yellowness-blueness
same color, based on knowledge about the absorption
measuring redness-greenness (a*) and yellowness-
(b*) scales.9 Black, which is 0 percent reflecting,
and scattering properties of the paint’s constituents.4
blueness (b*), there is also a vertical axis measuring
would have a value of 0 on all of the scales. The rec-
This sort of technology is used today in the custom
lightness (L*). Following the selection of a standard-
tangular CIELAB coordinates can also be transformed
paint-dispensing systems often found in paint and
ized light source and a standardized observer, spectral
to cylindrical coordinates that describe the hue
hardware stores. A sample a customer desires to
data (such as that shown in FIG. 2 ) can be converted
(hab) and chroma (C*ab) of a color. Chroma is the
216
Berns
percent reflecting) white would have a value of 100
FIG. 2
In situ spectrophotometric
measurements of thirty spots
on La Grande Jatte (A).
The measurements have been
divided up into groupings of
blue (B), green (C), orange
(D), purple (E), pink and red
(F), white (G), and yellow (H).
V
80
B
G
Y
O
R
V
80
B
G
Y
O
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V
80
B
G
Y
O
R
60
60
60
60
40
40
40
40
20
20
20
20
0
0
430
380
480
530
580
630
680
730
B
480
530
580
630
680
730
B
G
Y
O
R
B
G
Y
O
R
60
60
40
40
40
20
20
20
0
380
F
430
480
530
580
630
680
730
G
Y
O
R
530
580
630
680
730
380
430
480
530
580
630
E
V
80
60
0
480
D
V
80
B
0
430
380
C
V
80
0
430
380
V
80
B
G
Y
O
R
0
380
G
430
480
530
580
630
680
730
380
430
480
530
580
630
680
730
H
Rejuvenating Seurat’s Palette Using Color and Imaging Science
217
680
730
FIG. 3
FIG. 4
FIG. 5 (BOTTOM)
CIELAB color space. Like a
the relative lightness or dark-
Ewald Hering’s color wheel. It
In situ measurements of
the greatest change is visible
traditional color wheel, CIELAB
ness of a color. Shown here is
shows the psychological mix-
thirty colors sampled from La
in lighter colors, such as the
is divided into axes of paired
the color gamut, or range, of a
ing of adjacent primaries of
Grande Jatte (top) and those
yellows, creams, whites, and
complementary colors: the
typical CRT color display, like
red, yellow, green, and blue.
same colors after subtracting
lightest pinks.
a* axis measures redness-
that of a computer monitor.
the aging spectrum (bottom).
greenness while the b* axis
The large range of colors can
While there is a general bright-
measures yellowness-blueness.
be compared with that of La
ening in all of the examples,
CIELAB also has a vertical
Grande Jatte, shown in F I G . 1 1 .
axis (L*) that allows us to plot
Yellow
(Lightness) 100
80
60
L*
40
Red
20
Green
0
(Yellowness) +100
50
b*
0
+100 (Redness)
50
-50
0
a*
-50
(Blueness) -100 -100 (Greenness)
218
Berns
Blue
FIG. 6
Portion of La Grande Jatte
before digital un-aging (left)
and after (right).
degree of departure of a color from a gray of the
from La Grande Jatte by using a spectrophotometer.
total size of the painting—we employed a mosaic
same lightness. So, for instance, a tomato and a
This provided valuable information about particular
system to image the entire work. We captured
brick may have the same lightness, but the tomato
parts of the painting but measured a very small per-
twenty-five discrete image tiles, five across and five
has greater chroma; that is, the tomato is a more
centage of the work as a whole. In order to perform
down.12 Like the human visual system, the color
chromatic (often described as “intense”) red color
the ultimate correction and digital rejuvenation of
camera is trichromatic, so each of the more than
than the brick. Colors that appear luminous, as Seu-
La Grande Jatte, we needed measurements of all
one billion measurements corresponded to three
rat intended his to be, are simultaneously light and
of the colors in the painting. Therefore, we estimated
filtered channels referred to as red, green, and blue
their CIELAB coordinates with a color-managed
(RGB).13 However, depending on the specific color
digital camera. The Art Institute’s Imaging Depart-
filters and built-in color processing, the quality of
Measuring the Color of La Grande Jatte
ment created use-neutral files that were equivalent
the color image can vary considerably. Therefore,
Using Imaging
to a 1:1 representation of the painting. Because the
we created a custom color profile that converted
As noted above, we were able to determine the
camera captures 8,000 x 5,344 pixels at its highest
this RGB information into CIELAB coordinates.14
CIELAB coordinates for each of the measurements
resolution—corresponding to only 4 percent of the
high in chroma.
10
11
The CIELAB image measurements of La Grande
Jatte are plotted in
FIG. 11.
The overall darkening
of the painting is evident, since the maximum
lightness is below what can be achieved with fresh
lead white in linseed oil (about an L* of 97). We
also see that the range of yellows is rather limited
and that the greens are dark. Compared with the
colors achievable with fresh oil paints, the color
gamut of the painting is modest.
Digitally “Un-aging” La Grande Jatte
The CIELAB coordinates for the range of colors in
the painting are necessary for realizing our larger
goal of recapturing the appearance of La Grande
Jatte during the late 1880s, before the zinc yellow
began to deteriorate. A contribution to the change
in the appearance of the zinc yellow is the overall
darkening of the painting. In order to measure
the degree to which the natural aging processes
have affected the entire painting, we took a spectral measurement of an area of the work containing
Rejuvenating Seurat’s Palette Using Color and Imaging Science
219
V
75
B
G
Y
O
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FIG. 7
the complementary haloes surrounding many of the
Reflectance spectra of averaged yellow dots before and after
rejuvenation. All the spectra have nearly identical reflectances
below the transition wavelength (the point at which the curves
60
begin to part, about 470 nm). Replacing the darkened zinc yellow
with fresh zinc yellow greatly increases reflectance. Incorporating
Percent Reflectance
figures are more noticeable.
the first campaign results in an intermediate spectrum. Un-aging
45
increases reflectance, resulting in the “rejuvenated” yellow.
“Undarkening” the Spectra of Dots Containing
Zinc Yellow
While digital un-aging enables us to correct for the
natural process of overall darkening and yellowing
of paint films over time, it cannot compensate for
Average yellow dot
30
the unusual deterioration of the zinc yellow, which
Undarkened average yellow dot
was caused by a chemical change in the pigment
Undarkened average yellow dot with 25%
showthrough of underlying paint
15
(see Fiedler, p. 209). Conceptually, in order for the
Undarkened and un-aged average yellow dot
with 25% showthrough of underlying paint
0
380
brushstrokes of paint containing the darkened zinc
yellow to be “undarkened,” we must replace the
failed yellow with one that only underwent normal
430
480
530
580
630
680
730
aging, like the rest of Seurat’s palette. To do this,
Wavelength (nm)
we first had to identify the spectral properties of
the particular zinc yellow used, as well as the color
strength of the pigment and its concentrations
pure lead white—the sail of the boat in the top
and a slight increase in chroma (see
left corner. We compared this spectrum with a
extent of change caused by un-aging varied depend-
In order to get estimates, we took paint samples
fresh paint-out of lead white in linseed oil. The
ing on the particular color’s spectral properties; in
from the same dots that were measured spectropho-
spectral difference between the two generated the
general, however, lighter hues were more affected
tometrically. Inge Fiedler determined the paint
than darker ones.
mixtures microscopically and estimated the propor-
“aging spectrum” for the picture as a whole.15 The
F I G . 5 ).
The
in the mixtures with emerald green or vermilion.
aging spectrum can be added to the spectra of
Using this information, a digital image of the
fresh pigments to simulate the aging of Seurat’s
upper left portion of La Grande Jatte was processed
to relate microscopic analyses to macroscopic
palette. More importantly, it can be subtracted
with Photoshop, arriving at the simulation of its
reflection properties, and it was not feasible to take
from in situ measurements of La Grande Jatte to
un-aged appearance (see
un-age colors throughout the painting.
F I G . 6 ).16
Compared with
tions within each. However, it is extremely difficult
a sample of every darkened dot. We thus used a
the painting in its current state, there is a clear
theoretical color-mixing model as an analytical tool
The aging spectrum was subtracted from each
increase in lightness and chroma: it has become
to determine the amount of darkened zinc yellow
and
brighter, more luminous. There is also an increase
in any given mixture in order to replace it with the
their CIELAB coordinates were recalculated. As a
in contrast, making the differences between light and
proper amount of “fresh” (chemically unaltered)
result, the colors underwent an increase in lightness
dark passages more distinguishable. In particular,
zinc yellow.
of the spectral measurements shown in
220
Berns
FIG. 2
50
Art Institute conservators prepared reference
V
B
G
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FIG. 8
Reflectance spectra of the averaged green dots before and after
samples (paint-outs) of fresh zinc yellow, emerald
rejuvenation. Also shown is the match to the averaged darkened
green, vermilion, chrome yellow, and lead white
dots, made using the darkened zinc yellow, fresh emerald green,
40
thinned with linseed oil and painted on a single
conservators measured their spectral reflectances.
We referred to these samples to understand how
colors would have looked out of the tube on Seurat’s palette.
Because zinc yellow was available in a variety
fresh lead white, and the aging spectrum.
Percent Reflectance
panel with a white ground. After the paint-outs dried,
30
Average green dot
Match to average green dot
Undarkened average green dot
20
Undarkened average green dot with 25%
showthrough of underlying paint
Undarkened and un-aged average green dot
with 25% showthrough of underlying paint
of hues ranging from greenish yellow to reddish
10
yellow, we had to determine if the zinc yellow prepared at the Art Institute had the same spectral
properties as the one used by Seurat. In addition to
0
380
measuring the spectral reflectance of the fresh zinc
430
480
530
580
630
680
730
Wavelength (nm)
yellow, we also measured six darkened zinc yellow
dots from La Grande Jatte and averaged the results
(see
F I G . 7 ).17
75
When the spectra of the fresh and
V
B
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FIG. 9
Reflectance spectra of the averaged orange dots before and after
darkened yellows were compared, it became clear
rejuvenation. Also shown is the match to the averaged darkened
that the Art Institute samples were too green; to
correct for this we simply shifted its spectrum until
(the transition wavelength determines the shade of
yellow). In this manner, we estimated the spectral
properties of the zinc yellow that Seurat used.
This revealed that this particular pigment lacked a
green or red tint; as such it has a distinct place
lead white, and the aging spectrum.
Percent Reflectance
the transition wavelengths of the two nearly matched
dots made using the darkened zinc yellow, fresh vermilion, fresh
60
45
Average orange dot
Match to average orange dot
Undarkened average orange dot
30
Undarkened average orange dot with 25%
showthrough of underlying paint
among the range of hues he employed.
Changes in concentration or tinting strength
Undarkened and un-aged average orange dot
with 25% showthrough of underlying paint
15
alter the level of light absorption. Below 470 nm,
the amount of reflectance is due to absorptions by
the yellow pigment and aging. This principle enabled
0
380
430
480
530
580
630
680
730
Wavelength (nm)
Rejuvenating Seurat’s Palette Using Color and Imaging Science
221
FIG. 10
The colors of the averaged
paint layers. The bottom row
green (left), yellow (center),
shows the dots in the third
and orange (right) dots. The
row after un-aging. The colors
top row shows them in their
are surrounded by the average
current state; the second, after
hue of the sunlit or shadowed
undarkening; the third, after
grass excluding these dots. In
undarkening and taking into
the fourth row, the backgrounds
account effect of underlying
also have been un-aged.
The zinc yellow dots from La Grande Jatte reflect
dots would have possessed had they not undergone
significantly less light beyond the transition wave-
the unusual darkening.
length than fresh zinc yellow dots would, thus mak-
We performed the same procedure used to
ing them appear darker. The spectrum for the
undarken the yellow and green dots on the orange
darkened zinc yellow is in fact quite similar to that
dots containing vermilion and zinc yellow. Three
for a yellow ocher mixed with black.
dots were measured spectrophotometrically and
Although we know that emerald green can also
analyzed microscopically, revealing a range in the
darken over time, research by Fiedler indicated
proportion of the pigments. We averaged the spec-
that the zinc yellow was the primary cause of the
tra and then matched them with darkened zinc
darkening of the green dots in La Grande Jatte;
yellow, fresh vermilion, fresh lead white, and the
spectral reflectance and colorimetric analysis con-
aging spectrum (see
firmed her findings. Six green dots were measured
the darkened zinc yellow with the fresh equivalent
spectrophotometrically and their spectra averaged
in order to arrive at the spectrum for the orange
(see
F I G . 8 ).
The dots were also analyzed microscop-
ically, revealing that they contain various propor-
F I G . 9 ).
Finally, we replaced
had it not deteriorated.
Before applying these undarkened dots to the
tions of zinc yellow and emerald green and, in
digital image of the painting, we had to take into
some cases, small amounts of lead white. We thus
account one last variable: the translucent nature of
attempted to match the spectrum of the averaged
paint. A paint layer is seldom perfectly opaque,
darkened green dots by combining the averaged
and thus its surface color may be influenced by the
darkened zinc yellow (see above), fresh emerald
color of the substrate on which it lies. This effect
green, fresh lead white, and the aging spectrum.
is often heightened in older paintings. In the case
The close similarity between the in situ measure-
of La Grande Jatte, the result of this translucency
ment of the darkened green dots and the match
is that the underlying paint layers of the artist’s
that we created (using darkened zinc yellow but
first campaign on the work affect the perceived
fresh emerald green) provides further confirmation
color of the dots. In the paint-outs, the bright white
us to estimate the strength of Seurat’s zinc yellow.
that the emerald green was not responsible for the
substrate is visible; thus, the undarkened dots based
The fresh zinc yellow was scaled18 so that its spec-
darkening.19 Using the same approach employed
on the paint-outs are lighter and higher in chroma
trum, when added to the aging spectrum, most
to undarken the zinc yellow dots, we determined
than they would really appear on the painting.
closely matched the six zinc yellow dots’ average
an appropriate concentration of fresh zinc yellow.
spectrum between 380 and 470 nm (see
F I G . 7 ).
To compensate for this, we needed to account
The correct concentrations of fresh emerald green,
for the influence of the underlying paint on the
This manipulated curve simulates a measurement
fresh zinc yellow, and fresh lead white, plus the
tone of the darkened dots. Fiedler determined that
of Seurat’s original zinc yellow before darkening.
aging spectrum, produced the spectrum the green
the first-campaign grass is composed primarily of
222
Berns
FIG. 11
FIG. 12 (BOTTOM)
The image measurements of
compared with the color
Four different perspectives
clear increase the range of
La Grande Jatte plotted on
gamut of a typical CRT com-
of the CIELAB color gamut of
colors. There is an overall
CIELAB. The color gamut
puter monitor (see F I G . 3 ),
La Grande Jatte after rejuve-
lightening and an extension
(range) is shown as a color
the range and variety of colors
nation. Compared with La
of the yellows and reds
solid, here viewed from four
employed in La Grande Jatte
Grande Jatte before rejuvena-
in particular.
different perspectives. When
is quite limited.
tion (see F I G . 1 1 ), there is a
+100
100
100
50
75
75
100
80
60
L*
40
b* 0
L* 50
L* 50
20
-50
25
0
+100
25
50
-100
-100
-50
0
50
+100
0
-100
-50
a*
0
50
+100
0
-100
b* 0
-50
a*
0
50
+100
+100
50
-50
b*
0
a*
-50
-100 -100
100
100
+100
100
80
75
75
50
60
L*
40
L* 50
L* 50
b* 0
20
0
+100
25
25
-50
50
-100
-100
-50
0
a*
50
+100
0
-100
-50
0
a*
50
+100
0
-100
b* 0
-50
0
b*
50
+100
+100
50
-50
0
-50
a*
-100 -100
Rejuvenating Seurat’s Palette Using Color and Imaging Science
223
chrome yellow, emerald green, and lead white. Using
darkened yellow, green, and orange dots on a
and there is a greater sense of depth between the
the theoretical paint-mixing model, we found the
reduced-resolution composite image of the entire
seated woman with the umbrella and the little girl.
concentrations of these fresh pigments, and applied
painting. This heightened the painting’s luminosity
the aging spectrum; this generated a spectrum
and significantly increased its overall color gamut
matching the CIELAB coordinates of the average
(see
first-campaign sunlit grass. We then arrived at
F I G S . 1 1 – 1 2 ).
Conclusion
Although we took a scientific approach here to
While the Select Color tool was useful in rejuve-
arrive at the appearance La Grande Jatte would
revised paint-out spectra (the spectra of the zinc
nating the image at a low resolution, the Imaging
have had in the 1880s, the results are still specula-
yellow, emerald green, and vermilion had they
Department found that its accuracy on full-resolution
tive. The darkening of the zinc yellow mixtures and
been painted over the first-campaign sunlit grass
image tiles was insufficient. The tool particularly
the aging of the painting as a whole are undeniable,
color rather than white) by mixing the spectra of
had trouble differentiating second-campaign green
but the exact extent of the changes is still under
the sunlit grass and the paint-outs in a one-to-three
dots from the first-campaign grass and, in some
investigation. The results of our efforts do seem
ratio. This ratio, selected subjectively, allowed for
cases, yellowish green from yellow dots. Therefore,
reasonable, however, since the effect of the rejuve-
a 25 percent show-through of the underlying paints.
in refreshing a single image tile from La Grande
nated painting is consistent with descriptions of
With these new paint-out spectra, we repeated the
Jatte (see
undarkening calculations for each average colored
which allows one to select contiguous image areas
dot, and subtracted the aging spectrum from each
with similar color. This process provides a dot-
20
(see
F I G S . 7 – 9 ).
We used these revised spectra to
calculate CIELAB coordinates, which were rendered
as an image using Photoshop (F I G .
1 0 ).
The drab
F I G . 1 ),
we used the option Magic Wand,
by-dot mapping of the painting, but because it is
so labor-intensive it was only feasible to apply it
to a small portion of the work.21 For each selected
ocherlike dots become a vivid yellow, the olive dots
set of dots, Photoshop was used to change the
turn a vibrant yellowish green, and the reddish
CIELAB coordinates of each dot to their rejuve-
brown ones are now a lively light orange. These
nated coordinates.22
results are consistent with early observations about
the colors and luminosity of the painting.
The most dramatic change in this portion of
the painting is in the sunlit grass, where the dark
speckle has become points of brilliant yellow and
Digitally Rejuvenating the Dots Containing
yellow-green that increase the luminosity of the
Zinc Yellow
landscape. In the darker areas, vivid orange high-
Before the dots could be rejuvenated, they needed
lights sparkle within the shadows. These dots now
to be isolated from the entire image. The Imaging
appear playful, providing a sense of movement.
Department at the Art Institute used the tool Select
The overall brightening of the sunlit grass increases
Color in Photoshop, which can find all image areas
the contrast between light and dark passages: the
with a similar color, to select and replace the
shadow cast by the little girl is more apparent,
224
Berns
Seurat’s masterwork before it underwent its unfortunate color change.
FIG. 13
FIG. 14 (BOTTOM)
The center panel is La
Colors of constant hue organ-
Grande Jatte in its current
ized by lightness and chroma.
state. In the left panel, light-
The left-most column depicts
ness contrast has been
neutrals. Moving to the right,
increased. In the right panel,
chroma increases.
color contrast has been
increased.
Glossary
one of the possible modes of
interaction of light and matter in which all
the energy of the incoming light interacts
with the bulk of the material and it is all dissipated into it, that is, it is absorbed.
ABSORPTION:
the degree of departure of a color
from a gray of the same lightness (see FIG. 14 ).
L I G H T : radiation is a form of energy, part of
the family that includes radio waves and Xrays, as well as ultraviolet and infrared radiation. Radiation we can see is called light. We
can see wavelengths between about 400 nm
(violet/blue) and 700 nm (red).
CHROMA:
C O L O R G A M U T : the range of colors of a
coloration system, usually defined by a standardized color space. The color gamut of
an object, such as La Grande Jatte, can also
be determined (see F I G S . 1 1 – 1 2 ).
attribute by which a perceived
color is judged to be distinct from neighboring colors (see F I G . 1 3 ). When the distinction
is caused by differences in hue and chroma,
it is color contrast. When the distinction is
caused by differences in lightness, it is lightness contrast.
CONTRAST:
the similarity of a color to one of the
colors, red, yellow, green, and blue, or to a
combination of adjacent pairs of these colors
considered in a color wheel.
HUE:
attribute by which a perceived color is judged to have the property
of a light source. For colors to appear
luminous, they must be simultaneously light
and high in chroma.
LUMINOSITY:
O P A Q U E : when the scattering of a material
is sufficiently large that light cannot pass
through (some absorption is often present),
the material is said to be opaque.
one of the possible modes of
interaction of light and matter in which the
direction of the incoming light changes.
S C AT T E R I N G :
S P E C T R A L R E F L E C TA N C E : the ratio of the
reflected light to the incident light under
specified geometric conditions as a function
of wavelength.
T R A N S L U C E N T : when some but not all of
the light passing through a material is scattered, the material is said to be translucent.
Lightness
C O L O R S P A C E : colors are usually described
by three variables. These can be organized in
a three-dimensional fashion forming a color
space. The position of a color within a color
space, when defined numerically, provides an
unambiguous color specification. CIELAB is
an example of a color space: when variables
L*, a*, and b* are used, the organization is
rectangular; when L*, C*ab, and hab are used,
the organization is cylindrical.
the equivalency of a color to
one of a series of grays ranging from black to
white (see F I G . 1 4 ).
LIGHTNESS:
when light passes through
a material without changing its direction, the
material is said to be transparent.
T R A N S PA R E N T :
electromagnetic radiation
can be described quantitatively either as
energy of photons or as energy of waves; in
the latter case, wavelength is the unit for
measuring that energy. The nanometer (nm)
is a convenient unit of length. One nanometer
is 1/1,000,000,000 meter.
W AV E L E N G T H :
Chroma
Rejuvenating Seurat’s Palette Using Color and Imaging Science
225
and emerald green would certainly
Acknowledgments
the surface roughness that can contribute
I wish to thank Francisco Imai, Mitchell
to the overall scattering of light. A highly
dard observer (see note 6) were used for
Rosen, and Lawrence Taplin of the Mun-
varnished Old Master painting, for exam-
all the colorimetric calculations. D50 was
sell Color Science Laboratory for their
ple, could have a minimum reflectance
selected for two reasons: first, for color
like Precision M11 by JENOPTIK. We
assistance in creating the camera profile,
approaching 0 percent.
printing, D50 is a graphic arts standard;
lit the painting with four Hensel strobes.
second, D50 represents sunlight.
A spectral camera would have been the
Photoshop custom curves, and 3-D
4 The seminal work was published by
7 CIE illuminant D50 and the 1931 stan-
achieve luminosity.
11 The camera used was the German Eye-
best instrument for making these meas-
CIELAB renderings. At the Art Institute,
D. R. Duncan, “The Colour of Pigment
I am grateful to Francesca Casadio, Inge
Mixtures,” Journal of the Oil Colour
vom Lichtsinn (Berlin, 1920), trans.
urements. It would have provided us
Fiedler, Allison Langley, and Frank Zuccari
Chemist Association 32 (1949), pp. 296,
Leo M. Hurvich and Dorothea Jameson
with an image of the painting at every
of the Conservation Department for pro-
321. Because of computing limitations,
as Outlines of a Theory of the Light
wavelength, and if the camera had high
viding the paint samples and additional
instrumental-based color matching was
Sense (Cambridge, Mass., 1964). This
spatial resolution, each dot would have
measurements of La Grande Jatte.
not practiced until the 1960s.
results in a color wheel with these oppo-
many measurements since each measure-
nent colors opposite one another, as
ment corresponds to a pixel. When La
Thank you also to Alan Newman, Chris
5 Trichromatic theory is usually associated
8 Ewald Hering, Grundzüge der Lehre
FIG. 4.
Perceptually, these hues
Grande Jatte was analyzed, however, a
Gallagher, and Siobhan Byrns of the
with Thomas Young (“On the Theory
shown in
Imaging Department for performing the
of Light and Colours,” Philosophical
are unique and unambiguous. All other
system was not yet ready for such a large
digital photography and labor-intensive
Transactions 12–48 [1802]) and Her-
hues can be defined as ratios of adjacent
undertaking. Developing artwork spectral-
color selection and rejuvenation. Finally,
mann von Helmholtz (Treatise on Phys-
ones. An orange might be 50 percent
imaging systems is a research topic on
my appreciation goes to Robert Herbert
iological Optics, trans. from the 3rd
each of red and yellow. When creating a
which I am currently working; see Roches-
for his many helpful comments and
German ed., ed. James P. C. Southall,
color scale between opponent colors, the
ter Institute of Technology, “Art Spectral
enthusiasm.
[Rochester, 1924]).
midpoint is neutral. These complements
6 “Colorimetry” is a synthesis of two words:
are psychological, not physical, based on
Imaging,” http://www.art-si.org.
12 The Imaging Department drew a chalk
either light or colorant mixing. Physical
line parallel to the picture plane and
1 Fiedler 1984.
meaning “to measure.” Color specifica-
primaries can result in very different
used dual plumb bobs on either side of
2 The measurement of spectral reflectance
tions using numerical and instrumental
complements, and hence the many dif-
the camera to ensure the accuracy of
is central to the study and practice of
methods were first standardized in 1931
ferent color wheels.
the planes. Markers along the outside of
color science as it exists today. Color sci-
by the Commission Internationale de l’É-
ence, simply stated, quantifies our percep-
clairage (CIE), resulting in standardized
color-vision theory, its axes do not corre-
tions of color in a standardized fashion.
light sources, spectrophotometers, and
spond exactly to the unique hues. As a
Two instruments were used to measure
an average observer. Today, the following
consequence, experience is required to
and built-in color processing, the quality
La Grande Jatte: a Gretag-Macbeth Eye-
publication (or its most current revision)
relate CIELAB coordinates with specific
of the color image can vary widely with
One and a Gretag-Macbeth SpectroEye,
governs colorimetry: CIE No. 15.2,
color perceptions. The color space is
respect to treating the imaging system as
both with 4.5 mm circular measurement
Colorimetry, 2nd ed. (Vienna, 1986).
used most frequently to describe differ-
an analytical instrument, in similar fash-
apertures.
See Roy S. Berns, Billmeyer and Saltz-
ences in color (e.g., lighter, redder, higher
ion to the small-aperture spectropho-
man’s Principles of Color Technology,
in chroma) rather than their absolute
tometer measuring a sample’s CIELAB
the aging of the painting. The minimum
3rd ed. (New York, 2000), to discover
perceived color.
values; see Roy S. Berns, “The Science of
reflectance, in the case of La Grande
more about the many industrial uses
Jatte of about 5 percent, is a function of
of colorimetry.
Notes
3 The maximum reflectance is limited by
226
Berns
“color” and the Greek word metrein,
9 Although CIELAB is based on opponent
the painting’s perimeter were used as
a guide.
13 Depending on the specific color filters
10 This property cannot be achieved with
Digitizing Paintings for Color-Accurate
all pigments, but red lake, zinc yellow,
Image Archives: A Review,” Journal of
Imaging Science Technology 45 (2001),
Kubelka-Munk equations. Nonlinear
pp. 373–83.
optimization was used to determine
dots should have been recalculated using
or very light, so the assumption was rea-
concentrations that resulted in specific
the estimated spectrum for the shadowed
sonable in this case. Furthermore, per-
a GretagMacbeth ColorChecker DC con-
CIELAB coordinates or reflectance spec-
grass; however, the resulting spectrum
forming the translation on the CIELAB
sisting of 240 color patches, and a 60-
tra as required in order to achieve the
was nearly the same lightness as the
coordinates rather than on the camera
color-patch custom target consisting of
objective of rejuvenating Seurat’s palette.
darkened orange. Consequently, we
signals also improved the rendering,
the 30 chromatic pigments of the Gamblin
These mathematics are an integral
implemented the same technique used
since CIELAB has a perceptual basis.
Conservation Colors mixed with titanium
component of instrumental-based color
for the yellow and green dots.
white at two different proportions. We
matching, such as routinely performed
measured each target with a spectropho-
in paint stores.
14 We used two targets to verify our results:
tometer and calculated the CIELAB
16 We derived functional relationships so
20 Strictly, the paint-outs for the orange
21 Work is currently under way to perform
the dot-by-dot digital rejuvenation on the
painting as a whole. The Art Institute
coordinates for each color. We used the
that the un-aged colors, defined as
image and spectrophotometric data of
CIELAB coordinates, could be predicted
the ColorChecker DC to create a mathe-
from the in situ measurements. We then
matical transformation that converted
used these to build custom Photoshop
by translating CIELAB coordinates, an
the RGB data to L*a*b*. The profile con-
curves. These curves were an approxi-
additive shift in values. These transla-
sisted of three nonlinear functions that
mation, since the relationship between
tions were implemented in Photoshop by
transformed the encoded camera signals
spectra and colorimetry is complex; how-
creating custom curves, one for each dot
to linear photometric signals. These
ever, the colors un-aged by Photoshop
color. There is of course a range of colors
functions were similar to typical gamma
were very similar to those un-aged by
within each of the three types of dots,
curves of about 1/2.2, a common camera
subtracting the aging spectrum. The
due to varied proportions in the mixtures
encoding. A (3 x 4) transformation
success of this approximation resulted
and the thickness and opacity of the
matrix converted the linearized signals
in part from the limited color gamut of
paint used. We verified these ranges by
to CIE XYZ tristimulus values for illumi-
the painting.
statistically evaluating the CIELAB values
nant D50 and the 1931 standard
17 Microscopic analysis revealed that these
none of these three colors was very dark
hopes to incorporate it into the exhibition that occasioned this publication.
22 The darkened colors were rejuvenated
of each color on the images of the paint-
observer. This matrix incorporated an
dots were greater than 97 percent zinc
ing. The translation operation maintains
offset. The average performance of this
yellow with small amounts of vermilion
the color range. The limitation to this
profile was 2.6 and 2.7 CIEDE2000 for
and trace amounts of lead white and
approach is the assumption that the gamut
the ColorChecker DC (characterization
several dark particles. Their spectra did
of colors is independent of the average
target) and artist paints (verification tar-
not show absorptions caused by the
color; in other words, that as colors were
get), respectively. For a numerical exam-
presence of the vermilion or other pig-
undarkened, they would have the same
ple, see Berns (note 6), appendix.
ments. Therefore, these pigments were
color variability. Theoretically, this
15 The scaling was based on the absorption
assumed to have a negligible affect both
assumption is false, since the range of
and scattering properties, not the direct
spectrally and colorimetrically and were
colors that can be made from pigments
spectral reflectance properties. The color-
not further considered.
has fixed endpoints of black and white.
mixing model employed for all the spec-
18 See note 15.
As colors become very dark or light, their
tral calculations was the single-constant
19 See Fiedler 1984, p. 49.
range, by definition, reduces. Fortunately
Rejuvenating Seurat’s Palette Using Color and Imaging Science
227