Superficial and Shroud-like coloration of linen by short
laser pulses in the vacuum ultraviolet
Paolo Di Lazzaro,1,* Daniele Murra,1 Enrico Nichelatti,2
Antonino Santoni,1 and Giuseppe Baldacchini3
1
ENEA Research Center of Frascati, Department Applications of Radiation, P.O. Box 65, Frascati 00044, Italy
2
ENEA Research Center of Casaccia, Department Optical Components Development,
via Anguillarese 301, Rome 00123, Italy
3
Present address: via G. Quattrucci 246, Grottaferrata 00046, Italy
*Corresponding author: paolo.dilazzaro@enea.it
Received 23 July 2012; revised 19 October 2012; accepted 20 October 2012;
posted 22 October 2012 (Doc. ID 173160); published 14 December 2012
We present a survey on five years of experiments of excimer laser irradiation of linen fabrics, seeking a
coloration mechanism able to reproduce the microscopic complexity of the body image embedded onto the
Shroud of Turin. We achieved a superficial, Shroud-like coloration in a narrow range of irradiation parameters. We also obtained latent coloration that appears after artificial or natural aging of linen following
laser irradiations that, at first, did not generate any visible effect. Most importantly, we have recognized
photochemical processes that account for both coloration and latent coloration. © 2012 Optical Society
of America
OCIS codes: 140.2180, 140.3390, 300.6280, 350.6670.
1. Introduction
The front and back images of a scourged man, barely
visible on the linen cloth of the Shroud of Turin (see
Fig. 1) possess particular physical and chemical characteristics [1] such that nobody was yet able to create
an image identical in all its microscopic details, as
discussed in a number of papers [2–21].
The inability to replicate the image on the Shroud
makes it impossible to formulate a reliable hypothesis on how the body image was made. As a partial
justification, scientists complain the Shroud has
been seldom accessible. Indeed, the most recent indepth experimental analysis of the images on the
Shroud was carried out in 1978 by the multidisciplinary team of the Shroud of Turin Research Project
(STURP). They used the most advanced instruments
available at that time, which were supplied by
1559-128X/12/368567-12$15.00/0
© 2012 Optical Society of America
various manufacturers, having a commercial value
of over two million dollars. The Shroud was examined
by ultraviolet (UV), visible, and infrared spectrometry, x-ray fluorescence spectrometry, microscopy,
thermography, pyrolysis mass spectrometry, lasermicroprobe Raman analyses, microchemical testing,
and fiber sampling [3–13]. These analyses did not
find pigments or artist’s media on the Shroud, except
for some iron oxide particles, micrometer-size cinnabar, and small traces of vermilion (HgS) [13]. However, there is a large body of scientific evidence
[8,11,12,16] that the microscopic observations reported in [13] cannot support the Shroud image is
a painting. It is likely the microscopic debris particles
have been transferred by contact of pigments from
artist’s copies of the Shroud that have been “sanctified” by pressing the two images together [17].
After years of exhaustive study and data evaluation, the STURP team achieved the following results.
(a) X-ray, fluorescence, and microchemistry results preclude the possibility of paint being used
20 December 2012 / Vol. 51, No. 36 / APPLIED OPTICS
8567
(h) The image fading has three-dimensional
(3-D) information of the body encoded in it [14].
Fig. 1. (Color online) Photograph of the Shroud of Turin and
its negative black/white obtained by Jasc software. The image
is clearly a negative, not a positive. The dimensions of the
Shroud are 441 cm in length and 113 cm in width. From
http://www.sindone.org.
as a method for creating the image [7,8,11,12]. UV
and infrared evaluation confirm these studies
[3,5,6,10]. The body image is not painted or printed.
(b) Both kinetics studies and fluorescence measurements support the image was formed by a
low-temperature process. In fact, the temperature
was not high enough to change cellulose within the
time available for image formation, and no char was
produced [12,14]. As a consequence, the body image
was not made by a heated bas-relief.
(c) The Shroud’s image is superficial as the color
resides on the outer surface of the fibers that make
up the threads of the cloth [8,11]. Recent measurements on image fibers of the Shroud [19] confirmed
that the coloration depth is approximately 200 nm,
which corresponds to the thickness of the primary
cell wall of the linen fiber [22]. In a single linen
thread, there are some 200 fibers.
(d) The colored (image) fibers are brittle, show
“corroded” surfaces, and are more fragile than uncolored fibers [8,11]. When illuminated by UV light of a
lamp, image fibers emit a reduced fluorescent light
compared with fibers out of the image [3].
(e) The coloration of the Shroud image was
formed by an unknown process that caused oxidation, dehydration, and conjugation of polysaccharide
structure of fibers to produce a conjugated carbonyl
group as the chromophore [8,11,12]. To some extent,
the color is a result of an accelerated aging process of
the flax.
(f) The image seen at the macroscopic level is an
areal density image. This means shading is not
accomplished by varying the color but by varying
the number of colored fibers per unit area at the
microscopic level [5,9,11,12].
(g) The blood tests positive for human blood, and
there is no image on the Shroud beneath bloodstains
[12,23]. UV illumination allows observation of typical fluorescence of bilirubin around the main bloodstains [10], which would be consistent with a
hemolytic process caused by torture. Independent
analyses of forensic pathologist Baima Bollone confirmed STURP’s findings [24].
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AllattemptstocreateaShroud-likeimagehavefailed
to reproduce adequately the above characteristics.
Some researchers have obtained coloration/images
that look similar [5,18,20,21], but no one has created
images that match all microscopic and macroscopic
characteristics of the Shroud image. The answer to
the question of how the image was produced or what
produced the image is still unknown. This is the main
point of the “mystery of the Shroud.” Regardless of
the age of the Shroud, either medieval (1260–1390)
as dated by radiocarbon test [25,26] or older as it
results from other measurements [27], and whatever
the importance of historical documents about the
existence of the Shroud before 1260 [28,29], the most
important question remains the same: how did the
image of a man get on the Shroud?
In this paper we approach this unsolved problem,
summarizing the main results of experiments carried out at the ENEA Frascati Research Centre
aimed at identifying the physical and chemical processes able to generate a Shroud-like coloration.
2. Radiative Hypothesis
The results of STURP measurements, briefly summarized in Section 1, have important consequences
when seeking possible mechanisms of image formation. Let us discuss some of these consequences,
assuming that (according to the STURP conclusion
http://www.shroud.com/78conclu.htm) the Shroud
covered a man when the body image and blood stains
were formed.
• The front and back images do not show the
typical deformations of a 3-D body put in contact with
a two-dimensional cloth. As a consequence, we deduce that the image was not formed by contact with
the body. This observation suggests the Shroud was
loosely draping the body, that is, it was not wrapped
around and in full contact with the whole body. This
mode is also supported by the gradations of image
brightness that correlate with expected cloth–body
distances: in fact, there is a geometrical relationship
that corresponds to a body shape and a cloth draping
naturally over that shape [14]. Moreover, this “draping mode” is consistent with the lack of image of the
body’s sides. These considerations, combined with
the extreme shallowness of the image color, the absence of pigments, and the subliminal, microscopic
complexity of image at the fiber level, make unlikely
obtaining a Shroud-like image by contact methods
(i.e., by chemicals), either in a modern laboratory
[20], or a fortiori by a medieval forger. The hypothesis
of the medieval forger is also ruled out by the anatomical consistency of blood and serum versus wounds,
including the presence of bilirubin, which is invisible
to the naked eye. This subliminal feature is only
visible by UV fluorescence photography [10] and
requires knowledge of anatomy and of forensic medicine [30] not available in the Middle Ages.
• Under the blood there is no image. This
means that the blood stains occurred physically on
the Shroud before the body image [8,12]. As a consequence, the image was formed after the deposition of
the corpse. Moreover, we observe that all the blood
stains have sharp outlines and are flawless, and this
poses a question if the corpse was removed from
the cloth.
• On the Shroud there are no signs of putrefactions, which occur at the orifices about 40 h after
death. This means that the image does not depend
on putrefaction gases and the corpse was wrapped
in the Shroud not longer than two days.
In order to satisfy the conditions posed by these experimental observations, some papers [14,15,18,19]
have suggested that an electromagnetic energy incident on a linen fabric could reproduce the main characteristics of the Shroud image, namely the absence
of pigments, the shallowness of the coloration, the
image in areas not in contact with the body, the gradient of the color, and the absence of image under the
blood stains.
The first attempt to reproduce a Shroud-like coloration by electromagnetic energy used a CO2 laser emitting infrared radiation (wavelength λ 10.6 μm).
These experiments produced an image on a linen fabric
similar to the Shroud at macroscopic level [21]. However, microscopic analyses showed a bulky coloration
and many fibers carbonized, which are not compatible
with the Shroud image [1,12]. In fact, the CO2 infrared
radiation excites the vibrational energy levels of the
irradiated material, with consequent release of thermal energy, which heats the linen threads in the bulk
of the fabric. On the contrary, it is well known that the
energy carried by short-wavelength radiation breaks
the chemical bonds of the irradiated material without
inducing a significant heating (photochemical reaction). Moreover, linen has a molar absorptivity, which
increases when decreasing the radiation wavelength.
Consequently, the smaller the wavelength, the thinner
the material necessary to absorb all the radiation.
Therefore, we have chosen the UV radiation to obtain
at least two of the main characteristics of the Shroud
image: a thin coloration depth and a low-temperature
image formation.
3. Experimental Results by UV Laser Radiation
Figure 2 shows the setup of the laser irradiations.
The laser emits radiation pulses that are focused
by a lens onto a linen fabric fixed on a frame. The
energy per unit area (fluence) and the power per unit
area (intensity) of the laser pulses on linen are varied
by changing the surface of linen irradiated, i.e., by
moving the fabric with respect to the lens that
focuses the laser radiation, as shown in Fig. 2. During the five year irradiation experiments, temperature and relative humidity in the laboratory ranged
between 18°C and 25°C and between 50% and 70%,
respectively.
When we irradiated linen with our Hercules excimer XeCl laser (λ 0.308 μm, single pulse energy
5 J, time duration of each pulse 120 ns), we could
not get any coloration. Linens irradiated with high
fluence/intensity were carbonized, while at intermediate and low fluence/intensity values we did
not observe any change.
Then we irradiated the linen with the radiation
emitted by another XeCl laser that emits pulses 4
times shorter and an energy per pulse 12 times smaller than the Hercules laser. The irradiated area was
chosen to have the same values of laser fluence incident on linen of the previous irradiations. In this configuration we achieved a permanent coloration of the
linen in a narrow range of pulse duration, intensity,
number of laser pulses, and time interval between
successive pulses.
In summary [31–33], we have shown that the
combination of laser parameters necessary to color
Fig. 2. (Color online) Setup of excimer laser irradiations of linens. The laser pulses are focused by the lens ℓ on the linen fabric L. The laser
fluence/intensity on the linen is varied by moving L along the optical axis of the lens. Part of the laser pulse is reflected by the beam splitter
BS and monitored by the photodiode PD and the oscilloscope OS connected to a personal computer PC, which processes the data, taking
into account the shot-to-shot energy fluctuations. The distance between the XeCl and the ArF excimer lasers and the lens is about 1 m and
25 cm, respectively.
20 December 2012 / Vol. 51, No. 36 / APPLIED OPTICS
8569
F T N=A ×
Fig. 3. (Color online) Photomicrograph of the cloth irradiated
with 100 XeCl laser pulses. Intensity fluence 16 MW=cm2
(0.5 J=cm2 ) per pulse.
the linen (the time width of the laser pulses, the
intensity/fluence, the number of pulses, the repetition rate) is very narrow. In fact, to obtain the coloring of the flax, UV pulses must be shorter than 50 ns,
and small changes of any laser parameter may lead
to lack of linen coloration. However, the hue of the
color (brown to dark yellow, depending on the intensity and number of laser shots; see Fig. 3) was darker
than the yellowish image of the Shroud. The linen
coloration was superficial, but the depth of color
was still larger than that of the Shroud image.
The analysis of the above results suggest that short
pulses in the vacuum UV (VUV) would allow a coloration more similar to that of the Shroud. Our choice
was the ArF excimer laser.
4. Experimental Results by VUV Laser Radiation
The ArF laser (λ 0.193 μm, 0.08 J=pulse, 12 ns,
1 Hz) emits radiation in the VUV spectral region
with smaller energy and shorter pulse duration than
XeCl lasers. Using the same setup shown in Fig. 2,
the linen was irradiated in a wide range of laser
parameters [34–37] as summarized in Table 1, which
reports the observations of the irradiated flax as a
function of the number N of consecutive laser pulses,
of the spatially averaged fluence F of each laser
pulse, of the total fluence F T N × F, and of the spatially averaged intensity I of each pulse, defined as
follows:
F 1=A ×
Table 1.
N
30
100
50
200
200
402
600
500
8570
ZZ
Fx; ydxdy;
(1)
s
ZZ
Fx; ydxdy;
where A area of the flax irradiated by the laser
and Fx; y fluence at point x, y of the transverse
area s of the laser beam. An equation analogous to
Eq. (1) can be written for I.
Table 1 shows that the coloration of linen is proportional to the total laser fluence F T and is not related
to the intensity I nor to the fluence F of each pulse.
This surprising behavior is explained assuming that
each laser pulse interacts with a linen slightly modified by the previous laser pulse. This cumulative
effect becomes visible only when F T > 22 J=cm2 ,
which is the threshold value for coloration. In particular, the yellow color in Fig. 4 is obtained when
F T ≈ 25–27J=cm2 . When F T > 51 J=cm2 , linen is
ablated, and when F T > 66 J=cm2 , linen is vaporized
and holed.
An interesting property of the irradiated linen is
the hue of color, which continuously varies from light
yellow to yellow–sepia when increasing F T . Then we
obtain a fine adjustment of the values of RGB and of
the chromatic coordinates (http://en.wikipedia.org/
wiki/RGB_color_model) by varying F T , e.g., simply
by changing N. As an example, let us consider the
third row of Table 1. In this case, 50 laser pulses produce a light yellow linen coloration. This means that
each laser pulse varies the contrast and the RGB
value of the color by a very small amount, equal in
average to 1=50 ≈ 2%, and consequently, we have a
very fine control of the chromatic coordinates. In fact,
a variation of 2% cannot be appreciated, considering
that after 50 pulses (i.e., at 100% of color variation)
the color is barely perceptible. Similar arguments
can be extended from the second to the seventh
row of Table 1.
Equations (1) and (2) show that the F values in
Table 1 are averaged over the irradiated area.
Because of the nonflat-top spatial profile of the laser
fluence, however, the local values of Fx; y may differ
from the average F. Consequently, we can observe all
the possible effects on the linen in the same area. For
example, Fig. 5 shows damaged threads in the linen
region at the center of the laser beam [where Fx; y)
is higher], while, about 1 mm away, Fx; y is smaller,
and there are yellow colored threads. Near the outer
edge of the laser beam, Fx; y is too small to affect
linen threads.
Summary of Main Visual Inspection Results on Linen as a Function of the ArF Laser Irradiation Parameters
I (MW=cm2 =pulse)
F (J=cm2 =pulse)
F T (J=cm2 )
Macroscopic Findings on Linen
35
14
36
10.5
11.2
6.6
6
13.3
0.420
0.168
0.432
0.126
0.134
0.073
0.066
0.146
12.6
16.8
21.6
25.2
26.8
29.3
39.6
73.0
No change
Coloration only visible at grazing incidence
Light yellowing
Yellow color
Yellow color
Yellow–sepia
Yellow–sepia
Ablation
APPLIED OPTICS / Vol. 51, No. 36 / 20 December 2012
(2)
s
at λ 0.308 μm; see Fig. 6(b) [32,33]. As the threads
of our linen fabric have an average diameter of
300 μm, we deduce that the light at λ 0.193 μm
penetrates 2% to 9% of the diameter of the linen
yarn, depending on the specific conditions of
irradiation.
Among the analyzed fibers, we found one showing
a colorless inner part (see Fig. 7), and in this case it is
possible the color affects only the outermost film of
the same fiber, the so-called primary cell wall, which
is about 0.2 μm thick. This result is close to the thinnest coloration depth observed in the Shroud image
fibers [19].
Fig. 4. (Color online) Photomicrograph of a warp thread of
flax irradiated with ArF laser at a total laser fluence F T
26.4 J=cm2 . The thread was crushed with forceps to separate
the fibers and highlight their yellow color. At the center of the
thread, there is a uncolored zone due to a weft thread that shadowed the laser radiation [36].
Fig. 5. (Color online) Area of the linen irradiated by the ArF laser
beam shows different characteristics that depend on the local
values of Fx; y. (1) Colored area, (2) ablated area, and (3) area
irradiated below the threshold for coloration [36].
Concerning the thickness of coloration, photomicrographs [an example is shown in Fig. 6(a)] show
coloration depths ranging between 7 and 26 μm in
threads irradiated with different intensities [35].
This is a range of thicknesses 11 to 3 times thinner
than the coloration depth achieved by the radiation
5. Latent Coloration
We cut half of the laser spot on linen irradiated with
F T 16 J=cm2 , i.e., below the threshold for coloration; see Table 1. As a consequence, the irradiated
linen did not appear colored. We then heated one
of the two parts by an iron at a temperature of 190
10° C for 10 s, and a coloration appeared immediately
after heating. Figure 8 shows that the heating process, which simulates aging, colors only the surface
irradiated below threshold and does not color the
nonirradiated area. Moreover, when heating linen irradiated in the conditions of the first row of Table 1,
no latent coloration is observed after heating. That
is, in the latter case, we are below the threshold
for latent coloration. Therefore, we deduce that the
range for obtaining a latent coloration is F T ≈
13–20J=cm2 . Finally, when heating a colored linen,
we observe a more evident coloration, with a stronger
contrast with respect to the surrounding, nonirradiated area.
Using the short-pulse XeCl laser, we have obtained
a latent coloration similar to that of Fig. 8, which appeared after a natural aging of more than one year,
maintaining the linen at room temperature in a dark
environment [32].
The importance of these results is twofold. On one
hand, there is the scientific interest of UV and VUV
light that breaks chemical bonds in a way to favor
the oxidation and dehydrating effect of heat (aging),
finally resulting in linen coloration. This dual mechanism will be discussed in Section 8.A. On the
other hand, there is the interest of historians,
Fig. 6. (Color online) Photomicrographs of the cross section of two linen threads, irradiated by (a) ArF and (b) XeCl laser beams, respectively, “from top” of photos. In (a) VUV colors a very thin topmost part of the thread, corresponding to few fibers. In (b) UV colors more than
one half of the section of the thread. Both threads have an average diameter of 300 μm [35].
20 December 2012 / Vol. 51, No. 36 / APPLIED OPTICS
8571
Fig. 7. (Color online) Microscope image of a single linen fiber
colored after ArF laser irradiation. The mechanical damage in the
central part allows observation of small broken pieces of colored
primary cell wall (see inside circles) on a colorless inner part of
the fiber. The average diameter of the fiber is 20 μm.
50% of fluorescent light emitted by fibers out of the
image [3]. This quenched fluorescence is one of the
most peculiar characteristics of the Shroud image.
Figure 9(a) shows a linen fabric after ArF laser
irradiation when illuminated with a UV lamp.
The area irradiated by the laser emits a much
smaller blue fluorescence with respect to the linen
fabric. This result suggests that the VUV radiation
of the laser has changed the electronic structure
of the cellulose to reduce the typical fluorescence of
the linen. The image threads of the Shroud behave
the same way.
Similar to the laser coloring process, the quenched
fluorescence of the irradiated threads takes place
only in a narrow range of the irradiation parameters.
For example, Fig. 9(b) shows that laser pulses having
a doughnut-shaped Fx; y profile inhibit the fluorescence only in an ellipsoidal ring irradiated by the correct value of F T . Outside this annular region, F T is
too weak to inhibit the fluorescence. This means that
the fluorescence induced by UV provides accurate
and selective information on the local intensity profile able to generate a Shroud-like coloration.
7. Further Experiments
A. How Much Different is Our Linen from the Shroud?
Fig. 8. (Color online) Linen fabric cut after irradiation below
threshold for coloration. (1) Irradiated area after heating. (2) Irradiated area, not heated. (3) Nonirradiated area. Latent coloration
is limited to the area of the linen irradiated below threshold and
appears only after artificial aging of the upper part of irradiated
linen [36].
attracted by the possibility that, whatever may
have caused the Shroud image, the coloration may
not have been immediately visible, i.e., it may have
“developed” over time.
6. Fluorescence by UV Illumination
Linen fabrics emit blue fluorescence when illuminated by UV light. However, spectroscopy measurements show the Shroud fibers of the image emit only
In our experiments, one wonders about the differences between our linen fabric and the linen of the
Shroud, besides the age. As our experiments involve
photons and optics, we measured some additional optical characteristics of our linen to be compared with
the linen of the Shroud. We used a Perkin–Elmer
Lambda 950 spectrophotometer equipped with a
15 cm diameter integrating sphere [38]. The interior
of the sphere is covered with a plastic material
known as Spectralon, whose characteristics of reflection are almost 100% Lambertian and constant
over the whole spectrum between UV and visible.
Additionally, this instrument has an internal calibration of the Spectralon, which allows the direct measurement of the absolute reflectance spectra. We
measured the hemispherical absolute spectral reflectance Rλ (i.e., the percentage of light reflected by
our linen with respect to the incident light), and
the results are shown in Fig. 10, together with the
spectral reflectance measured on the Shroud as
Fig. 9. (Color online) Blue fluorescence induced by UV lamp on linen after irradiation with ArF laser. (a) In the working point of the third
row of Table 1. The irradiated area shows a reduced fluorescence. (b) By using a laser beam having a “holed” spatial profile of the intensity.
In this case the fluorescence reduction is not uniform over the whole laser spot.
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APPLIED OPTICS / Vol. 51, No. 36 / 20 December 2012
Fig. 10. The solid curves show the absolute reflectance of the
linen of the Shroud in areas of no image as a function of the wavelength [3]. The dashed curve shows the absolute hemispherical
reflectance of the linen used in our experiments [35].
reported in [3]. Figure 10 shows a strong similarity of
our linen with respect to the Shroud. We note a small
difference in the spectral region between 520 and
600 nm, showing our linen is less yellowish than
the Shroud, possibly because of the different age.
Most important, the absolute reflectance of the two
linens at the laser wavelengths we used, 193 and
308 nm, is almost the same. Thus, when irradiated
in the UV and VUV, our linen behaves like the linen
of the Shroud.
Using the same spectrophotometer, we measured
the hemispherical transmittance Tλ of the linen
(i.e., the light transmitted by our linen with respect
to the incident light) as a function of wavelength.
Then we can deduce the spectral absorbance Aλ
as follows:
Aλ 1 − Rλ − Tλ;
(3)
that is, Aλ is the amount of light absorbed by linen
as a function of wavelength. The results are shown
in Fig. 11.
B.
Can UV Laser Radiation Make Linens Older?
The process of dehydrative oxidation that produces
the Shroud image can be considered a kind of premature aging of linen [8]. To check if the excimer laser
Fig. 11. Plot of the absolute value of the absorbance of the linen
versus the wavelength, according to our experimental values inserted in Eq. (3). The thickness of the plot gives the error associated to the measurement.
irradiation produces a similar aging effect, we
observed some linen fibers placed between crossed
polarizers in a petrographic microscope to detect
changes induced by laser irradiation in the crystal
structure of the fibers. The petrographic microscope
allows us to observe a pattern of isochromatic lines
that depend on age of the sample, mechanical stress,
and presence of defects. When the fibers are aligned
to the axis of polarization of the analyzer, we see a
dark image: in this case the fibers are placed “to extinction,” and there is no birefringence. If a part of
the fiber aligned to extinction is damaged, it becomes
birefringent and appears bright because damaged regions have a different crystalline orientation than
the fiber. Figure 12 shows a partially irradiated fiber
of linen immersed in oil as observed in cross polarization to identify the stressed regions of the fiber.
The nonirradiated region has the usual aspect of a
recent linen. In contrast, the irradiated (colored)
region of the fiber shows bright spots and tracks
corresponding to stressed areas coupled with highfragility zones. The dehydration of the fiber cellulose
is in fact associated with an increment of the fiber
fractures pointed out by birefringence at the location
of the defects.
A similar behavior is observed on old linen fibers
like those used to wrap Egyptian mummies. We can
therefore infer that short and high-intensity UV
pulses change the crystalline structure of cellulose
in a similar manner as aging and low-intensity radiation (radon, natural radioactivity, secondary particles from cosmic rays) accumulated in a long-term
period do.
C. Is the Coloration Induced by Excimer Laser Irradiation
a Photochemical or a Thermal Effect?
In order to verify whether the UV and VUV light
interacts with the linen by photochemical or
thermal processes, we used the infrared camera
ThermoShot
F30
(http://www.nec‑avio.co.jp/en/
products/ir‑thermo/pdf/catalog‑f30‑e.pdf) equipped
with microbolometers sensitive in the spectral range
8–13 μm. This camera is able to measure the surface
temperature of objects with the uncertainty of 0.2°C.
The camera was aligned in front of the linen
during laser irradiation, monitoring in real time the
Fig. 12. (Color online) Petrographic microscope observation of a
linen fiber. In the middle there is a partly colored fiber of linen in
oil 1.515 between crossed polarizers. The left part of the fiber is the
nonirradiated region, which is enlarged in the inset below. On
the right there is the region irradiated by XeCl laser, enlarged
in the inset above. From [32].
20 December 2012 / Vol. 51, No. 36 / APPLIED OPTICS
8573
temperature of the whole linen fabric as shown in
Figs. 13(a) and 13(b). During laser irradiations, the
room temperature was between 20°C and 21°C,
and the linen region irradiated by the UV XeCl laser
was heated up to 33°C, while the linen irradiated
by the VUV ArF laser was heated up to 25°C. It is
known that thermal effects can color the linen only
when the linen temperature approaches 200°C [11],
and we can conclude that excimer laser coloration is
a photochemical process that does not involve significant thermal effects.
8. Analysis of Results
Our results show that excimer lasers are powerful
tools to simulate the physical and chemical processes
that might have caused the peculiar coloration of the
Shroud image. In order to gain a deeper insight into
these processes, we need to detail some chemical and
physical properties of linen fabrics and of the excimer
lasers.
A.
Chemical Processes
Each thread of linen consists of about 200 fibers, rodlike structures with an average length of 30 mm and
average diameter of 20 μm. Each linen fiber has an
inner part (the secondary cell wall) of cellulose, and a
thin (0.2 μm) outer skin (the primary cell wall) composed of hemicellulose (predominantly xyloglucan)
bound with pectin to cellulose [22]. Let us recall that
hemicellulose is a polysaccharide similar to cellulose,
but it consists of shorter chains (500–3000 sugar
Fig. 13. (Color online) (a) (Left) Photo of the linen during XeCl
laser irradiation. (Right) Same picture seen in infrared light.
The color scale at the bottom shows that the warmest region of
the linen (in the middle of the laser spot) reaches 33°C, while
the nonirradiated area is at the room temperature of 20°C.
(b) (Left) Photo of the linen during ArF laser irradiation. (Right)
Same frame seen in infrared light. The color scale reveals that the
warmest area of the irradiated linen is at 25°C, while the nonirradiated area is at the room temperature of 21°C.
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units) as opposed to 7000–15,000 glucose molecules
per polymer seen in cellulose. While cellulose is crystalline, hemicellulose has an amorphous structure
with little strength. To some extent, hemicellulose
can be considered a degraded form of cellulose.
The STURP results show the image chromophore
is a conjugated carbonyl produced in the polysaccharide structure of fibers by a dehydrative oxidation
process [8,11,12,39]. The color of the Shroud image
is a result of an accelerated aging process of the
linen, similar to the yellowing of ancient papers [40].
Figure 14 shows two different chemical paths possibly involved in the formation of the image on the
Shroud.
The different thickness of coloration obtained with
the XeCl and ArF lasers (see Fig. 6 and Sections 3
and 4) may be due to the different λ, respectively
emitted. In fact, the shorter the wavelength, the larger the energy absorbed per unit volume. However,
Fig. 11 shows there is only a difference of 11% between the flax absorption at 0.193 μm and at
0.308 μm [34,35]. As a consequence, we must find
an additional mechanism to explain the different penetration depth of light in the fibers and the different
hue of color, i.e., yellow after ArF laser irradiation at
0.193 μm and light brown after XeCl laser irradiation
at 0.308 μm; see Figs. 3 and 4. This additional mechanism could be promoted by the absorption band
below 0.2 μm of alkene groups (─C═C─) [41] typical
of degraded cellulose and of organic impurities of the
primary cell wall of linen fibers. The VUV absorption
of these groups may trigger a reaction chain that
leads to photo-oxidation (aging) and to new alkenes
and carbonyl groups. After a proper irradiation dose,
new conjugated C═C and C═O groups are formed,
increasing delocalization and thus shifting the
absorption band to longer wavelengths in the blue–
green region of the visible spectrum, to finally produce the yellowish Shroud-like coloration shown in
Figs. 4, 5, 6(a), and 7. Note that the XeCl wavelength
is too long to fit in the absorption band of alkenes, so
that it is not able to start the many-step process
schematized in Fig. 14.
In this frame, the formation of latent images described in Section 5 can be explained by oxidation
and dehydration of the cellulose (caused by heat or
by natural aging) amending the new chemical bonds
induced by laser irradiation, thus facilitating the formation of conjugated unsaturated structures that
are an essential part of the chemical transitions in
Fig. 14. The synergy between heat and UV light is
detailed in [42], showing how the process initiated
by exposure to UV radiation is accelerated and
reinforced by heat.
B. Physical Processes
Let us consider the different role of laser intensity
and fluence in the coloration of the linen. We showed
in Section 3 that XeCl laser pulses having the same
fluence but different pulse durations (i.e., different
intensities) produce different colorations. This
Fig. 14. (a) Main molecular structure common to both cellulose
and hemicellulose. There are two possible transitions a → b →
c and a → d → e that generate chromophores after oxidation and dehydration. The C═C and C═O double bonds in (c) and
(e) act as chromophores and are responsible for the yellow color of
the fibers of image on the Shroud of Turin.
suggests that the intensity is the key parameter.
However, Table 1 shows that consecutive laser pulses
sum their effects, and the key parameter is F T , that
is, the number of photons per unit area. This apparent dichotomy evidences that we are observing a
complex photochemical process, where intensity
and fluence play in turn a dominant role, depending
on the duration of the pulses, the number of photons
per unit area, and the number and repetition rate of
laser pulses.
Let us now analyze why it is so difficult to get a
coloration limited to the primary cell wall of flax
fibers (see Fig. 7). As mentioned in Section 4, the
fluence/intensity spatial profile of the excimer laser
beam is not uniform, showing high-frequency spatial
fluctuations, which can be detected and measured
by a CCD camera with high spatial resolution;
see Fig. 15.
The fluctuations in Fig. 15 have an irregular period, with gradients of intensity/fluence up to
350 MW=cm2 per centimeter (4 J=cm2 per centimeter). The value of laser intensity (fluence) incident
on two points of the linen at, say, 1 mm distance can
vary up to 35 MW=cm2 (0.4 J=cm2 ). The huge value
of the intensity/fluence gradient can explain why it is
possible to get the “right” value of intensity for submicrometer coloration only in a very limited area,
which is difficult to be found by photomicrographs.
Fig. 15. (Color online) One-dimensional intensity/fluence profile
of our laser beam measured by a CCD camera Andor Model
DV-430UV, with a 22 μm single-pixel resolution. The inset shows
an enlargement of the high-frequency spatial fluctuations. Note
that the contribution of CCD noise to the spatial fluctuations on
the plateau is negligible, of about 4 × 10−3 .
As mentioned in Section 1, point (f), the shading of
the Shroud image is not accomplished by varying the
color but by varying the number of colored fibers per
unit area [5,12]. In addition, the image area has a
discontinuous distribution of color along the threads
of the Shroud [9]. Some of these features can be found
in our irradiated linens; see, e.g., Fig. 16, which
shows colored fibers next to uncolored ones in the
same thread.
However, we did not fully achieved a “half-tone
effect” comparable with that observed on the Shroud
[5,12]. In principle, it would be possible to replicate
exactly this characteristic by laser pulses having a
spatial intensity distribution “sawtoothlike” with
variable period. This distribution can be achieved
by state-of-the-art diffractive optics that arbitrarily
modulate the spatial distribution of laser beams [43].
9. Summary, Consequences, and Remarks
In this paper, we summarized the current state of
knowledge on the Shroud image and the reasons of
the difficulty to create an image that matches its
peculiar superficiality and chemistry at the microscopic level. After countless attempts, the inability
to replicate the image on the Shroud prevents the formulation of a reliable hypothesis on the process of
the image formation. Because of these scientific
and technological difficulties, the hypothesis of a
medieval forger does not seem reasonable. We then
summarized the experiments done at the ENEA
Research Center of Frascati, which have demonstrated the ability of VUV light pulses lasting few
nanoseconds to generate a Shroud-like coloration
on linen that matches many (although not all) characteristics of the Shroud image. By the way, the ability of VUV light to generate a Shroud-like coloration
helps to clarify the controversy between two scientists of the STURP team: Jackson, who foresaw
the possibility of coloring flax by VUV radiation [15],
and Rogers, who believed that laser pulses would
have heated and vaporized flax, without any coloration effect [44]. Rogers’ opinion was based on the
Fig. 16. (Color online) Photomicrograph of linen threads after
ArF laser irradiation. Single colored fibers are visible next to
uncolored fibers, like in the Shroud image.
20 December 2012 / Vol. 51, No. 36 / APPLIED OPTICS
8575
failure of experiments made at Los Alamos using
excimer lasers, but our results demonstrate that
their failure was due to parameters (e.g., laser pulse
width) outside the narrow range of values able to
generate the permanent linen coloration.
Let us summarize in the following the main results
we achieved.
I. We obtained a linen coloration only in a
narrow range of laser parameters. In particular, the
temporal duration of the single laser pulse must be
shorter than 50 ns [31,32]. While a short laser pulse
width is necessary for obtaining a coloration, the
VUV laser spectrum allows a coloration limited to
the outer surface of the threads.
II. The most interesting results were obtained
with VUV light. The permanent linen coloration is
a threshold effect, i.e., the color is obtained only when
F T > 22 J=cm2 ; see Table 1. When the F T value is
above threshold, the linen is ablated and/or vaporized, while when F T < 13 J=cm2 the linen does not
change color. Even when F T is in the coloration
range, not all the irradiated fibers are colored (Figs. 5
and 16) due to the spatial fluctuations of energy density of the laser pulses shown in Fig. 15.
III. We triggered a photochemical coloration process. In fact, the thermal heating associated with UV
and VUV radiation is within a few degrees centigrade and therefore irrelevant for the purpose of coloring by scorching linens; see Fig. 13. This result fits
with the “cold” coloration process of the Shroud
estimated in [8,11,12].
IV. The hue of color depends on the wavelength
and on the number N of laser pulses, which is proportional to F T . Irradiations at 0.308 μm generate a
brownish coloration, while the 0.193 μm photons produce a yellow color (see Fig. 4) comparable to the color of the Shroud image. In both cases, the contrast
slowly increases with the number of laser pulses,
allowing an accurate control of the RGB value by
varying F T .
V. The different hue of color obtained by UV
and VUV radiation is due to different chains of photochemical reactions respectively triggered. In particular, the VUV radiation at 0.193 μm is absorbed by
alkene groups in degraded cellulose, whose number
increases with F T , thus inducing a photolysis of the
cellulose, which promotes the formation of chromophores; see Fig. 14. These chromophores determine
the yellow coloration of the fibers [8,12,39–41].
VI. We observed an irradiated fiber whose coloration was possibly confined in the primary cell wall
[35,36], which is comparable with the thinnest coloration depth observed in the fibers image of the
Shroud of Turin [8,11,19].
VII. Aging can be a concause of linen coloration. In
fact, after laser irradiations that, at first, do not generate a visible coloration of linen, a latent coloration
appears either by artificial (Fig. 8) or natural aging of
linen [32,35]. Latent coloration is interesting for the
synergy between UV, oxidation, and the dehydrating
effect of heat (or of aging), which triggers the
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APPLIED OPTICS / Vol. 51, No. 36 / 20 December 2012
coloration process, and also for historians, attracted
by the possibility that, whatever may have caused
the Shroud image, the coloration may have developed over time.
VIII. The partial inhibition of fluorescence induced
by VUV laser radiation (Fig. 9) is an additional feature of our coloration similar to the Shroud image.
The induced fluorescence is also capable to selectively recognize the uniformity of F T incident on
linen; see Figs. 9(a) and 9(b).
IX. Both UV and VUV radiations coloring linen
are compatible with the absence of image under
the bloodstains on the Shroud because in this spectral region light is absorbed by very thin layers of
blood hemoglobin. According to [45] the UV light
may be responsible for another special feature of the
Shroud: the red color of blood stains after so much
time since their deposition.
X. Using a petrographic microscope, we have
observed some defects induced by UV radiation in
the structure of irradiated linen fibers (see Fig. 12),
similar to very old linen fabrics [11,46].
XI. Absolute reflectance measurements show
that, when irradiated in the UV and VUV, our linen
behaves like the linen of the Shroud; see Fig. 10.
In summary, our results demonstrate that a short
and intense burst of directional VUV radiation can
color a linen cloth so as to reproduce many of the
peculiar characteristics of the image on the Shroud
of Turin, including the hue of color, the shallow penetration depth of the color, and the inhibition of
fluorescence.
The Shroud image has characteristics that we
have been able to reproduce only in part, for example
the gross shading structure that is determined by the
ratio of yellow to uncolored fibers in a given area; see
point (f) in Section 1 and Fig. 16. As discussed in
Section 8.A, sophisticated diffractive optics could replicate these features, but this effort is far beyond
our intention. In fact, our purpose was not to demonstrate that a battery of 10,000 lasers can accurately
reproduce the image on the Shroud. Our main purpose was to perform accurate and reproducible
experiments apt to understand the physical and chemical mechanisms that might have played a role in
the generation of the Shroud body image, using a
powerful and versatile tool such as the laser, regardless of the source of energy that may have caused this
image. In this frame, our experimental data can be
helpful to scholars seeking a linen coloration by corona discharge [18] or electrostatic discharge and
radon emitted during seismic events [47], which involve UV and VUV light but are difficult to control
and characterize.
This is not the conclusion; we are composing pieces
of a fascinating and complex scientific puzzle. The
enigma of the body image of the Shroud of Turin
is still “a challenge to our intelligence” [48].
The authors thank Professor Giulio Fanti
(University of Padua) for photomicrographs of fibers,
for photos of Fig. 13, and for useful discussions about
the manuscript contents.
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