Laser Stimulated Imaging of Large Scale Fluorescence in Caves

Abstract

Calcium carbonate fluorescence has been a hallmark of speleothems for many decades and experienced by thousands of cavers with UV lights. The development of near-UV high power lasers has literally opened up new ‘vistas’ in our ability to fluoresce cave formations. Described here for the first time, is a portable, 405 nm laser scanning system that can fluoresce and image wide angle cave panoramas up to 50 meters away. The resulting images show large scale changes in luminescent colors which can be analyzed for growth successions and relationships. The colors themselves are not specifically diagnostic but do represent differentiation in the impurities found in the water that incorporate into the crystal structure at the time of formation. First and foremost this is a discovery process that highlights odd or unusual features that beg further investigation using laboratory techniques.

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NSS NewS, July 2017 11
Abstract
Calcium carbonate fluorescence has
been a hallmark of speleothems for many
decades and experienced by thousands of
cavers with UV lights. The development
of near-UV high power lasers has literally
opened up new ‘vistas’ in our ability to
fluoresce cave formations. Described here
for the first time, is a portable, 405 nm
laser scanning system that can fluoresce
and image wide angle cave panoramas up
to 50 meters away. The resulting images
show large scale changes in luminescent
colors which can be analyzed for growth
successions and relationships. The colors
themselves are not specifically diagnostic but
do represent differentiation in the impurities
found in the water that incorporate into the
crystal structure at the time of formation.
First and foremost this is a discovery process
that highlights odd or unusual features that
beg further investigation using laboratory
techniques.
Introduction
Solution caves, like those found in the
mountain regions of South-east Arizona,
were created by natural processes spanning
thousands of years. They are a product of
slightly acidic meteoric, vadose, and phreatic
water penetrating the joints in soluble rock
[1]. Speleothems are secondary cave deposits
that are composed of calcite or a combina-
tion of the over two hundred known cave
minerals precipitated from cool or hot water
Laser Stimulated Imaging of Large Scale Fluorescence in Caves
Thomas G. Kaye1, Jessica Garcia2
1: Foundation for Scientific Advance-
ment, tom@tomkaye.com,
2: Southeast Arizona Group, National
Park Service, jessica1_garcia@nps.gov
entering the CO2-rich cave environment
[2]. The water can come in close proximity
to certain ions during speleogenesis and
have the potential to produce a fluorescent
reaction.
The fluorescence recorded in this study
was imaged in the limestone walls and
speleothems found in the Southeast Arizona
region. The study focused on caves found
in the Huachuca and Chiricahua mountains.
The unique geology of these caves provides
a surplus of potential ions capable of absorb-
ing the laser energy and re-emitting energy
in the form of fluorescence. The Huachuca
and Chiricahua mountains are unique areas
geologically.
At Coronado National Memorial
(Coronado NMem) within the Huachuca
Mountains, there are caves composed of
Paleozoic and Mesozoic limestones and
dolostones. The cave-bearing units are
surrounded by interbedded sandstones,
conglomerates, shales, and silt deposits,
generally encased in quartz monzonite [3,
4]. During the Huachuca’s geologic history,
magmatism produced heated, mineral-rich
fluids that altered the surrounding rocks,
producing mineral deposits, unique vari-
eties of ions, and causing large sections
of limestones to be metamorphosed into
massive marble blocks before speleogenesis.
Increased faulting and fracturing beginning
in the Mesozoic have implications for sub-
surface water travel. Slightly metamorphosed
caves are known to have both fluorescent
and non-fluorescent minerals, like calcite and
its polymorph aragonite, dolomite, epidote,
vesuvianite and small garnets. Coronado
NMem caves are also known to contain
more rare fluorescent minerals and poten-
tial chemical activators, including willemite,
rhodochrosite, brochantite, and scheelite [5].
In the Chiricahua Mountains, the caves
are composed of fossil-bearing Paleozoic and
Mesozoic limestones. The hypogene Crystal
Cave in the Cave Creek area is particularly
unique. A large percentage of the cave walls
are covered with quartz crystals, almost like
walking through a geode. The speleogenesis
of the cave is believed to have preceded a
silicification period due to the drusy and
crystal quartz found coating the calcite rock
walls throughout the majority of the cave.
There also appears to have been geochemi-
cally distinct fluids throughout speleogensis
as seen by the formation of the network
maze system, geodes, vugs, and mammilary
crusts[6]. Fluorescence can be seen in the
limestone walls beneath the quartz coating
and in the mammillary crusts at lower cave
elevations. Cerussite, barite, sphalerite,
calcite, aragonite, gypsum are a few of the
potential fluorescent minerals and elemental
fluorescent activators, such as lead, molyb-
denum, manganese, and tungsten are found
throughout the Chiricahua Mountains [7] and
can contribute to the strong fluorescence and
phosphorescence seen in the cave.
Laser stimulated fluorescence provides
a new portable, affordable way to perform
long distance, non-invasive geochemical
analysis of caves. The concentration and
shade of the fluorescence color is a product
of the wavelength of light introduced, any
filters, the fluorescent properties of the speci-
men, and present activating ions. Absorption
and reemission isn’t always done by the same
contaminant. For example, a very common
elemental activator in calcite is divalent
manganese, fluorescing an orange-red color.
The manganese often works with co-acti-
vators to absorb the photons and transfer
the energy over to Mn2+ for emission and
co-activators are most commonly Pb2+,
organic impurities, and REE’s [5]. Using this
technique we can visualize changes in forma-
tions and cave walls which could give insight
into speleogenesis and cave geochemistry.
Methods
The general process to record a fluores-
cent image is as follows:
All incident lights are turned off to
achieve total darkness.
A near UV laser is projected through a
‘line’ lens that forms a vertical blue laser line
on the cave wall.
A hand switch scans the vertical laser
line left and right over the field of view by a
motorized stage that mounts the laser.
Once the laser is moving, the camera
is triggered and does a 30-60 second time
exposure.
A filter in front of the camera blocks
ALL of the laser light so only fluorescence
is recorded by the camera.
The raw image is color equalized in
Photoshop to bring out the maximum differ-
ences in the colors.
High power near ultraviolet (UV) laser
diodes are now available in the one watt
range. These bright light sources can be
used in portable devices in remote locations.
A custom third generation system has been
developed that consists of an enclosure
housing a 405 nanometer (nm) 1 watt laser
Figure 1— In cave setups, 405nm violet
and 455nm blue laser scanners anking a
Nikon D810 camera with yellow tinted laser
blocking lter. Exposing dual wavelengths
and breaking the laser uorescence down
into a spectra can discriminate between
organic and inorganic luminescent centers
in the formations.

12 NSS NewS, July 2017
diode driven by a 12 volt battery. An internal
direct current motor allows lateral scanning
of the enclosure with a hand switch. A laser
line lens spreads the beam into a vertical line
and once scanned back and forth, exposes
the entire field of view during a time expo-
sure in total darkness (Fig. 1). As a third
generation device, the size and weight has
been minimized to less than 700 grams in a
13x11x6cm enclosure. The entire apparatus
is mounted via a pad on the motor axis to a
camera tripod. During exposure, the vertical
laser line is scanned left and right by a hand
switch controlling the scan motor. In this
fashion, the laser can illuminate and fluoresce
entire walls at great distance limited only by
the laser power.
While most modern DSLR cameras can
be used for this purpose, the Nikon D810
currently has the best picture resolution due
to the removal of the fringing filter in this
model. At 36 megapixels is also at the top
of the range for pixel count and works very
well in low light. 30 second time exposures
are usually employed to record images. Noise
Reduction is set ‘on’ so the camera imme-
diately takes a closed shutter image known
as a ‘dark frame’ that is subtracted from the
data image to greatly reduce noise.
Normally the laser light would over-
whelm the fluorescence, so a laser blocking
filter is used in front of the camera lens. For
general work, a long-pass 425nm filter is
mounted in a standard filter ring and screwed
to the front of the lens. Our team has devel-
oped quick release magnetic filter holders
to facilitate quick removal for a while light
image, followed by the laser scan.
Once the image is recorded, the RAW
camera files are processed in Photoshop
to balance the colors. Color equalization is
required in order to visually see the geochem-
ical color differences caused by the ions [8].
The second technique employs a trans-
mission grating in front of the lens with the
dispersed spectrum perpendicular to the
laser line. In this configuration the laser is
scanned while the DSLR is in movie mode.
The frames of the video can be processed
individually and a spectrum can be extracted
from each pixel. A second laser with a differ-
ent frequency is used over the same area to
detect spectrum shifts with wavelength that
are diagnostic.
Due to the inaccessibility of these
formations scale bars could not be included
in the images. The vast majority were shot
at a distance of 2-8 meters and the imaged
areas are generally a meter or more. Some
wide angle panoramas have been imaged
at over 30 meters recording the majority of
the cavern.
Results
The power of this
system lies in the fact
it can fluoresce vast
sections of caves in a
single image many tens
of meters away. Figure
2 shows a panorama
shot in complete dark-
ness at 20 meters. All of
the light that the image
recorded comes from
fluorescence. The violet
laser light is the only illu-
mination source and it is
blocked from the image
by a filter at the camera
lens. The laser is a much
more powerful source of
photons than a typical ultraviolet flashlight
or UV fluorescent bulb so it can reveal fluo-
rescence not seen with weaker light sources.
The first images from the panoramas
showed clearly that a large range of fluores-
cent colors exists even in tightly clustered
formations (Fig. 3). There are large scale
similarities throughout the examined caves
as expected, but localized irregularities
were intriguing. The simple explanation of
changes in groundwater as the source of
the colors is not easily explained in some
examples outlined below. Areas with clusters
of formations as small as 10-20 centimeters
can exhibit a wide range of colors. In this
study straws, stalactites, helictites and draper-
ies all exhibited multiple fluorescent colors.
Only rarely does one color exist at one end
of a formation cluster segmented from the
rest (Fig. 4). More commonly, especially with
soda straws and helictites, multiple colors
exist throughout the cluster (Fig. 5).
Comparing colors from cave to cave, all
the caves investigated within 30 kilometers
show the same basic combination of colors.
This is best exemplified by the light blue
soda straws that exist in every cave imaged
(Fig. 6). It was determined that the light
blue formations are the most recent based
on several lines of evidence such as draper-
ies with blue forming at the drip line (fig. 7).
The blue straws also appear to be in the best
condition with minimal red spots suggesting
they are the youngest.
Red spots and zones on the calcite
appear to have a different origin (Fig. 6).
They are found in isolated spots on the
formations and do not follow the typical
banding patterns that form through evapo-
ration. Subsequent lab investigations from
specimens collected under permit from the
Fig. 3—Total darkness, uorescence only, image exhibiting wide a range of colors. Area covered
is approximately 4 meters wide. Inset image is under natural white light. All the colors in this
image come from uorescence generated inside the formations. The laser used to stimulate the
uorescence is blocked by a lter so no laser light or outside illumination is present in the image
Fig. 2—Wide angle shot of cave wall 20 meters away. The yellow ball
at the top of the stalagmite is uorescent paint. The laser used to
stimulate the uorescence is blocked at the camera. All the colors
in the image come from uorescence, there is no incident light. 30
second time exposure in the dark, near UV laser.

NSS NewS, July 2017 13
cave floor, suggests they are the result of iron
oxidizing bacteria [9].
Discussion
Since fluorescence in speleothems
is caused by contamination of the crystal
lattice [10], it is in fact an instantaneous
geochemical fingerprint [8]. Since the same
fluorescent color could be caused by differ-
ent activators in the mineral, color by itself
can only tell similarities or differences in
the geochemistry. Since similar colors were
found throughout the study area, consistent
mechanisms must be at work. However, at
smaller scales of less than a meter, dramatic
differences in color are found suggesting
other mechanisms beside ground water are
incorporating fluorescent ions into the grow-
ing formations.
Soda straws under fluorescence provide
interesting data that are difficult to explain.
Blue, red and sometimes green straws are
typically found intermixed in a small zone.
This study found that the blue formations
were the most recent. This would lead to
the conclusion that the blue straws must
also be the “youngest” in the group. This
is contradicted by the fact that ALL the
straws in the groups studied were found to
be actively dripping and presumably all still
growing. In many cases the “older” straws
were similar lengths as the “younger” blue
straws which should not be the case if they
had longer time to grow (Fig. 6). This is an
area of active study for our group but we can
offer no hypotheses at this time.
Figure 8 shows a small cluster of helic-
tites with white fluorescing tips. All the
helictite groupings in this cave were imaged,
but only this group exhibited color banding
at the tips. This fact makes it highly unlikely
that the color change is due to groundwater,
but a different explanation is lacking. This
image represents the fact that many findings
from wide area fluorescent imaging have
no simple explanation and generate more
questions than answers. These helictites are
in a highly protected cave and will never
be sampled. No other helictites in any cave
studied showed these features so the authors
are interested in hearing from anyone else
finding similar helictites.
Yellow is often seen associated with
“carrot” formations (Fig. 9). They appear
to be overgrown straws but why they would
exhibit the yellow fluorescence by them-
selves is another mystery only revealed by
laser imaging. Yellow color in other types
of speleothems is dominant in some caves
but not in others. In our study area anec-
dotally the yellow fluorescence was typically
associated with the rarer and more unusual
formations.
Figure 10 shows a distinctive purple
color that is reported to be fungus. Anecdotal
reports had this fungus covering a large
portion of the room but subsequent laser
scanning shows this to be the only example
at this time. Cave microbiomes are an area
of expanding interest. This image highlights
the fact that unusual organic substances on
formations stand out under fluorescence.
Fig. 4—A rare example of a specic uorescent color showing up
at one end of a formation cluster. This study uncovered the blue
formations as the most rescent growth. Usually scattered within the
cluster, this image shows that the water ow to this area changed
over time and is most recently owing on the left side of the image
Fig. 6—Fluorescent light blue soda straws were found in all caves in
the study area. They are determined to be the most recent from color
succession in other formations. The red blotchy areas are under study.
Indications are they are episodic outbreaks of iron oxidizing bacteria.
Fig. 5. Helictites and soda straws showing an array of colors in the same cluster. Most of the
helictite clusters imaged showed different colors scattered throughout the group. The purple
in this image was unusual.

14 NSS NewS, July 2017
In order to attempt to solve these
apparent contradictions, chemical analysis
is required. Sampling formations requires
special permits but even then, in most cases
they are not easily accessible in cave ceilings,
off trail etc. The projected laser line can act
like the slit in a spectrograph. By adding
a dispersion grating to the camera lens, a
spectrum can be recorded for each pixel
along the laser line (Fig. 11). By scanning
the laser line across the cave while recording
video, a spectrum can be recorded for each
individual pixel.
By employing two frequencies of laser
in subsequent scans, it can be determined if
the luminescent centers in the formations
are being generated by organic or inorganic
molecules. If the spectral peaks shift with
the laser frequency they are organic, if they
do not, then they are inorganic [11]. The
dual laser system has been employed in the
hypogene Crystal Cave to analyze the lumi-
nescent centers and they were found to be
inorganic due to lack of spectral shift. Similar
formations scanned in Coronado cave
showed organic ions incorporated in those
speleothems. This is the current leading edge
for this research and under development is
software to automatically process the video
stream into a colored map based on the
generated spectral differences for each pixel.
Conclusions
Laser stimulated fluorescence for the
first time allows analysis of large scale
fluorescence in caves. In many ways it is a
solution looking for a problem. The initial
analysis of multiple caves in the SE Arizona
area shows large scale similarities as well as
local unique occurrences. While the laser
has the ability to reach remote and sensitive
locations in the caves without trespass, the
difficulties in sampling these cave formations
remain. The spectroscopic scanning offers
the most promise for broad analysis of these
untouchable formations.
The results from this initial study clearly
demonstrate there are many questions to
Fig. 7. Blue drip line indicates the light blue colors are the most
recent formations. The position of the blue zone at the edge of this
formation shows it is recent. The blue as the last to form was a
consistent nding in all the studied caves.
Fig. 8. Helictites showing differentiated uorescence at the tips.
This is a stand-alone group that shows white tips. Other groups in
the same cave did not show this color banding, whose formation
remains a mystery. Image area aprox. 30cm.
Fig. 10. Fungus showing
unusual purple color
in the center of image.
Anecdotal reports told
of a fungus growing in
this cave. This was the
only example found,
but demonstrates
that fluorescence can
differentiate unusual cave
biology.
Fig. 9 (left). Fluorescent
yellow colors were most
prevalent in the carrot
formations seen here in
the center of the gure.
Yellows were typically rare
but usually found in these
types of formations. They
could be found directly
alongside contrasting
color soda straws. 30
second exposure in total
darkness 405nm laser
stimulation.

NSS NewS, July 2017 15
be answered. The unusual colors found
in certain localized areas defy a simple
explanation. Bacteria has been known to
affect some formations such as moon milk
[12], but bacteria may have larger more
significant effects in the formation of other
speleothems. Spectral analysis and labora-
tory testing will be the next phase in the
study. In future work we hope to examine
ancient cave art which could benefit from
long distance multispectral analysis. This
technique requires us to think about fluores-
cence in a new way to expose novel data on
speleothems.
Acknowledgments
Peter Lipa, Steve Willsey, Stephanie
Kangas, Peter and Debra Ceravolo, Ellen
Preiss, Brenda Hanes, Erika Way, Peter
Kane, John Maier, Carla Kanak, Nicole
Davis, Brian McMillan, Bob Zimmerman,
National Park Service.
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Fig. 11. The blue line above is the laser on the wall of a cave. If one were to hold a prism up to
your eye, you would see the blue line and a rainbow spectrum coming off, which is in effect
what is happening here. With a transmission grating in front of the camera lens, a uorescent
spectrum can be recorded off the laser line on the cave wall. In this way spectra can be recorded
for every pixel along the laser line while it scans the wall. This allows for stand-off spectral
analysis of speleothems.