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Chapter 1.1 Laser - Fundamentals Alessandra Andreoni1,
Gerard Słiwinski2 1 Dipartimento di Fisica e Matematica,
Universita’ degli Studi dell’Insubria, Como, Italy 2 Photophysics & Laser Department, Contents 1.1.1
Introduction This chapter is devoted to description of the
various lasers quoted in this CD-booklet and used by conservators and
conservation scientists for different aims. In lasers, as much as in discharge lamps, the light is emitted by atoms, ions or molecules that decay from a high to a low energy level. The peculiarity of lasers is that this transition is stimulated by photons identical, in all features, to those that the excited material would emit spontaneously to loose the excess energy (Fig. 1.1.1). In other words, in the spontaneous emission process occurring in discharge lamps the excited atoms, ions or molecules release photons of different wavelengths and directions of propagation. In lasers, providing photons at one of these wavelengths and propagating at one specific direction ensures that the stimulated light will only be emitted at the same wavelength and along the same direction. This is the basic reason why the majority of lasers emit beam of monochromatic light.
Already at the beginning of the last century Plank and Einstein had discovered that the phenomenon of stimulated emission is the exact reverse of light absorption. In particular, as depicted in Fig. 1.1.2, photons with energy equal to the difference between the energies of the excited and low levels of the material, E2→E1 , have the same probability of being absorbed and promote the transition E1→E2 as of stimulating the emission of an identical photon and cause the decay E2→E1. Obviously, absorption diminishes the photon flux while stimulated emission increases it. If the latter phenomenon has to prevail, the majority of atoms, ions or molecules must be in the excited energy state rather than in the low-energy state, that is the photons must find the material in the situation that is called condition of population inversion (Fig. 1.1.2).
Since the population inversion depicted in
Fig. 1.1.2b corresponds to a rather unnatural situation, a
strong source of excitation (pumping mechanism) is always present in a
laser device to transform the material that would normally absorb light
(Fig. 1.1.2a), into one (active material) capable of
amplifying light by stimulated emission. Gaseous active materials, such
as N2 molecules in the Nitrogen laser, mixtures
of noble/halogen gases in the excimer lasers, Neon in the He-Ne laser
or Ar+ in the Ar-ion laser, are pumped by
electric currents; solid active materials like Nd+3
in the Yttrium Aluminum Garnet matrix (Nd:YAG laser) are optically
pumped by either intense Xe-lamps or light-emitting diodes. If one observes that stimulated emission tends to destroy the population inversion (since each event of stimulated emission creates a new photon but causes one decay to the low-energy state in the active material) it is not surprising that, in some lasers, the condition of population inversion cannot be indefinitely maintained. In general, the higher is the rate at which the photon flux density is amplified (high-gain lasers), the earlier absorption starts to prevail over stimulated emission and the laser material stops being an active material, that is the laser goes below threshold. Lasers in which the condition of population inversion can be fulfilled only for short times are called self-terminating lasers. Typical examples are the Nitrogen and the excimer lasers, which cannot be operated as continuous wave (cw) sources of light. They can only emit pulses with duration of up to few nanoseconds and very high peak power. The fact that a high-gain laser goes quickly below threshold is deliberately exploited in Q-switched lasers. For instance, Nd lasers are not self-terminating lasers, so much so that cw Nd lasers exist, but by suitable Q-switching techniques (see section 1.1.4) their gain can be made so high that they behave as self-terminating lasers and emit pulses of several ten nanoseconds duration and tenths-Gigawatt peak power. In conclusion, as long as the active material
of a laser is in the condition of population inversion, the active
material emits “collimated photons” due to
stimulated emission besides the spontaneously emitted photons. The
latter are emitted with random directions of propagation. If the active
material is shaped so as it has similar sizes in all directions, the
stimulated photons are “collimated” along the
direction of the first stimulated transition that occurs in the active
material. But if the shape is elongated in one direction (i.e.
bars/rods for solid active materials, tubes for gaseous active
materials) the stimulated emission is maximally favoured along
the long-size direction of the material. As depicted in
Fig. 1.1.3, this is due to the obvious fact that the
stimulating photons encounter more active material when they travel in
this direction.
In self-terminating lasers a single passage of the light in the active material is often enough to deplete the excited-state to such an extent that the condition of population inversion is not anymore fulfilled. When the rate of stimulated emission processes is not as high, it is convenient to further exploit the remaining population inversion by sending the photons back to the active material by means of a mirror, M1, located on the right-hand side in Fig. 1.1.4a and again by a second mirror, M2 in Fig. 1.1.4b. To work properly, the two mirrors must be perfectly aligned to each other and parallel to the direction of the active material rod or tube, for solid or gas lasers, respectively. In practice the only lasers having either no mirror or only one mirror are the self-terminating lasers, such as the Nitrogen laser and the excimer lasers that were mentioned above.
In the majority of cases, lasers have two
mirrors, mainly because one passage of stimulated emission causes
little numbers of decay E2→E1,
see Fig. 1.1.2b, in comparison with the pumping rates that are
achievable. Moreover, if the pumping process is so efficient to ensure
population inversion be indefinitely preserved, the laser with
two mirrors can operate for the time durationequal to that of the pump,
up to cw. Obviously, in a laser with two 100% reflecting mirrors, the
stimulated-emission photons would simply bounce back and forth between
the mirrors and the beam would be confined between them. To provide an
output beam, usually one of the two mirrors, whose ensemble is called
the “laser resonator”, is a partially transmitting
one and constitutes the “output mirror” of the
laser resonator, through which the laser beam can leave the resonator
as shown in Fig. 1.1.4c. It is customary to characterize the output of
cw lasers by their power, which is measured in W (Watt) and expresses
the energy delivered per unit time, while for pulsed lasers the most
significant quantity is the energy of the pulse, which is measured in J
(Joule). Since the power is, by definition, the energy delivered per
unit time, a laser pulse of energy E [J] and
duration τ [s] has an “instantaneous”
power P = E / τ [W],
where the word “instantaneous” means that our
pulsed laser gives the power of P [W] only for a
time interval of duration τ [s]. However for a pulsed laser operating at a given repetition rate, specified as the number of pulses per second, n, we may define an “average” power value <P> as the energy delivered in 1 s: <P> = n·E [W]. As an example we consider a Nd:YAG laser (Q-switched and amplified) with pulse energy E = 800 mJ, pulse duration τ = 12 ns operating at the repetition rate of 10 pulse per second, i.e. n = 10 [Hz or s-1]. The “instantaneous” power is P = E / τ = 800 mJ /12 ns = 800·10-3J /12·10-9s ≈ 67·106 W= 67 MW (megawatt). The “average” power is <P> = n·E = 10 s-1 800 mJ = 10 s-1 0.8 J = 8 W. The “instantaneous” power is often called peak power. It is obvious that for cw lasers we have only one definition of power: for instance a 4-W cw Ar-ion laser simply delivers energy equal to 4 J every second. Thus to bring to a target an energy of, say, 20 J we need an exposure of 5 s (= 20 J / 4 W) with this laser and of 2.5 s with the previous Nd:YAG laser (2.5 s = 20 J/ 8 W).
In many applications of lasers, including those in conservation, it is important to control the energy (power) arriving on a given area of the sample to be treated/investigated. To be quantitative on these points, we define two specific quantities, namely the fluence, which expresses the energy per unit area and is measured in J/cm2, and the intensity, which expresses the power per unit area and is measured in W/cm2. Besides the fact that a laser beam is only ideally a non-diverging light, it must be pointed out that both fluence and intensity received by a sample upon delivery of a given beam, either continuous wave or pulsed, strongly depend on the angle of incidence. The examples in Fig. 1.1.5 are illuminating. Suppose that a pulse from the Nd:YAG laser (Q-switched and amplified) of above, with pulse energy E = 800 mJ and peak power 67 MW, is shaped into a beam of 2 mm2 cross-section. It makes a (round) spot of 2 mm2 area onto a sample that lies normal to the beam, but an oval spot of double area if it hits the sample surface under a 30 deg angle. Hence it delivers a fluence F = 800·10-3J / 0.02 cm2 = 40 J/ cm2 in the former case, but a fluence F = 20 J/ cm2 in the latter. Similarly for the intensity: I = 67 MW/ 0.02 cm2 = 3350 MW/ cm2 and I = 67 MW/ 0.04 cm2 = 1675 MW/ cm2 for the sample perpendicular and inclined as in Fig. 1.1.5. For a cw source, such as the mentioned Ar-ion laser of 4-W power and say 2 mm2 beam cross-section, the problem concerns the intensity, whose values are I = 4 W/ 0.02 cm2 = 200 W/cm2 and I = 100 W/cm2 respectively for the sample perpendicular and inclined as in Fig. 1.1.5. The general rule to calculate F and I correctly is that energy and power must be divided by the spot area at the sample. Such area is given by the beam cross-section divided by the “sinus” of the angle evidenced in Fig. 1.1.5. For readers interested in completing the above
description by means of e-media, several internet sites offer
interactive education courses on laser fundamentals, e.g. http://www.mic-d.com/curriculum/lightandcolor/lasers.html
1.1.2
Why lasers in conservation? In the beginning of the third millennium and
after about half a century of the laser R&D, more than 10,000
laser transitions are known. A number of them have been used in
successful practical developments which resulted in a variety
of laser sources offered on the market. Nowadays, it is hard to imagine
a domain of the human activity without the use of lasers. Applications
in metrology (precise measurements and standards), research,
telecommunication and data transmission, industry (materials
processing, prototyping), medicine (surgery, dentistry,
bio-stimulation) and entertainment (high density optical read/write
audio and video devices) are established. The laser reading, copying
and pointing devices are in common use. The schematic overview of
recent laser applications is shown in Fig. 1.1.6.
Applications of lasers in the restoration of
artworks advanced through extensive research over past decades and
covered the laser ablation for surface cleaning and also very
sensitive, in most cases non-destructive diagnostics of materials and
their identification. The recent application fields are: - surface
cleaning, - removal of the over paintings and damaged/polluted varnish layers, - analysis of
pigment composition and original constituents, - analysis of the chemical and elemental
composition of the surface and underlying layers in order to: - non-destructive marking and holographic
micro-catching used as “labelling” of the works of
art. The use of the laser beam for the surface
cleaning and conservation represents a technique complementary
to these applied traditionally by conservators, and of capabilities
studied extensively since about thirty years when lasers became the
reliable radiation sources in laboratories. Numerous experiments, case studies, and
published works confirm that lasers can serve as very
efficient, non-contact processing tools in conservation
practice. They allow avoiding problems arising typically when
the chemical or mechanical methods are in use. The potential of laser interaction bases on the
fact, that each material absorbs photons (light) of a given energy. The
characteristic energy quantum of absorbed photon, which is equivalent
to the particular wavelength λ (absorption wavelength): E = h (c/λ), with h and c
being the Planck constant and the speed of light, respectively, depends
on the chemical composition of a given material (substance) and
represents its spectroscopic “fingerprint” in the
form of the absorption spectrum. The number of photons absorbed depends in
general on the illumination parameters such as light
intensity, angle of incidence, and conditions referred to the
surface. These conditions are characterized by a spectroscopic quantity
called absorbance which depends on the material, its roughness,
coverage by contaminants etc. It is worth mentioning that spectroscopic
data of most materials are known and are easy accessible via vendors of
the chemical data bases. The interaction of laser emitting a given
wavelength results in the characteristic response (absorption or
reflection) of the irradiated material. The laser beam parameters such
as focus diameter, its position and location can be selected and the
laser light can be guided to the processing area by an articulated arm
or the elastic fibre-guide very precisely. Moreover, for most lasers
the pulsed operation is possible, i.e. the energy can be delivered in
portions. In this manner a fully controllable interaction with the
object is assured. Important conclusions can be drawn from the above.
First, a choice of λ for the substance of interest assures the
interaction selectivity which is very advantageous in conservation
works. Moreover, the depth of the penetration, area of the action and
therefore the risk of any kind of damage is controlled.
1.1.3
Characteristics of the laser beam There are several important features of the
light extracted from lasers - obviously in the form of narrow beams -
which are distinctive when compared to other light sources: Monochromaticity - the resonant response of the lasing medium to
excitation and also selective amplification provided by the laser
cavity results in emission characterized by a very narrow spectral
range (dλ). This means that - except in special
cases - a laser emitting for example the green light, say
around λ = 540 nm does emit this light only, in
contrary to the previously mentioned quartz bulb lamp, which emits a
broad spectrum (200 nm < λ < several micrometers) covering the UV,
visible and near-infrared range (NIR), where the last one is
experienced as the pure heat radiation. High brightness - because of the same reasons mentioned above
and thanks to optimized excitation conditions, a large number of
atoms/molecules of the active medium is excited and stimulated to emit
at the same wavelength. For estimate the density of the medium of 1018
molecules per cm-3 and 1022
cm-3, and the laser efficiency differing from
20% to 0.3% can be considered, for the gas and solid state lasers,
respectively. The unusually high brightness of laser compared to
conventional light sources can be easily imagined if one considers the
rough number of quanta of 1014 emitted by a
laser starting to operate, whenever a blackbody heated up to 1000 K
emits about 10 photons only. Coherence - follows from the phase matched oscillations
of resonantly excited and emitting species. The temporal and spatial
coherence means that at the laser output mirror there is practically no
phase shift between individual photons. Consequently, the shorter the
beam path corresponding to a measurable phase shift the better
the quality of the laser source. The coherence of the laser beam can be
again compared to the absolutely incoherent light emitted by the bulb
lamp. Low divergence - results from the quality, alignment of
mirrors, mode content – i.e. homogeneity of oscillations
corresponding to laser emission, and finite apertures of the optical
resonator elements. However, all laser beams diverge and become larger
as they travel. The beam divergence Θ [rad] is given by simple relation: (Beam divergence) = (Constant) •
(Wavelength) / (Beam diameter) Typically, the value of Θ does not exceed 1-1.5 mrad which means that at
a distance of 1m the beam of a diameter of 6 mm will expand to 7-7.5
mm. A useful figure of merit that makes it possible
to compare the beam quality of different lasers is the product of beam
diameter at the laser output mirror times beam divergence. The beam
quality is important in numerous applications and together with the
value of decides on the focusing ability. It corresponds to the minimal
spot size of the laser beam which can be in practice as small as 3λ. In the applications to conservation of cultural
heritage the monochromaticity favours the selectivity of the laser
interaction for desired specific targets, both in diagnostic and
conservation procedures. The high brightness, which means, in practice,
high intensities in narrow minimally diverging beams, allows to achieve
high sensitivity in all analytical laser spectroscopy techniques that
are applied to diagnostics, e.g. LIF,
LIBS, Raman, also with remote sensing,
e.g. LIDAR. Moreover a high
brightness is a pre-requisite in all cleaning procedures based on
material ablation. Coherence is exploited in interferometric methods
used for documentation and diagnosis, such as holography and 3D
recording.
In dependence on the active medium which serves
for generation of the coherent beam, the commercially
available lasers of interest for conservation practice are: - gas lasers –
containing gases or gas mixtures as working medium, e.g. CO2/N2/He,
He-Ne, N2, or the halogen (F2,
Cl2) doped mixtures for production of excimers -
excited by electrical discharges; - solid-state
lasers, where the laser light is generated and amplified in the
mono-crystalline or amorphous, rare-earth ion doped
rods, optically pumped by intense light sources such as lamps or other
lasers; - diode
(semiconductor) lasers in which a direct conversion of the electrical
excitation energy into laser radiation takes place. In general, for a given laser material the
content of the laser active molecules is around 0.1%. This means that
at material densities of N = 1018
molecules/cm3 and 1022 cm-3
typical for the gas and condensed phase lasers, respectively, the
corresponding density of laser active molecules could be as high as 1015
cm-3 and 1019 cm-3
in both cases. These values are the maximum attainable of the
population inversion ΔN mentioned previously (Fig. 1.1.2) and being
simply a number equal to the difference in population of the
ground and excited levels characteristic for a given laser material.
Finally, taking into account all the parameters described above one can
estimate the laser output power extracted due to the excitation (i.e.
pumping) pulse using the relation
where T is the transmission
coefficient of the output mirror of laser resonator, ΔN – population inversion, t
- the laser pulse length closely related to the radiative lifetime of
the excited molecules, L – active
material length in resonator, and F – the
material cross-section; the product L·F
represents simply the volume of the active material. The active media
of the most representative gas and solid state lasers and their
emission wavelengths are shown in Fig. 1.1.7.
There are no doubts that some gas lasers such
as the He-Ne or CO2 ones are most mature and
well established on the market. However, during last decade a
considerable development in the field of solid state lasers and diode
lasers is observed. They become reliable, proven tools that compete on
an economic and application basis with the gas laser technologies. At
present, the Nd:YAG lasers, and also the excimer lasers are most
frequently applied for artwork conservation purposes. However, in the
initial phase of the research on artwork cleaning by laser the solid
state ruby lasers were successfully used, too. The achievements of laser technology and
optoelectronics as observed by users can be validated not only via
current broadening of the application field or the extension of the
emission wavelength range covered. The increasing peak power or laser
energy attainable in a single pulse represents the recent tendency to
study the short-range atomic and nuclear interactions experimentally.
This requires well localized selective excitation by extremely intense
sources of coherent radiation operating on the femtosecond and shorter
time scale. The development of lasers and optoelectronic devices
observed via the consecutively invented and applied pulse shortening
techniques is schematically shown in Fig. 1.1.8.
Due to the completely different interaction
effect of the femtosecond laser pulse compared to the micro- and
nanosecond one also the final results of some key processes require
more attention to be paid. For instance, the material ablation or
changes of its chemical structure can be markedly influenced
by the pulse width variation in the range of 6-9 orders of magnitude
when applying the ns and fs lasers for conservation and diagnostics of
historical objects, too. A short description of some laser sources
already investigated and recently applied for the conservation and
artwork diagnostics is given below.
The name excimer covers the electronically
excited species such as monomers, dimers and other complexes
which exist in the electronically excited state only. Excimers are
characterized by short radiative lifetimes of the order of nanoseconds
and large cross sections for stimulated emission which guarantee an
efficient laser operation. Consequently, the principle of excimer
lasers emitting mainly in the UV spectral range bases on electronic
transitions of such complexes as ArF (emission wavelength λ = 193 nm), KrF (248 nm), XeCl (308 nm) which
are created in gas mixtures under high voltage discharge excitation.
These mixtures contain originally small amounts of halides together
with a noble gas (Ar, Kr, or Xe) and a prevailing part of another inert
buffer gas. The entire laser unit consists of discharge chamber (gas
tube) in an optical resonator, HV charger, and the system serving for
pumping and mixing of gases. The electrical high voltage discharge is
transversal with respect to the length direction of the gas tube.
Therefore the output beam of an excimer laser normally has a
rectangular cross-section. The pulse energies range from several tens
of mJ up to 1 J for the powerful units at pulse repetition rates up to
about 100 Hz. The pulse duration is typically 10-15
ns. Applications of the excimer lasers in the
spectroscopic surface diagnostic, pigment analysis and
ablative laser cleaning of stone objects and also varnish
removal on paintings are well documented in the literature.
It is a laser structurally similar to an
excimer laser except for the fact that the active medium is a gas of
stable N2 molecules. In summary: λ = 337.1 nm (near-UV) Active material: gas of N2 molecules Pumping: electrical discharge across the laser tube Operation mode: pulsed (self-terminating) Pulse duration: τ ≈ 0.2 ns
to 10 ns, repetition rate: from single shot to 100 Hz Pulse energy: E ≈ 0.05 mJ to 10 mJ Applications: Due to its wavelength and to the
short pulse duration it is used as the excitation source for
time-resolved fluorescence analysis of materials (organics, such as
pigments). The high peak power available (i.e. typical instantaneous
power value P = 1 MW) allows time-resolved
fluorescence mapping (Chapter 4.4).
It is a gas laser in which the active species
is an Ar ion that is formed by the electrical discharge that also
provides the population inversion. The gain is much lower than in the
previously mentioned gas lasers. That is why the active material is
contained in long capillary tubes (typically 1 m length) between two
highly reflecting mirrors. In summary: λ = 514 nm (green) or 488 nm (blue) or 364 nm
and nm (near-UV) depending on the mirror spectral reflectance Active material: gas of Ar+ (ionized
noble gas, hence atomic) Pumping: electrical discharge along the laser tube Operation mode: cw Power: P ≈ 0.5 W to 10
W, depending on spectral lines, pump discharge power and tube length. Applications: The green line is used for
holography, all lines are
used for Raman (Chapter 4.1).
Note: A schematic of the beam generation in the Argon-ion laser device
is demonstrated schematically at http://www.mic-d.com/java/argonionlaser/index.html
The solid state Nd:YAG laser is of particular
interest. This laser operates at the fundamental wavelength of
1064 nm and by the use of additional modules for frequency conversion
in non-linear crystals also the second, third and fourth harmonic at
532 nm, 355 nm and 266 nm can be obtained, respectively.
Transmitting of the beam energy to the processing area is possible in a
convenient way by using the articulated arm or flexible fibre-guides.
The Nd:YAG laser can be operated both cw or pulsed, depending on the
pump. For the pulsed regime operation, which is the most relevant to
conservation, the repetition rates are easily controllable in the range
10-100 Hz or even up to several kHz in some recently developed units,
and pulse energies above 1 J can be routinely obtained at 1064 nm. The
duration of non-processed pulses (free-running regime) are in
the ms range, and shortening of the pulse duration
by three orders of magnitude, i.e. down to several nanoseconds, is
commonly achieved by means of the laser cavity Q-switching technique.
This kind of temporal modulation results in the formation of pulses
characterized by a fast leading edge: the strong and short initial peak
(several ns) containing of about 80% of the total pulse energy is
followed by a low-level pedestal of longer duration. As the
pulse energy is temporally compressed in such a
“giant” pulse, the Q-switched Nd:YAG laser emits
peak powers (instantaneous power) in the range of 100 MW - 1
GW. Its excellent beam quality allows obtaining intensity values up to
109 W/cm2 by means of
focussing. The Q-switched operation can be advantageous in cases where
large radiation intensities are applied locally. In summary: λ = 1064 nm (NIR) and harmonics (532 nm, 355 nm
and 266 nm) by frequency conversion. Active material: solid-state, rod/slab of YAG (Yttrium Aluminum Garnet) doped with Nd+3 ions. Pumping: optical by flash lamps or laser diodes (for
pulsed or cw). Operation mode: typically pulsed (ms duration if free running). With cavity
Q-switching: 10 ns typical duration. Pulse energy: typically about 1 J (at 1064 nm). Applications: The fundamental output is used for cleaning;
the harmonics for spectroscopy-based diagnostics. Note: The fundamental output is in the NIR
(near infrared), hence not visible. For this reason the Nd delivery
system, usually based on an optical fibre, also carries a red He-Ne
laser beam as the aiming beam.
The He-Ne laser is very similar to the Ar laser as to the construction. The emitting species, which undergoes population inversion, is Ne, while He is added to allow pumping. It is characterised by an excellent spatial coherence. In summary: λ = 632.8 nm (red) with possibility of other
emission lines depending on the mirror spectral reflectance Active material: gas of Ne (in the presence of He) contained in
a capillary tube. Pumping: electrical discharge along the laser tube or
by radiofrequencies. Operation mode: cw. Power: P ≈ 0.5 to 25
mW, depending on pump discharge power and tube length. Applications: aiming beam for NIR and UV lasers. Also used
for 3D reconstruction.
λ = 820 nm (NIR) and wavelengths nearby,
depending on semiconductor active material and the temperature. Active material: solid-state, p-n junction in a semiconductor. Pumping: electrical by an injected current both pulsed
and cw. Operation mode: typically pulsed with duration depending on the
current injected. Pulse energy and average powers are rather low. Applications: Laser Doppler
vibrometry and other interferometric techniques for the early
diagnosis of detachments from substrates, 3D reconstruction.
1.1.5
Is conservation by laser cost-effective? Expenses are one of the important factors of
the conservator’s intervention and application of laser
obviously makes this point even more meaningful despite the laser
represents an efficient, fully controllable and selective tool. For
this reason an example of the laser cleaning of sandstone monument is
briefly considered here and the expense factors of this laser
application are compared with their counterparts of the conventional
restoration technique. An investment based on the Nd:YAG, Q-switched
laser system commercially available on the European market is taken
into account. The overall costs of laser cleaning are assumed
to consist of three groups of components, i.e. the investment, personal
cost, and overheads of 30%. The amortisation period of 5 years is taken
into account and the personnel payment is assumed to be twice the
average local salary due to specific qualification required. The cleaning rate R of the
system is given by the time required for cleaning of the unit surface
– here 1 dm2, and its value
depends on laser parameters and on the encrustation to be removed with S being the spot area
in cm2 of the laser beam at optimal fluence, N
- the pulse number applied to surface S, and f
- pulse frequency in Hz; the numerical factor results from units of the
cleaning rate, which are minutes per dm2. Finally, by combining the surface cleaning data
from literature with parameters of the laser system such as the laser
fluence and pulse repetition rate, and under assumption of the
encrustation thickness of 0.1 mm the expenses of laser cleaning can be
estimated. Despite different unit prices depending on the system
applied the expenses of about 0.2 - 0. 5 € per dm2
of the laser-cleaned surface are lower by about an order of magnitude
than these related to the chemical and mechanical methods. Considering
different objects it may be than concluded that application of laser is
cost-effective particularly in case of cleaning of the sculptures,
bas-reliefs or complicated architecture details. Moreover, the usage of
laser markedly reduces time of the conservator’s work.
A. E. Siegman: Lasers. Univ. Science Books, Mill Valley, California,
1986. O. Svelto: Principles of Lasers. Plenum Press,
New York and London, 1998. Ch. K. Rhodes (Ed): Excimer lasers. Springer,
Berlin, 1991. F. P. Schaefer (Ed): Dye Lasers. Springer, Berlin, 1989. C. C. Davis: Lasers and Electrooptics. Cambridge Univ. Press, 1996.
1.1.6.2 Providers and Useful Websites COST G7 website pages:
Alessandra Andreoni Gerard
Słiwinski |
