the artwork is exhibited or stored. The measurements carried out in the museum
environment require careful risk assessment to minimize the potential hazard of an
accidental damage of studied objects. The first mobile Raman instrument used by
Vandenabelle et al. [34] in the museum facility was a noncommercial fiber optic
Raman spectrometer adjusted to the analysis of the vulnerable art objects. Over the
years, the increasing number of the CH studies made a significant impact on the
development of commercially available Raman spectrometers. Recently, the portable Raman spectrometers and some handheld spectrometers allow to record
excellent quality Raman spectra comparable with laboratory dispersive Raman
microspectrometers. Jehlicka et al. [35] compared seven portable Raman spectrometers successfully applied to the study of 17 beryl specimens including gemstones: emeralds (eight samples plus two synthetic emeralds) and aquamarines
(three samples).
Regardless of where the Raman analyzes are performed (museum or laboratory)
and the size and type of the device used, the laser power and its irradiance (measured in W m
−2 ) have to be kept sufficiently low in order not to cause any damage.
Also, the time of the analysis should be as low as possible in order to reduce the
total absorbed dose of light. Moreover, very often the access to the objects is strictly
limited and analysis cannot be repeated so the quality of the spectra should be
efficient and has to deliver all the data required on the analyzed objects. During the
measurements carried out in the laboratory environment, it is possible to perform
analysis with the longer time of the acquisition and increase the spectral
signal-to-noise ratio. Also, the good wavelength stability and spectral calibration
are crucial. There are additional aspects that need to be considered in the analysis—
spectral range and spectral resolution used. Investigating inorganic materials, like
minerals (rock crystals, anatase, rutile) or mineral pigments (azurite, smalt, ocher),
it might be sufficient to record the bands at the low wave number in the spectral
range up to 2000 cm
−1 . But in analysis of paintings, the presence of m(CH)
stretching region at around 3000 cm
−1 might reveal the presence of some organic
substances, so recording of the higher spectral region of Raman spectra would be
beneficial.
For many cultural heritage studies, the lasers with low excitation power are
needed, so the special attention should be given to the choice of the laser wavelength adequate to the particular study. The Raman scattering intensity depends on
the laser wavelength (with the exponent 4), laser intensity and detector and optics
efficiency [13]. The most popular are green (514 nm, 532 nm) and red (785 nm)
lasers. It is important to remember that for the red laser usually the higher exciting
power is required (200–300 mW) and the green laser needs lower power (80–250
mW), thanks to the associated increase in the Raman effect and detector efficiency
[13]. The green laser usually is used in the investigations of inorganic materials like
minerals and gemstones, while the red laser is mostly applied in the studies of
organic materials like binding media, varnishes, or dyes.
The first reports presenting the application of the Raman microprobe in the
investigations of the painted manuscripts and colored archaeological materials [36]
and for the identification of azurite and malachite [37] were published by Guinea in
486
T. Łojewski and B. Łydżba-Kopczyńska
environment require careful risk assessment to minimize the potential hazard of an
accidental damage of studied objects. The first mobile Raman instrument used by
Vandenabelle et al. [34] in the museum facility was a noncommercial fiber optic
Raman spectrometer adjusted to the analysis of the vulnerable art objects. Over the
years, the increasing number of the CH studies made a significant impact on the
development of commercially available Raman spectrometers. Recently, the portable Raman spectrometers and some handheld spectrometers allow to record
excellent quality Raman spectra comparable with laboratory dispersive Raman
microspectrometers. Jehlicka et al. [35] compared seven portable Raman spectrometers successfully applied to the study of 17 beryl specimens including gemstones: emeralds (eight samples plus two synthetic emeralds) and aquamarines
(three samples).
Regardless of where the Raman analyzes are performed (museum or laboratory)
and the size and type of the device used, the laser power and its irradiance (measured in W m
−2 ) have to be kept sufficiently low in order not to cause any damage.
Also, the time of the analysis should be as low as possible in order to reduce the
total absorbed dose of light. Moreover, very often the access to the objects is strictly
limited and analysis cannot be repeated so the quality of the spectra should be
efficient and has to deliver all the data required on the analyzed objects. During the
measurements carried out in the laboratory environment, it is possible to perform
analysis with the longer time of the acquisition and increase the spectral
signal-to-noise ratio. Also, the good wavelength stability and spectral calibration
are crucial. There are additional aspects that need to be considered in the analysis—
spectral range and spectral resolution used. Investigating inorganic materials, like
minerals (rock crystals, anatase, rutile) or mineral pigments (azurite, smalt, ocher),
it might be sufficient to record the bands at the low wave number in the spectral
range up to 2000 cm
−1 . But in analysis of paintings, the presence of m(CH)
stretching region at around 3000 cm
−1 might reveal the presence of some organic
substances, so recording of the higher spectral region of Raman spectra would be
beneficial.
For many cultural heritage studies, the lasers with low excitation power are
needed, so the special attention should be given to the choice of the laser wavelength adequate to the particular study. The Raman scattering intensity depends on
the laser wavelength (with the exponent 4), laser intensity and detector and optics
efficiency [13]. The most popular are green (514 nm, 532 nm) and red (785 nm)
lasers. It is important to remember that for the red laser usually the higher exciting
power is required (200–300 mW) and the green laser needs lower power (80–250
mW), thanks to the associated increase in the Raman effect and detector efficiency
[13]. The green laser usually is used in the investigations of inorganic materials like
minerals and gemstones, while the red laser is mostly applied in the studies of
organic materials like binding media, varnishes, or dyes.
The first reports presenting the application of the Raman microprobe in the
investigations of the painted manuscripts and colored archaeological materials [36]
and for the identification of azurite and malachite [37] were published by Guinea in
486
T. Łojewski and B. Łydżba-Kopczyńska