Advances in Chemical Engi neering and Science , 2011, 1, 224-230
doi:10.4236/aces.2011.14032 Published Online October 2011 (
Copyright © 2011 SciRes. ACES
Alternative UV Sensors Based on Color-Changeable
Martina Vikova, Michal Vik
Departmentof Textile Chemistry, Technical University of Liberec, Liberec, Czech Republic
E-mail: {martina.vikova, michal.vik}
Recieved August 16, 2011; revised September 7, 2011; accepted September 20, 2011
Photochromism is a chemical process in which a compound undergoes a reversible change between two
states having separate absorption spectra, i.e. different color [1]. In our previous work we have published
some solutions of problems of measuring photochromic textile sample by standard commercial spectropho-
tometric systems [2]. Main problem with measurement of kinetic behavior of photochromic pigments by
standard spectrophotometer is relatively long time period between individual measurements (5 s) and impos-
sibility of measuring whole color change during exposure without interruption of illumination of sample
during measurement. It means, standard commercial spectrophotometers enable off-line measurement of ki-
netic behavior during exposure period and quasi on-line measurement during reversion period. Based on this
problem, it is only possible to obtain precise data during reversion—decay process and growth process (ex-
posure) is affected by high variability of data. Following this knowledge, we developed original experiment-
tal system with short time scanning of color change of photochromic samples during growth and decay pe-
riod of color change. In this study it is presented new view on the relationship between intensity of UV-A
radiation and color change half-life t1/2. Via this relation, it is demonstrated the possibility of the flexible tex-
tile-based sensors construction in the area of the radiation intensity identification.
Keywords: UV Radiation, Photochromism, Spectrophotometer, Textile, Sensor
1. Introduction
It is well known, that ozone depletion in the earth’s at-
mosphere has made the headlines on many occasions and
most people would be aware of the significant problem
that exists. Sunburn, skin cancer, premature aging, and
suppression of the immune system are some of the
harmful effects of acute and cumulative exposure to ul-
traviolet radiation (UVR). A decrease of 1% in ozone
would lead to increases in the solar UVR at the earth’s
surface and may eventually lead to a 2.3% increase in
skin cancer. Preventing the damages of UV-rays caused
by exposure to sunlight is difficult. Most of the health
problems due to UV-rays are not visible on a short issue,
and so, there is a feeling that there is no risk, except
sunburns by staying for a long time in the sun [3-5]. An
easy to understand and use UV-sensor would help people
know what exact protection they need according to the
amount of UV they are exposed to.
The idea of this work is to create a two part flexible
UV-sensitive measurement device. One part would be
the UV-sensitive area, changing color when exposed to
sunlight, which contains in its spectrum the wavelengths
of UV-rays, and a colored scale (4 or 5 steps scale), cor-
responding to different amounts of UV-intensity. When
exposed to the sunlight, the UV-sensitive part would
change color. By comparison with the stable part—the
scale—it would indicate the user the amount of UV-ra-
diations he is exposed to in the form of health protection
advices, letting him know when some extra protection is
required (wearing a t-shirt, put sunscreen on...) or if the
sun exposure should simply be stopped after a certain
time in the sun.
Color-changeable part of UV textile sensor is based on
application of the photochromic pigments. “Photochro-
mism is a reversible transformation of a chemical species
induced in one or both directions by absorption of elec-
tromagnetic radiation between two forms, A and B, hav-
ing different absorption spectra” [6-8].
Figure 1 shows that the thermodynamically stable form
A is transformed by irradiation into form B. The back reac-
tion can occur thermally (Photochromism of type T) or
Figure 1. Difference in absorption spectra between two
forms of photochromic specie.
photo chemically (Photochromism of type P). The most
prevalent organic photochromic systems involve unimo-
lecular reactions: the most common photochromic mole-
cules have a colorless or pale yellow form A and a colored
form B (e.g., yellow, red, violet, green or blue). This phe-
nomenon is referred to as positive photochromism [9].
Other systems are bimolecular, such as those involving
photocycloaddition reactions. When λmax(A) > λmax(B),
photochromism is negative or inverse.
The unimolecular processes are encountered, for exam-
ple, with spiropyrans, a family of molecules that has been
studied extensively [10-22]. Solid photochromic spiro-
pyrans or solutions (in ethanol, toluene, ether, ketone, es-
ters, etc.) are colorless or weakly colored. Upon UV irra-
diation, they become colored. The colored solutions fade
thermally to their original state; in many cases, they can
also be decolorized (bleached) by visible light.
In general, the photochromic processes involve a one-
photon mechanism [23,24]. B is formed from the singlet
(1A*) or triplet (3A*) excited states or both. B, the photo-
product, may also be formed from an upper excited state
populated by absorption of two photons.
Figure 2 describes two-photon photochromism [25-27].
The transition probability to populate the final state (hence
to obtain the photoproduct) depends on the product of the
photon irradiances Ep(1) and Ep(2) of the two excit
Figure 2. Stepwise two-photon photoc hr omic reaction.
ing beams. It is, therefore, advantageous to utilize lasers
emitting high photon irradiance, such as those generating
picoseconds or sub-picoseconds pulses. Two absorption
processes may be distinguished:
Simultaneous absorption of two photons via a virtual
Stepwise (or sequential) two-photon absorption where
the second photon absorption takes place from a real
The determination of the photochromic parameters, such
as the number, nature, and kinetic and spectral properties of
the transient species formed under irradiation is not a trivial
task because the photoproducts are too labile to be isolated
in many cases [28]. As an illustration, the kinetic behavior
of the unimolecular systems is considered (e.g., spiro-
pyrans, spiroxazines, dihydroindolizines, which are of ma-
jor importance for applications to ophthalmic lenses) [29].
Under continuous monochromatic irradiation, a photo-
chromic system can be considered to be at no equilibrium
and open. The evolution of the concentrations of the react-
ing species (starting compounds, photo isomers, and deg-
radation products) can be described by an appropriate set of
differential equations. The only simplifying hypothesis that
is used for their establishment is that the well-stirred mix-
ture obeys Beer’s law; nevertheless in color science is ob-
viously used Kubelka-Munk function, because measured
media are turbid [30,31]. Kubelka-Munk function, as well
as Beer’s law, is monochromatic. For description of kinetic
behavior of photochromic samples we used first order ki-
netic model as is shown in following equations [32]:
()(0)( )( )
  (1)
K is the Absorption Coefficient the limiting fraction of
absorption of light energy per unit thickness, as thickness
becomes very small.
S is the Scattering Coefficient the limiting fraction of
light energy scattered backwards, subscript defines time
relationship: (t)—actual time, (0)—time on the beginnin-
gand, ()—time in infinity.
k is rate constant and t time.
In Figur e 3, the main levels of shade intensity (I0, I, I1/2)
are shown. Shade intensity I is calculated from Equation
Special attention is given to the half shade intensity I1/2,
because this value is related to the half-life of the photo-
chromic reaction, the half-life of photochromic color
change t1/2. The half-life of photochromic color change t1/2
is a measured of the rate of the color change and is calcu-
lated based on Equation (3):
Copyright © 2011 SciRes. ACES
Figure 3. Evolution of reflectance under continuous irra-
diation and decay of reflectance when irradiance is stopped.
ln 2
2. Materials and Methods
Main task of our research work is to develop a simple
textile sensor, which is sensitive to UV light and kinetic
study of behaviour and main attention was given to pos-
sibility use the technology of textile printing by screen
printing-PTP. For experiment were used five commer-
cial photochromic pigments P1-P5. Examples of chemi-
cal structure are given in Figures 4-8.
Figure 4. 3,3,5,6-Tetramethy l-1-propylspiro [i ndoline-2,3'[3H]
pyrido [3,2-f][1,4]benzoxazine] (P1).
Figure 5. Methyl 2,2,6-tris(4-methoxyphenyl)-9-methoxy-
2H-naphtho-[1,2-b]pyran-5-carboxylate (P2).
Figure 6. Methyl 2,2-bis(4-methoxyphenyl)-6-acetoxy-2H-
naphtho-[1,2-b]pyran-5-carboxylate (P3).
Figure 7. 1,3,3,5,6-Pentamethyl(indoline-2,3’-[3H] naphtho
[2,1-b] [1,4] oxazine) (P4).
Figure 8. 3,3-Diphenyl-3H-naphtho[2,1-b]pyran(P5).
In our previous work we have published some solu-
tions of problems of measuring photochromic textile
sample by standard spectrophotometer system [33]. Main
problem with the measurement of kinetic behavior of
photochromic pigments by standard spectrophotometer is
relative long time period between individual measure-
ments (5 s) and impossibility of the measurement whole
color change during exposure without interruption of
illumination of sample during measurement. A method
where the sample is exposed in one place and after ex-
posure moved to another place for measurement of actual
color, causes a time delay that is dependent on move-
ment velocity between the place of exposure and the
place of measurement. In such methodology, the results
show poor reproducibility of measurement of the expo-
sure phase of the photochromic color change. That
means, the standard commercial spectrophotometers en-
able off-line measurement of kinetic behavior during
exposure period and quasi on-line measurement during
Copyright © 2011 SciRes. ACES
reversion period. Result is the exposure phase data varia-
tion in comparison to small variation of decay phase data,
as is shown on Figure 9. Following this knowledge was
developed the original on-line experimental system with
short time scanning of color change of photochromic
samples during growth and decay period of color change.
The basic difference between the “LCAM-Photochrom
3” system and a standard spectrophotometer is in the
continuous illumination of the sample against a flash
source that is used as standard. In such a measuring sys-
tem, it is possible to use a flash discharge lamp. The re-
flectance values obtained are independent of whether it is
Xenon, Mercury or other strobe source. This construction
allows measurement of the color change of the sample in
fast scans without disturbing the drift of the light source.
The system developed allows 5 ms intervals between
each measuring scan. Nevertheless, it was found that 5
seconds was a sufficient interval for the tested samples.
3. Results and Discussion
We have described in previous paragraphs differences
between commercial and our experimental measuring
system. Figure 9 shows our best results for pigment P1,
which we obtained from commercial system when we
used special conditions for minimizing of time delay
between excitation illumination and measurement. We
have obtained individual experimental points during ex-
position period—growth process of color change inten-
sity via individual exposure of each point. You can see
differences between experimental points and model of
exposure. As we mentioned, this graph documents best
results for adjusted commercial system, the discrepancies
was obviously between model and experimental points 3
- 5 times higher, except for time consumption of whole
experiment and pure reproducibility (variation 23% -
44%)—mainly in dynamic phase of kinetic curve.
Figure 9. Off-line measurement of growth and decay proc-
esses of colour change intensity for pigment P1, intensity of
illumination = 714.6 µWcm2.
On the other hand Figure 10 shows results from our
experimental system for same condition of illumination
intensity and pigment P1. It is evident that model of
experimental points fits better than from standard sys-
tem, aside from shorter time consumption (20 min
against 200 min).
Differences between absolute levels of Shade Inten-
sity I (integ K/S) from each measuring system are oc-
curring by differences between spectral ranges of used
systems and calculation. For standard spectropho-
tometer (SF300UV, Datacolor Int.) previous method of
color change intensity calculation based on integration
of K/S values per whole visual spectrum range of
wavelengths was used.
Main reason for which we made this experiment, it
is to design of simple UV textile based sensor. When
the relationship between the half-life of color change
t1/2 and the effect of pigment concentration is tested, the
result is a practically independent linear relation for the
exposure period of photochromic color change as
shown in Figure 11. This means that the time of color
change during exposure is not influenced by increased
concentration of the pigments (a virtually constant
value of t1/2).
It can be seen from Figure 12 that the rate of the
color change during the reversion phase is slightly de-
pendent on pigment concentration, mainly for the two
pigments with a slow rate of color change. Thus, it is
possible to conclude that the half-life of color change
is, for the exposure period of color change, practically
independent of concentration. On the other hand, for
the reversion period it can see that there is a decrease
of t1/2with increasing concentration.
We can see on Figure 12 that speed of color change
during reversion phase is slightly dependent on pig-
Figure 10. On-line measurement of growth and decay proc-
esses of colour change intensity for pigment P1, intensity of
illumination = 714.6 µWcm–2.
Copyright © 2011 SciRes. ACES
Figure 11. Dependence of Half-life of CCI on concentration
of pigments, intensity of illumination = 714.6 µWcm–2
Figure 12. Dependence of Half-life of CCI on concentration
of pigments, Intensity of illumination = 714.6 µWcm–2
ment concentration, mainly for two pigments with slow
speed of color change. It is possible to say that half-life
of color change is for exposure period practically in-
dependent on concentration; on the other side for rever-
sion period we can see decreasing of t1/2 with increasing
of concentration. One explanation, which may be pro-
posed, is that this relationship is based on a faster recon-
version to the original chemical structure of the photo-
chromic pigment at higher concentration (likewise light
fastness of dyestuff).
A strong negative linear relationship was found be-
tween half-life and irradiance for all photochromic pig-
ments. On the Figure 13 is shown, that linear increasing
of irradiation intensity affect linear decreasing half-life
of color change t1/2, aside from that have higher illumine-
tion intensity also on shade intensity alone. That means
higher illumination intensity is observed as deeper shade
of sensor.
Figure 13. Half-life of color change t1/2 relation on intensity
of UV-A radiation.
tested photochromic pigments. Based on this kinetic
model the construction of an optical yield Oy formula for
the reaction of photochromic textile was proposed:
 
ed ed
kt kt
 
 
As a result of experimental observation and the calcu-
lation of the dependence of optical yield Oy on UV ra-
diation intensity, it was found that the optical yield of the
photochromic reaction Oy depends linearly on the UV
radiation intensity, as is documented in Figure 14. This
correlation could be used for calibration of potential
Smart textile sensors with photochromic pigments ap-
plied as indicators of UV radiation intensity.
4. Conclusions
In our study we showed new aspects of the relationship
between UV radiation intensity, color change half-life of
exposition and half-life of relaxation, reversion respect-
tively. These relations demonstrate the possibility of
The kinetic study and experimental data sets obtained
clearly verify the first order kinetic model for the all Figure 14. Dependence of Oy on UV-A irradiance PPT-P1.
Copyright © 2011 SciRes. ACES
flexible textile-based sensors construction in the area of
radiation intensity identification. We demonstrate also
differences between photochromic pigments behavior
concerning to spectral sensitivity. First order exponential
functions, which are used in kinetic model calculation, fit
well the kinetics of color change intensity of photochro-
mic pigments. They give good fits to the growth curves
as well as to the relaxation one’s. Based on this result,
the possibility has been demonstrated in principle that
photochromic textiles can be usable for the preparation
of a sensorial system, which allows simple visual as-
sessment of the amount of UV radiation. This system can
be designed for example as a simple rule scale, where for
comparison there is a constant colored part made from
UV stable pigments or dyestuffs. Individual parts of the
constant scale can be judged on having the same color as
the photochromic part at a specific intensity of UV radia-
tion. The observer will be able to estimate the amount of
UV radiation corresponding to a color match between the
photochromic color changeable part and visual stable
part of the textile UV sensor.
5. Acknowledgements
We acknowledge support of the Technical University of
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