Dublin Institute of Technology
ARROW@DIT
Articles
School of Physics
1-1-2008
A visual indication of environmental humidity
using a color changing hologram recorded in a self developing photopolymer.
Izabela Naydenova
Dublin Institute of Technology, izabela.naydenova@dit.ie
Raghavendra Jallapuram
Dublin Institute of Technology, j.raghavendra@dit.ie
Vincent Toal
Dublin Institute of Technology, vincent.toal@dit.ie
Suzanne Martin
Dublin Institute of Technology, suzanne.martin@dit.ie
Recommended Citation
Naydenova, Izabela; Jallapuram, Raghavendra; Toal, Vincent; and Martin, Suzanne, "A visual indication of environmental humidity
using a color changing hologram recorded in a self -developing photopolymer." (2008). Articles. Paper 3.
http://arrow.dit.ie/scschphyart/3
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APPLIED PHYSICS LETTERS 92, 031109 共2008兲
A visual indication of environmental humidity using a color changing
hologram recorded in a self-developing photopolymer
Izabela Naydenova,a兲 Raghavendra Jallapuram, Vincent Toal, and Suzanne Martin
Centre for Industrial and Engineering Optics, School of Physics, Dublin Institute of Technology, Kevin Street,
Dublin 8, Ireland
共Received 15 November 2007; accepted 21 December 2007; published online 23 January 2008兲
A reflection hologram for visual indication of environmental humidity has been studied. The
hologram is recorded in a self-developing photopolymer and changes color when exposed to a
change in humidity and is fully reversible. The range of color change, reversibility, and the response
time of the hologram have been studied in a controlled humidity environment. Fully reversible
holograms with response times from few seconds to tens of minutes have been designed. Extremely
sensitive bright visual humidity indicators, capable of dramatic color change within a few seconds
of breathing on them are demonstrated. © 2008 American Institute of Physics.
关DOI: 10.1063/1.2837454兴
Relative humidity 共RH兲 is of special interest for many
industrial and domestic applications. In order to improve
product quality, monitoring of the relative humidity during
many industrial processes is required. Usually, the relative
humidity is measured and controlled by humidity meters that
detect change in a physical property of a thin film, such as
capacitance, resistivity or thermal conductivity. These humidity control systems require a power source, and are relatively complex and expensive. It is also often important to
monitor relative humidity during product transport and storage utilizing simple and inexpensive indicator devices incorporated into packaging. An example of such a device is a
card containing spots of silica gel that indicate a change in
relative humidity by a color change. However, they often
have a slow response time and limited accuracy. Disposable
holographic sensors that change their properties when exposed to different environmental conditions could provide an
easily interpreted, visual response with an added advantage
of faster response time and better accuracy.
The sensitivity of some holographic recording materials
to humidity has been known for many years.1–3 In silver
halide materials for instance it has been used to achieve color
control in reflection holograms.1 Special care to avoid exposure to high humidity is required for other materials2,3 as it
destroys the recorded hologram. The design of a hologrambased humidity sensor requires a unique combination of
properties of the holographic recording material—high sensitivity to humidity and accompanying resistance to damage
by humidity. To achieve this combination of properties appears to be very challenging as no successful fabrication of
such a holographic device has been reported so far. A humidity sensor using a wavelength-dependent holographic filter
with fiber optic links was reported by Spooncer et al. in Ref.
4. The sensor medium, gelatin, was characterized by a strong
hysteresis in humidity response, temperature dependence,
and variable sensitivity and reported as unsuitable for use as
a sensor. Although not designed specifically for the detection
of humidity, sensors operating on a similar principle have
also been demonstrated in Ref. 5. In the present paper, to the
best of our knowledge we report for the first time the suca兲
Electronic mail: izabela.naydenova@dit.ie.
cessful fabrication and characterization of a visible, highly
sensitive, and fully reversible humidity sensitive reflection
hologram recorded in a self-processing photopolymer.
In general, a hologram is produced in a photosensitive
material by exposure to an interference pattern produced by
two coherent light beams. The spatial variation in light intensity or state of polarization is recorded in the material as a
variation in refractive index, absorption or thickness. In the
most general case, the recorded pattern is a diffraction grating that produces diffracted light when illuminated with light
of appropriate spectral or polarization characteristics and in
the appropriate direction. The properties of the diffracted
light—intensity, wavelength, phase or state of polarization
depend on the properties of the recorded diffraction pattern
and are useful for the design of holographic sensors. A
significant advantage of such a sensor would be the possibility of a visual indication when exposed to the environment to
which it is sensitive. The availability of a large variety of
holograms and holographic materials makes the design of
holographic sensors a very flexible process. The present
paper focuses on the humidity sensing properties of a reflection phase hologram recorded in an acrylamide-based photopolymer.
Figure 1 illustrates the basic principle of the humidity
sensitive hologram. The high spectral selectivity of the reflection holograms6 means that light of a specific color is
diffracted when the hologram is illuminated with white light.
The color observed depends on the holographic fringe spacing ⌳ given by
FIG. 1. 共Color online兲 Principle of operation of the humidity sensitive reflection hologram.
0003-6951/2008/92共3兲/031109/3/$23.00
92, 031109-1
© 2008 American Institute of Physics
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031109-2
Naydenova et al.
⌳ = /共2n0 sin 兲,
Appl. Phys. Lett. 92, 031109 共2008兲
共1兲
where is half of the angle between the recording beams
inside the recording medium, is wavelength of the recording light and n0 is the refractive index of the recording material. According to Eq. 共1兲, there are two possible mechanisms for change in the reconstructed or diffracted
wavelength 共and, thus, in the observed color兲 for a hologram
that is disposed in a medium that can easily absorb or release
water as the relative humidity of the environment changes.
Firstly, dimensional changes of the recording medium occur
due to the ease with which the material swells or shrinks,
leading to change in the fringe spacing. Secondly, if there is
a change in the overall refractive index of the medium due to
the absorption of moisture, the optical path length between
recorded fringes will change. The relative contributions of
the two mechanisms will depend mainly on the properties of
the recording medium. The ultimate effect is a change of the
observed color of the diffracted light as a result of change of
the environmental humidity.
The material used in this study is a self-processing
acrylamide-based photopolymer developed at the Centre for
Industrial and Engineering Optics, Dublin Institute of
Technology.7–10 The humidity indicators were fabricated using an optimized photopolymer composition for recording
in reflection mode11 containing 0.8 g acrylamide, 0.25 g
N , N⬘methylene bisacrylamide, 1.5 ml triethanolamine,
17.5 ml of 10% w / v polyvinylalcohol stock solution, and
3 ml of 0.11% w / v of Erythrosin B stock solution. Layers
with thicknesses of 30– 180 m were obtained by varying
the volume of the photopolymer solution gravity settled on a
50⫻ 50 mm2 glass slide between 0.5 and 2 ml. The samples
were dried for 24 h. The frequency doubled output from a
NdYVO4 laser was used to record the reflection holograms
at 532 nm wavelength using a standard two beam geometry,
allowing for control and equalization of the intensities of the
two beams. The total exposure required to achieve 30% diffraction efficiency did not exceed 100 mJ/ cm2. Humidity indicators incorporating a holographic image of an object were
recorded as Denisyuk type holograms.
A controlled environment chamber with humidity control system 共Electro-Tech systems model 503-20兲 was used
to characterize the reflection holograms. The relative humidity in the chamber could be maintained at a set point in the
range of 5%–100% RH with accuracy better than ⫾1% RH.
The optical test setup was assembled inside the humidity
chamber. Light from an AvaLight-HAL-S light source was
fiber guided into the humidity chamber and then collimated
and used to probe the holograms. The diffracted light from
the hologram was coupled into a second fiber by a lens and
guided to a spectral analyzer, AvaSpec-2048. To obtain the
calibration curve for a specific hologram, the relationship
between the wavelength peak in the spectral response of the
hologram and the relative humidity in the chamber was determined. The response time of the hologram was measured
by recording the change of the spectral peak position with
time after achieving a preset humidity. The change in the
peak position during the time needed to achieve the preset
humidity for layers with thickness above 30 m was negligible. The response time was also studied by recording the
holographic images with a digital camera Sony Cybershot
DSC300 at different times after the change in the relative
humidity was introduced.
FIG. 2. 共Color online兲 Spectral response of a hologram recorded in a 30 m
thick layer 共squares兲 and theoretical fit 共red line兲.
The typical spectral response of a reflection holographic
grating recorded in a 30⫾ 3 m thick photopolymer layer is
presented in Fig. 2. The thickness of the hologram, obtained
by fit of the experimental data with the well known
function12 describing the wavelength selectivity of a reflection hologram, was 28 m, in good agreement with the results from independent measurement of the layer thickness.
As expected such a holographic optical element6,12 is highly
selective with respect to the wavelength of the diffracted
light. The full width at the half maximum is 2.8 nm. This
explains monochromatic appearance of the reconstructed image when the recorded object is relatively flat 共Fig. 3兲 and
there is a limited range of spatial frequencies in the recorded
hologram.
To calibrate the humidity response of the hologram, the
change of the spectral peak position was determined for different relative humidities. A characteristic example of the
humidity response of a reflection holographic grating recorded in 30 m layer is shown in Fig. 4. It can be seen that
when the relative humidity changes from 5% to 80% at
23 ° C the position of the peak in the hologram’s spectral
response increases by 130 nm. To identify the mechanism
causing this effect, one can differentiate Eq. 共1兲 and, assuming that the average refractive index is unchanged, one can
calculate that the observed change in the reconstructed wavelength can be produced by approximately 10% change in the
fringe spacing and consequently in the volume of the hologram. Such a change in volume can be easily expected taking
into account the highly hygroscopic nature of the polyvinyl
alcohol binder and the polyacrylamide formed during the
recording of the hologram. Alternatively using the same
equation and, assuming that the fringe spacing remains constant, the change in the average refractive index required to
cause a 130 nm shift in the diffracted wavelength is in order
of 0.3–0.4. This value is extremely high and suggests that the
first mechanism is mainly responsible for the observed color
change. Further studies using a white light interferometer to
FIG. 3. 共Color online兲 Color appearance of a hologram exposed to different
humidity levels.
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031109-3
Naydenova et al.
Appl. Phys. Lett. 92, 031109 共2008兲
FIG. 5. 共Color online兲 Color appearance of a reflection hologram before and
after breathing on it.
FIG. 4. Humidity response of a reflection hologram recorded in 30 m
thick photopolymer layer.
measure the dimensional change will determine the exact
contribution of each process.
Initial studies in a controlled humidity and temperature
environment show no substantial temperature dependence of
the response of the hologram. When the temperature changes
from 23 to 50 ° C at constant humidity of 30%, the spectral
peak shifts by 1.6 nm. More detailed studies on the temperature dependence of the humidity response are in progress.
In order to study the reversibility of the humidity response, the hologram was exposed to seven cycles of change
in relative humidity, from 20% to 80% RH. The position of
the spectral peak was repeatable within 1 nm, a change in the
visual appearance of the hologram that is too small to be
detected. Further studies involving larger numbers of cycles
are underway.
The response times of samples recorded in photopolymer layers with different thicknesses were also studied. It
was observed that the thickness of the layers has a significant
influence on the time required to achieve the final color appearance at specific relative humidity. For 30 m thick layer,
it took 10 min to reach a fixed color when the humidity was
changed from 50% to 60% RH and for the 90 m thick layer
as long as 50 min was necessary. After optimization of the
thickness of the recording layer, a response time of as fast as
a few seconds was achieved. Pictures from a very sensitive
hologram are shown in Fig. 5. The hologram changes color
in less than 30 s after breathing on it and the initial color
appearance is restored within 5 min.
In conclusion, we have successfully recorded a high diffraction efficiency, photopolymer based, humidity sensitive,
reflection hologram, which changes color when exposed to
different relative humidities, mainly due to swelling or
shrinkage of the photopolymer medium. The reversibility,
low dependence on temperature, and fast response times are
very promising for the design of humidity indicators providing easy to interpret visual information. The response times
can be controlled by the thickness of the photopolymer layer
used to record the holographic humidity indicator.
The support of this research by Enterprise Ireland
through its Commercialisation Fund, Technology Development phase is greatly appreciated.
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