We use terahertz time-domain spectroscopy combined with effective-medium theory to measure moisture content and thickness of leather simultaneously. These results demonstrate that this method could become a standard quality control test for the industrial tanning process

A I Hernandez-Serrano

E Castro-Camus

E Garcia-Sanchez

M Alfaro

S C Corzo-Garcia

CiteSeerX

Quality control of leather by terahertz time domain spectroscopy
A. I. Hernandez-Serrano,1 S. C. Corzo-Garcia,1 E.
Garcia-Sanchez,1 M. Alfaro,1, 2 and E. Castro-Camus1
1Centro de Investigaciones en Optica A.C., Loma del Bosque 115,
Lomas del Campestre, Leon, Guanajuato 37150, Mexico
2Curtidos y Acabados Kodiak, S.A. de C.V. Priv. Acidos 102
Fracc. Industrial Sta. Crocce II Leon, Gto. CP 37440, Mexico
We use terahertz time-domain spectroscopy combined with
effective-medium theory to measure moisture content and thick-
ness of leather simultaneously. These results demonstrate that this
method could become a standard quality control test for the indus-
trial tanning process.
c⃝ 2014 Optical Society of America
OCIS codes: 300.6495, 160.4890, 160.1435.
Appears in: Applied Optics 53, 7872-7876 (2014). http://dx.doi.org/10.1364/AO.53.007872
1. Introduction
Tanning is the industrial process by which animal skins are converted into leather, which
subsequently is used to manufacture a wealth of products that range from shoes to car
seats. This process has seen relatively little changes since it was first used in ancient times.
Although some automatic machines have been introduced, the process has not changed
significantly. Most of the quality control is performed manually and even by subjective
inspection of the finalized leather. Two parameters of extraordinary importance are the
water content of the leather and its thickness. So far, the water content of leather is usually
determined either indirectly, by measuring its electrical conductivity. Or by gravimetry,
which is the process of taking a small sample of a leather hide, weighing it, drying it for
2min at 90-100 ◦C and weighing it again.[1] Both of these methods are manually performed
and either destructive or at least produce undesirable marks on the leather. The thickness
is usually measured using a mechanical micrometer.[2] In order to prevent damaging the
leather or producing a blockage of the conveyors that move the leather through the process,
a non-contact method to measure these two parameters is desirable.
Terahertz radiation has attracted enormous attention owing to its potential of application
in many fields. Advances in the technologies required to access this band of the electro-
magnetic spectrum have been reported in the last three decades by an active and growing
2
community.[3, 4] One of the characteristics that make this radiation of interest is the fact
that it is strongly absorbed by water,[5] therefore, terahertz can be used as a very sensitive
non-contact and non-destructive probe of water content. This has been demonstrated for
paper,[6] plants[7, 8] and other materials.[9] The transmission of terahertz radiation through
leather samples has been reported, however, no quantitative information relevant to the qual-
ity control of this product has been extracted from such measurements yet.[10, 11] In this
article we report transmission measurements of leather samples with different water content
and thicknesses. We used an effective medium model for the combination of leather and
water in order to quantitatively obtain the two variables simultaneously from the complex
transmission spectra.
2. Qualitative measurements
In order to have a qualitative idea of whether the terahertz transmission technique could
work as a moisture probe, a terahertz pulse was recorded after transmission through four
samples of leather with different moisture levels. The measurements were performed using
a terahertz time-domain spectrometer (THz-TDS) similar to the one described in Ref [12].
An intermediate state of the skins during the tanning process is called “Wet-Blue” (WB), as
suggested by its name, this material contains high amounts of water. One sample of WB was
measured as supplied from the factory (dripping wet, > 50%wt), while two others where left
drying for 15 minutes and 3 hours respectively before taking the measurement. In addition,
a fully processed leather sample, with a water content below 10%wt, was measured. The
waveform of the pulses transmitted through the leather are displayed in Fig. 1. As shown
in the figure, the pulse is completely absorbed in the sample with highest water content,
while for the other samples, the amplitude of the pulse increases as the water content
decreases, reaching a relatively large amplitude for the fully processed leather. This is a clear
indication that the water content is related to the terahertz transmission. Furthermore, this
demonstrates that in the latter stages of the process the leather is sufficiently transparent
to obtain good transmission spectra and, therefore, a good signal-to-noise ratio on the
measurement for quality control purposes.
3. Effective medium theory
Given that quality control of processed leather requires the determination of both the mois-
ture content and thickness of each leather hide, we tried using the multiple echo method
3
−2 0 2 4 6 8 10
−4
−2
0
2
4
6
8
Time (ps)
E
T
H
z 
(a
rb
. u
ni
ts
)
Reference (air)
"Processed" leather
WB Dry
WB Medium
WB Wet
Fig. 1. (Color online) Terahertz pulses transmitted through leather samples with different
humidity content. The amplitude of the pulse decreases as the water content of the sample
increases. The curves on the figure are on the same scale and have been shifted vertically
for clarity.
described by Duvillaret et al.[13, 14], however, the leather samples are not transmissive
enough to obtain measurable FabryPerot reflections after the main pulse. Therefore we de-
cided to investigate if effective medium theory can be used to obtain a quantitative measure
for both of these parameters from the terahertz transmission spectra.
The Landau-Lifshitz-Looyenga model[15] has been used before to calculate the effective
complex dielectric function mixtures of materials that can be considered homogeneous on
length scales comparable to the wavelength of the radiation. If two materials with complex
dielectric constants ϵw and ϵl are mixed at volumetric fractions aw and 1− aw respectively,
then the model predicts that the mixture will have a complex dielectric function given by
3
√
ϵmix(ω) = aw
3
√
ϵw(ω) + (1− aw) 3
√
ϵl(ω). (1)
The complex refractive index of the mixture is given by n+ iκ =
√
ϵmix; therefore, a mixture
sample of thickness d will have a transmission coefficient given by [16]
T (ω) = t12t21e
−ω
c
(κ+κscatt)dei
ω
c
(n−1)d, (2)
where t12 = 2ñ/(ñ + 1) and t21 = 2/(ñ + 1) are the Fresnel transmission coefficients at the
4
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.5
1
1.5
2
2.5
3
3.5
Frequency (THz)
P
er
m
it
ti
vi
ty
(ε
0)
Fig. 2. (Color online) The real (circles) and imaginary (squares) parts of the dry leather
dielectric function are plotted. The symbols represent actual measured points and the
continuous lines are polynomial fits that were used for the effective medium model. The real
and imaginary parts are approximated by the polynomials Re(ϵ) = 3.7× 10−39ϵ0THz−2ν2−
4.9× 10−27ϵ0 THz−1ν+3.19(ϵ0) and Im(ϵ) = 1.2× 10−2ϵ0 THz−2ν2− 7.4× 10−2ϵ0 THz−1ν+
0.0132(ϵ0).
air-sample and sample-air interfaces and
κscatt =
c
ωd
[
(
√
ϵmix − 1)
4πτ cos θ
λ
]2
(3)
is a correction to the imaginary part of the refractive index caused by scattering losses at
the rugous interfaces of the mixture.[17] Here τ is the rugosity of the sample’s interfaces
measured as the standard deviation of the surface height profile and θ is the incidence angle.
In order to apply this model to leather we need to determine the dielectric function of
“dry” leather. For this we baked a sample of processed leather for 2 hours at 95◦C. The
sample was immediately transferred to the TDS system. A transmitted and a reference pulse
where measured under a nitrogen atmosphere to prevent absorption from atmospheric water
vapor. The complex dielectric function was subsequently calculated. This was achieved by
Fourier transforming the reference (nitrogen) and sample waveforms. These two functions
were divided in order to obtain the complex transmittance function. From the phase and
amplitude of such function it is possible to obtain the real and imaginary parts of the
refractive index, and therefore of the dielectric function of the material. Further details of
the formalism are presented in Ref. [18]. The dielectric function for dry leather is shown in
5
0.2 0.4 0.6 0.8
3.5
4
4.5
5
5.5
6
Frequency (THz)
R
e(
ε e
ff
(ε
0)
)
0%
20%
40%
60%
80%
100%
(a)
0.2 0.4 0.6 0.8
0
0.5
1
1.5
2
2.5
3
Frequency (THz)
Im
(ε
ef
f(
ε 0
))
0%
20%
40%
60%
80%
100%
(b)
Fig. 3. (Color online) Real (a) and imaginary (b) parts of the dielectric function of leather
for various moisture levels calculated from the effective medium model.
Fig. 2. The real part of the dielectric function, related to the propagation speed of radiation
in the sample is almost constant. The imaginary part, that measures the absorption of the
material, shows small (<0.5 ϵ0) positive values. This is most likely related to vibrational
absorption of the proteins present in the sample.
By using Eq. 1 combining the dielectric function of water, obtained from a double Debye
model[19], and the dielectric function of dry leather it is possible to obtain the complex
dielectric function of wet leather with different humidities. Both the real and imaginary
part of such dielectric functions are shown in Fig. 3. As expected from the measurements
in the previous section, both the real and imaginary parts of the dielectric function of the
mixture have an important dependence on the water content. The real part almost doubles
and the imaginary part increases by a factor of 15 (at 0.2THz) when the water content goes
from 0% to 100%. It is important to notice that provided the volumetric proportions of
water and leather it is relatively easy to calculate the dielectric function of the mixture,
however, the calculation of the proportion of water and leather from the measured dielectric
function of the mixture is, in general, not trivial, for this an iterative fitting algorithm, such
as the one described in the following section is required.
6
0 0.5 1
−40
−35
−30
−25
−20
−15
−10
−5
0
ar
g(
T
)
Frequency (THz)
a)
0 0.5 1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
|T
|
Frequency (THz)
b)
Fig. 4. (Color online) Phase (a) and amplitude of the transmission coefficient of various
leather samples. The symbols represent measured values, while the continuous curves are
fits of the effective medium theory model. The symbols correspond to samples a humidity
and thickness of: 19%, 1.7mm (circles); 24%, 1.7mm (squares); 42%, 2.1mm (stars) and
45%, 2.2mm (triangles) respectively as measured by conventional methods.
4. Calculation of thickness and moisture
In order to obtain both the thickness and moisture content of leather samples, we imple-
mented an iterative algorithm that makes a complex least-square fit to an experimental
transmission spectra. This algorithm adjusts the parameters aw and d of Eq. 2 which de-
pend on Eq. 1. More details about the algorithm can be found in Ref. [7]. For consistency
with standard practices in the tanning industry, the humidity presented in this article refers
to the mass fraction that water represents of the leather sample expressed in percentage. The
conversion between volumetric and mass fraction can be found in the article just referenced.
In order to test the algorithm and validate the technique, four processed leather samples
were prepared. Different amounts of water were added to each one of them and they were
left in sealed plastic bags for 4 days in order to ensure the water was evenly diffused into the
tissue. Each sample was cut in two and each half was placed in separate sealed bags. A set
of samples was sent to the tannery facility to be characterized with conventional methods
and a second set was taken to the THz-TDS system. None of the two people performing the
characterization knew the amount of water added to each sample.
The characterization in the tannery was performed by gravimetry, using a commercial
7
OHAUS MB45 humidity analyzer. With this equipment, each sample was weighted, baked
at 200 ◦C and weighted again to measure its water content. The thickness was determined
using a micrometer with 0.1mm resolution. For the terahertz characterization a transmitted
waveform was recorded for each sample together with a reference waveform. All of these
measurements were done in air in order to test the technique under conditions as close
as possible to those of a real production line. The transmission spectra were calculated
and are shown in Fig. 4. Subsequently the iterative algorithm described in the previous
section was applied in order to obtain fits to the experimental points (also shown in Fig. 4),
and therefore, the thickness and water content of each sample was determined. In terms of
computation each calculation took ∼0.9 s, which is acceptable for quasi-real-time monitoring
in an industrial environment. As seen in the figure, there is an excellent fit between the
model and the experimental data for all the samples. For the fitting process, the parameter
τ ∼ 90µm was measured for the leather surface using a perfilometer. The angle of incidence
θ was assumed to be zero.
The results from the tannery and the parameters of the fitting process are plotted to-
gether in Fig. 5 for comparison purposes. There is excellent match in both variables with
mutual standard deviations between the measurements of 2% for humidity and 0.15mm for
thickness. It is important to notice that for samples with water content higher than ∼45%
the uncertainty of the terahertz measurement is expected to increase significantly given that
the amplitude of the transmitted pulse gets smaller and therefore the poor signal-to-noise
ratio of the transmission spectra results in a worse estimation of both parameters.
5. Final remarks
In this article we demonstrated that it is possible to use THz-TDS in combination with effec-
tive medium theory to simultaneously quantify the water content and thickness of leather.
The comparison between the measurements using THz radiation and conventional methods
match very well and the mutual deviation of such measurements is perfectly acceptable for
quality assurance purposes. The fact that this is a non-contact technique allows its imple-
mentation on production-line conveyors for automated quasi-real time characterization of
each hide produced which is impossible with the methods currently used. This tool promises
to be extremely useful for the quality control in the tanning industry.
8
1 2 3 4
0
0.5
1
1.5
2
2.5
T
hi
ck
ne
ss
 (
m
m
)
Sample
b)
 
 
Micrometer measurement
THz measurement
1 2 3 4
0
10
20
30
40
50
H
um
ud
it
y 
(%
)
Sample
a)
 
 
Gravimetric measurement
THz measurement
Fig. 5. (Color online) Comparison of measurements of humidity (a) and thickness (b) for
various samples obtained using conventional methods (gravimetry and a micrometer) and
the parameters obtained from the terahertz measurements. There is excellent correlation
between the conventional and THz measurements with a mutual standard deviation of 2%
and 0.15mm respectively.
6. Acknowledgements
Authors would like to thank the financial support from Curtidos y Acabados Kodiak, S.A.
de C.V. and CONCyTEG (Grant GTO-2012-C01-187407). A.I. Hernandez-Serrano, S. C.
Corzo-Garcia and E. Garcia-Sanchez would like to thank the support of CONACyT through
scholarships 354621, 327556 and 332470.
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Quality control of leather by terahertz time domain spectroscopy

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Quality control of leather by terahertz time domain spectroscopy

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