In Vivo Quantitative Evaluation of Gloss - Allured Books

Chapter 51
In Vivo Quantitative
Evaluation of Gloss
A real time polarization analysis technique is described that
differentiates components of scattered light in video images and
enables in vivo quantitative evaluation of gloss from hair and
skin during quality control and claims substantiation.
key words: Gloss, shine, skin, hair, light scattering,
polarization, imaging
Cosmetic manufacturers need to quantify the visual appearance of their products
objectively and accurately to study skin and hair, to improve the performance,
quality and the reproducibility of cosmetic products, and to substantiate their claims.
In vivo non-contact testing provides the most meaningful results since it is carried
out in realistic conditions directly on a model. For this chapter, in vivo gloss measurements
were conducted on commercially available hair and skin products.
Color and Gloss
Color and gloss are two important parameters that define the visual appearance
of an object. To simply explain the physics of the light-scattering phenomena, let’s
consider the case of a planar object illuminated by an incident light coming from
one direction (see Figure 51.1). The outgoing scattered light can be divided into
different categories. 1-3 Figure 51.1.
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Mirror-like reflected light or specular reflection: A portion of light is
optically reflected by the surface of the object at a symmetrical angle (mirror-like
reflection at specular angle) just like a bouncing ball. Specular light does not usually
penetrate the object, so in most cases it does not carry any information related to
the color pigment. Thus, the color of the specular light is related to the color of the
illumination. Indeed, one can easily observe a white reflection when looking at an
object from the specular angle. Artists call it a highlight, 3 to scientists it’s a specular
reflection or surface scattering, and cosmeticians term it gloss, shine or luster.
Volume scattering: A portion of light penetrates inside the object and is scattered/absorbed
by the color pigments. The volume-scattered light that is not fully
absorbed leaves the object carrying the color information of the pigments. Indeed,
when viewing an object from an angle other than the specular angle, one can observe
the color of the object. Volume-scattered light is also called diffused light.
Surface scattering: Finally, a portion of light is scattered at the surface of the
object because of the presence of surface defects. The surface-scattered light carries
the information of surface quality and surface defects. It is of particular importance
for the analysis of metallic surfaces, when there is no volume-scattered light.
Degrees of Gloss: Gd and Gd*
Based on the above parameters, we can define two gloss degrees (abbreviated
Gd and Gd*):
• Gd, ranging from 0% (low gloss) to 100% (high gloss), is the percentage of
specular light in the total amount of reflected light.
• Gd*, ranging from 0 (low gloss) to ∞ (high gloss), is the ratio of specular
light divided by the volume-scattered light in the direction of observation.
Gd* allows the classification of high gloss objects.
Both Gd and Gd* cover a large range of glosses and allow the evaluation and
classification of cosmetics products. Furthermore, because they take into account
the total reflectivity of the object, they are correlated to the visual assessment carried
out by the human eye. Dark objects, for example, appear glossier (because of
their low reflectivity) than light objects (high reflectivity).
Operating Principle
Our evaluation technique is based on differentiating and measuring the types
of scattering events. Depending on its origin, the light scattered exhibits various
polarization signatures. 1,2,4 Specular light, for example, has the property of preserving
the original polarization state whereas diffused light is fully depolarized. We
use an innovative real time polarization analysis 5,6 technique (see the Polarization
Analysis sidebar) to quantify the scattering events described above.
By using a polarimetric camera with a high polarization contrast parameter
value (>100) at a fast video rate (15-30 frames/sec), one can now measure the
following scattering parameters— 1,2,4 specular light, surface-scattered light, and
volume-scattered light, all in black and white— or if needed, in the red, green
and blue colorimetric space. Figure 51.2 presents three images delivered by this
imaging system a .

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Figure 51.2.
Polarization Analysis
Using a polarized illumination, specular and diffused components of
light can be retrieved by measuring parallel and crossed polarizations. This
separation has already been used and explained in past literature for hair
luster analysis. 7
Thanks to recent advancements in electro-optics technology, a fast polarimetric
video camera with a high polarization contrast (>100) was developed
in order to perform real-time analysis. The polarization contrast is actually a
parameter that quantifies how well the technique can break the scattered light
into its components. The higher the polarization contrast of the system used,
the better the analysis and the separation of the components. It is critical to
use a high polarization contrast (>100) technique in order to well separate
the components and carry out accurate gloss measurements.
The classical image is the real image delivered by a usual camera. It presents
both color and gloss information. It contains the information of totally reflected light
and is the sum of the parallel polarization (P) and the crossed polarization (C).
In the case of skin, the surface image does not provide any color information
and is black and white. It shows the distribution of the surface scattered light on the
complex shape of the face, highlighting the surface imperfections (pores, wrinkles).
In addition, it extracts and displays the specularly reflected light. Mathematically,
it is the exact difference between the P and the C image.
The volume-scattering image presents no specular light and shows the diffused
light and the true colors of the skin. It was calculated by multiplying by two the
crossed polarization image C.
Using the definitions previously given, the images are then processed and combined
to provide accurate and consistent gloss values Gd and Gd* on areas of the
object specifically chosen by the user. The primary innovation of this technique is
that it allows for the gloss characterization of curved and highly textured surfaces
which, until now, was not possible with classical gloss meters.

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Experimentation
The experimental set-up for gloss measurement includes: a polarimetric imaging
sensor; a polarized illumination system; and a computer with software dedicated
to fast polarization image acquisition, processing and display of results (Figure
51.3).
Figure 51.3.
The illumination can be based on fluorescent lights (cool white) or white Light
Emitting Diodes (LEDs), which make it compact, cold and easily controllable. The
software was developed to provide calibration, acquisition of polarization images,
gloss measurement, real-time analysis, display and recording of various images. A
chin rest or a head rest is also used to control the position of the face for optimum
repeatability.
Results
In vivo gloss measurement of foundation: Foundations presenting various
levels of gloss were tested with the system. A Region of Interest (ROI) in the “T”
area of the face was chosen for the measurement. The gloss degree was measured
on each pixel in the entire ROI; then the average of all the values was calculated.
Results of eight commercially available foundations are shown in Figure 51.4.
The gloss results correlate to visual observation and confirm the matte/glossy
nature of each foundation. The glosses ranged from 19% to 45% on the Gd scale,
which demonstrates large differences between commercially available products.
The accuracy of our measurement was better than 0.5 gloss unit on the Gd scale
without using any additional time averaging with the video imaging sensor.
Efficiency testing of hair product: We also quantified the evolution of hair
shine in the following five different conditions: uncombed; combed; combed with a
reference conditioner; combed with a shine improvement conditioner; and combed
with a shine improvement conditioner plus a liquid polish. Since hair is a highly
reflective object with low diffusion (Gd>70%), we chose to plot the results on a
Gd* scale as shown in Figure 51.5.

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Figure 51.4.
Figure 51.5.
The results show a large range of Gd* values depending on hair conditions and
the products that were applied. Combing drastically increases the reflected light by
50% and makes hair shinier. A classical conditioner increased the shine of combed
hair by another 10% while a second conditioner, claiming to improve the shine,
increased it by 40%. By adding a liquid polish, we could further increase the shine
of combed hair by 60% reaching a Gd* value of 7.85. The total uncertainty of the
measurement was 0.3 on the Gd* scale.
The Gd* measurements obtained correlate to the visual assessment and depend
on the hair color of the model. The present measurements were carried out on light
brown hair. Higher values of Gd* as well as larger gloss effects due to the application
of the products are expected in the case of dark brown or black hair.
Gloss distribution: We calculated the Gloss Degree (percentage of specular light
in the total amount of reflected light), pixel by pixel, in order to instantly represent
the spatial distribution of the gloss of a complex 3D surface such as a human face
using the formula Gd = (P – C)/(P+C). The color of the images of Figure 51.6 was
encoded by the Gloss Degree using the color scale shown in each image. This “gloss
mapping mode” allows the user to visualize immediately the gloss distribution and
efficiently communicate the differences between two products. Figure 51.6 shows