Epsilon™ Questions


1. What is contact imaging ?

Contact imaging is a method for acquiring an image by direct contact between the object of interest and the sensor(s). Of course, this is a very wide definition and includes ultrasonic imaging, atomic force microscopy, microradiography, etc.


The contact imaging device used in the Epsilon™ is a semiconductor fingerprint sensor. Its contact surface measures 12.8mm x 15mm and contains 76800 individual sensors, arranged in a rectangular array of 256 columns and 300 rows. These sensors respond to changes of capacitance and therefore to the dielectric permittivity (dielectric constant) of any electrically insulating materials that touch its surface. In terms of skin science, the capacitance response provides information about hydration, because water has an unusually high dielectric permittivity.

With contact imaging, there is no light, no optics, no focusing, no colours and no shadows. The “intensity” of the pixels is determined not by illumination, but by the property to which the sensor responds. The capacitance response of the Epsilon™ sensor provides a powerful means for studying skin hydration.

2. What is dielectric permittivity and why is it relevant ?

Dielectric permittivity, also known as dielectric constant, is a property of insulating materials that characterises their interaction with an electric field. It is relevant because the Epsilon™ sensor responds to capacitance and capacitance depends on dielectric permittivity. The reason we emphasise dielectric permittivity rather than capacitance is calibration. Dielectric permittivity is a material property that can be used for calibration, whereas capacitance is a device property for which no calibration standards are available. The common symbol used for dielectric permittivity is the Greek letter epsilon (ε) and this is where the Epsilon™ got its name from.


The dielectric permittivity of some common materials is listed in the table below.



Air 1.0
Petroleum Jelly 2.1
Ethyl Acetate 6.0
Ethylene Dichloride 10.4
Isopropyl Alcohol 17.9
Ethanol 24.5
Methanol 30.0
Nitrobenzene 34.8
Ethylene Glycol 37.0
DMSO 46.7
Glycerol 47.0
Water 80.1

As can be seen in the table above, water has a particularly high dielectric permittivity and this is what capacitance-sensing instruments such as the Corneometer® and the Epsilon™ rely on for measuring hydration.

3. What kind of sensor does the Epsilon™ use ?


The Epsilon™ uses a Fujitsu MFB200 fingerprint sensor (Fujitsu Ltd, Japan), which has 76800 sensing elements arranged in a rectangular array measuring 12.8mm x 15mm. The contact surface is coated with a 2µm thick protective layer of SiO2.


Each sensing element measures 50µm x 50µm and responds to the dielectric permittivity (dielectric constant) of the sample in contact with it. The native sensor response is digitised with 8-bit (0-255) resolution.

4. What is the image resolution and measurement depth of the Epsilon™ ?

The lateral resolution is 50µm, determined by the geometry of the sensing elements.


The measurement depth is <5µm, determined by the penetration of the electric field generated by the sensing elements into the contacting material.

5. Are there any patent restriction on my use of the Epsilon™ ?

No, there are no longer any restrictions. Since April 2016, Biox has an unrestricted, non-exclusive, world-wide licence from L’Oréal to exploit their SkinChip patents EP1438922 & EP1177766 relating to non-optical imaging on non-dermatoglyphic skin, hair and mucous membranes. Under this licence, all Epsilon™ images and measurements claimed in these patents can be used for all purposes, including commercial purposes such as claims support and advertising.

6. Can the Epsilon™ be used for skin hydration measurement ?

Yes, the Epsilon™ can measure skin hydration with greater accuracy and flexibility than conventional single-sensor probes such as the Corneometer®. Both the Epsilon™ and the Corneometer use the same capacitance measurement principle. Both the Epsilon™ and the Corneometer have a sensing depth that confines the measurement predominantly to the Stratum Corneum. However, the Epsilon™ has 76800 sensors whereas the Corneometer has one. That’s a game-changer because skin is heterogeneous and skin-sensor contact is variable. The reason why the Epsilon™ can be used for hydration measurement is its unique calibration technology that converts the highly non-linear sensor output into a linear and calibrated response. By contrast, the MoistureMap (CK Technology sprl, Belgium) can only visualise hydration and needs a Corneometer for measurement. Our research shows that Epsilon™ and Corneometer measurements correlate well. The Epsilon™ measures hydration using its calibrated dielectric permittivity (dielectric constant, ε), scale rather than some arbitrary scale such as Corneometer Units. This works because the dielectric permittivity of water is much higher (ε~80) than that of other constituents of skin.

There are (at least) four reasons why Epsilon™ hydration measurements are superior to Corneometer hydration measurements, as illustrated below.


1. Hydration Heterogeneity Assessment
Epsilon™ images provide a revealing visualisation of hydration distribution in the vicinity of the site of interest. These images can be processed to give quantitative measures of heterogeneity in terms of Standard DeviationCoefficient of Variation (CV%), or visually as images or histograms. This is illustrated in the volar forearm image shown below.


The two images left are the same, but with the 2.6mm diameter green Region of Interest (RoI) circle displaced by a horizontal distance of 3.8mm. The mean hydration within the left RoI is close to that of the whole image, with a dielectric permittivity of ε~15 and a CV~90%.

The mean hydration within the right RoI is clearly higher than that of the whole image, with a dielectric permittivity of ε~24.4 and a CV~70%.

From these data, the hydration of the two RoIs differ by more than 60%!

The Epsilon™ software displays colour-co-ordinated histograms that indicate hydration distribution for both the whole image (red) and the RoI (green). In this case, the hydration distribution within the left RoI is similar to that of the whole image, whereas there is a distinct shift towards higher hydration for the right RoI.

2. Correction for  Skin-Sensor Contact
The Epsilon™ software has a powerful ε-filter to correct for bad contact between the sensor and skin. Bad contact may be due to microrelief or wrinkles, hair or other obstructions. The example below illustrates how this works.
The image on the left is of an area of male ventral forearm, where the skin-sensor contact is impeded mainly by hair and the protruding sensor surround. The bad contact shows up as black because of the low hydration/ε of hair and air. The associated histogram shows the bad contact by a prominent peak at low values of ε. Note that the histogram uses a logarithmic scale, which makes the peak look less prominant than it is.
The ε filter controls allow you to remove both low and high ε pixels from the  image.
The image and its histogram show the effect of filtering pixels with ε-values below 3.5. Pixels removed by the filter are shown in grey in both the image and the histogram. Clearly, the ~43% of pixels that remain give more accurate skin hydration information (ε = 18.1) than the unfiltered image (ε = 8.3). Note that you can also use the Region of Interest (RoI) controls to focus on specific features within the filtered image.

3. Correction for Skin Surface Water
Skin surface water can be a problem when measuring hydration (i) in the vicinity of mucous membranes, occluded or damaged skin, (ii) in the presence of water-containing topical products, or (iii) in the presence of insensible perspiration or sweat. Skin surface water is not hydration, but its presence will increase hydration readings of conventional instruments. The example below illustrates how the ε-filter described in 2. above can deal with skin surface water.
The image on the left is of the second joint of a male left thumb. Its main feature is a size mismatch between the skin contact area and the sensor. However, the non-contacting pixels (76% of the total) have been removed by the ε-filter described in 2. above, as indicated by the grey colouring. For the remaining pixels, the mean hydration is ε~21.1 with a CV~60%.

The yellow/white spots are surface water from perspiration. The 2mm diameter RoI encloses an area  where surface water predominates. Within this RoI, the mean permittivity is considerably higher (ε~27.8, CV~78%) than for the whole area of contact.

In the left image, the surface water was filtered out for ε-values above 60, as indicated by the light grey colour in the image and histogram. For the whole image, this filter changes the mean ε from 21.1 to 20.1. For the RoI, the change is from 27.8 to 21.3.

The effect of the surface water alone was a ~5% measurement error for the whole contact area and a  ~23% measurement error for the RoI.


4. Time-dependent Measurementss
The Epsilon™ system can be used to study time-dependent phenomena by recording short bursts or long videos. This is illustrated below with a study of occlusive surface water accumulation in a tape stripping experiment. Scotch tape was used on a volar forearm skin site, repeatedly stripping the same site. Epsilon™ bursts (60 second total duration @ 1 frame per second) were recorded on the intact site and after every second strip.

Occlusion by contact between the skin and the Epsilon™ sensor during the 60 second burst measurements causes TEWL to be trapped as skin surface water. Trapped skin surface water causes the dielectric permittivity to increase, which shows up in the images as a colour change from dark red through to yellow.

Shown here are example images from the recorded bursts. Note that significant barrier damage becomes visible after just 2 strips, as indicated by the increased rate of surface water accumulation compared with intact skin. Also, the barrier damage after 14 strips is highly heterogeneous, with the bright yellow areas indicating greater than average damage.
These occlusion plots were calculated as whole-image averages. They clearly show (i) an almost unchanged hydration at t=0, irrespective of the number of strips removed, and (ii) increased barrier damage with number of strips removed.

These examples illustrate the capabilities of the Epsilon™ for static and dynamic skin hydration measurement on previously inaccessible sites such as hairy skin, scalp and skeletal joints, and under less than ideal conditions such as when sweat glands are active. 

7. Can the Epsilon™ be used for characterising Skin Micro-Relief ?

This facility is available in the the current release of the Epsilon™ software (Version 3), which you can download from the Supportsection of this website.

8. Can the Epsilon™ be used for characterising hair ?

Yes, both static and dynamic measurements can be performed. More soon.

9. How does the Epsilon™ compare with the MoistureMap ?

Both the Epsilon™ and the MoistureMap (CK Technology sprl, Belgium) are contact imaging devices. Both the Epsilon™ and the MoistureMap use semiconductor fingerprint sensors. Both the Epsilon™ and the MoistureMap are based on L’Oréal SkinChip research, eg [1-2].

The big difference between the Epsilon™ and the MoistureMap is its linear and calibrated response. This uniquely enables the Epsilon™ to  make quantitative capacitance-related measurements such as skin or hair hydration. By contrast, CK recommend that the MoistureMap should be used for visualisation only and that a Corneometer® should be used alongside the MoistureMap for quantitative measurement.

As for the rest, see below.

Epsilon™ Model E100
MoistureMap Model MM 100
L’Oréal Patent Licence Yes Yes
Linear Response to Capacitance Yes No
Calibrated Response Yes No
Hydration Histogram 256 levels of discrimination 5 levels of discrimination
Hydration Measurement Yes, 256 levels of discrimination No, visualisation only
Skin Micro-Relief Analysis Yes Yes
Fingerprint Sensor Fujitsu MFB200 Upek TCS1CT
Number of Pixels 256 x 300 = 76800 256 x 360 = 92160
Sensor Area 12.8mm x 15mm 12.8mm x 18mm
Pixel Spacing 50µm 50µm
Measurement Head Dimensions 27mm x 30mm 30mm x 43mm
Spring-loaded Sensor Yes Yes
Live streaming Yes Yes
Image Format Lossless TIFF Jpeg
Video Format Lossless AVI AVI
Video Analysis Hydration, Area None
Event Trigger Yes No
Footswitch Trigger Option Option
In-vitro Adaptor Included as standard Option at extra cost
Interface USB2 Proprietary interface box
Power USB2 Separate 12V, 4A Supply
Warranty 1 Year 1 Year


[1] JL Lévêque & B Querleux. SkinChip, a New Tool for Investigating the Skin Surface in-vivo. Skin Res Technol 9(4): 343-7, 2003.
[2] E Xhauflaire-Uhoda & G Piérard. Skin Capacitance Imaging. Chapter 13, Handbook of Cosmetic Science and Technology, 3rd Edition, 2009. ISBN 9781841847436.

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