Epsilon™ Linear & Calibrated Response


The Epsilon™ differs from other capacitance contact imaging devices such as the SkinChip (L’Oréal, France) and the MoistureMap (CK Technology sprl, Belgium) in its linear and calibrated response to near-surface dielectric permittivity (ε). The Epsilon™ uses a dielectric permittivity scale rather than some arbitrary capacitance scale, because dielectric permittivity is a material property that can be calibrated, whereas capacitance is a device property without independent definition. And once you have linearity and calibration, every pixel in the image provides a reproducible measurement that can be interpreted in terms of hydration.

The native response of the fingerprint sensors used in capacitance contact imaging devices is non-linear and variable, as illustrated by the solid (typical) and dotted (sensor to sensor variations) grey lines in the graph below.



At low ε, the readings can be off-scale, producing a uniform white image, even when the sensor surface is dirty. At high ε, the response is non-linear, with progressively decreasing sensitivity. This causes fingerprint lines to be imaged with a consistent dark grey colour for a wide range of skin hydration values. Therefore, fingerprint sensors are good for fingerprinting, but less than good for quantitative imaging. The linearised response (red line in the graph above) is achieved by (i) altering sensor operating parameters to keep the readings on-scale for the entire ε-range of interest and (ii) mapping the resultant output onto a linear scale by means of a mathematical model of the sensor’s characteristics. After linearisation, the sensor is calibrated to ensure consistent measurements from instrument to instrument and from time to time.