Patent Description:
Many skin treatment devices require the determination of a skin characteristic as part of their operation. Skin characteristics used by such devices can include skin tone, skin melanin index, and other parameters that relate to skin color (skin tone/skin pigmentation), skin patterns or skin features (e.g. the presence of hair, moles, scars, acne, etc.). Examples of treatment devices that benefit from determination of skin characteristics include photoepilation devices. Such devices use treatment sources such as laser, LED, flash lamp etc. for epilation. An example is an Intense Pulsed light (IPL) device. Further examples include devices for treating acne and skin lesions.

Intense Pulsed light (IPL) technology is a popular solution for many treatment applications such as, but not limited to, photoepilation, lesion treatment, photo-rejuvenation, in-home personal care, professional personal care and medical settings. IPL photoepilation devices usually apply light across a broad wavelength spectrum to the surface of the skin, targeting the melanin in the hair e.g. in the hair follicles. The hair/follicle absorbs the energy and therefore heats up, leading to an interruption of the hair growth.

A skin characteristic such as a skin tone is typically categorised into groups such as: 'white', 'beige', `light brown', `medium brown', `dark brown' and `brownish black and darker'. Typically, IPL photoepilation devices are not used with darker skin as the skin will absorb energy in the light pulse rather than the hair or follicle. In that case, if a skin tone of e.g. brownish black and darker, is detected, the device is programmed not to trigger a flash. Typically, "skin tone" is interpreted as relating to the melanin content ("level of brown"). A skin characteristic such as skin color or pigmentation can be used to detect skin erythema.

The determination of skin characteristics enables controlling the energy setting of the device e.g. based on a light/dark skin tone, and for ensuring device operational safety. For example, if an unsuitable skin tone is detected, the device automatically disables flashing. Alternately, a suitable energy level for treatment can be variably selected/adapted based on the skin characteristic.

<CIT> discloses a device for photobiomodulation treatment of the human body by means of infrared light. The device comprises a laser light source for generating the treatment light and an output control module configured to adjust the wavelength or intensity of the treatment light based on the output of a skin tone calculating module. The skin tone calculation module is configured to calculate the skin tone of the user's body by comparing the light reflected by the skin with preset data. The light reflected by the skin is measured by a plurality of light receiving units arranged at different distances from the light source. The preset data may be data representing a range for classifying the optical signals measured according to the distance from the light source. The skin tone is derived from the slope of a curve that represents the measured intensity of the reflected light as a function of the distance of the light receiving unit from the light source.

<CIT> discloses an apparatus for improving the PPG pulsation cycle signal quality in a blood glucose meter by using a skin color sensor. The skin color sensor comprises a skin reflection detection unit arranged for measuring the light irradiated towards the skin by means of a white LED and reflected by the skin of the subject to be measured. The skin color sensor further comprises a skin color estimation unit configured for converting the measured light into a color code and determining a skin color type based on the color code. The intensity of the light of the white LED to be generated for the PPG measurement is adjusted in dependence on the determined skin color type.

At least for the above, it is desirable to determine the skin characteristic with sufficient accuracy.

An apparatus (e.g. a sensor) which measures a skin characteristic may make use of at least one, e.g. two light sources, e.g. LEDs, that operate at two distinct wavelengths (e.g. at <NUM> nanometres (nm) - wavelength <NUM> and <NUM> central wavelengths - wavelength <NUM>, respectively). The light from these LEDs is emitted towards the skin and a detector measures the reflected light, resulting in detector signals S <NUM> and S2 for the two LEDs respectively. Based on the levels of skin reflectivity R for the two wavelengths a skin tone or pigmentation can be computed. The skin tone or pigmentation is a function of the ratio of reflection <MAT> and/or skin reflectivity <MAT> at each wavelength <NUM>, <NUM>.

A problem with such measurements is that the detected signal is influenced by both external factors such as ambient light and temperature, as well as internal reflection losses within the device. The measurements may further be affected by the detector response characteristics (e.g. how it behaves at low intensities). As a result, when said apparatus is used with a personal care device for skin treatment, the device may not effectively classify the skin tone. For example, on one hand, the device may not block certain skin tones when it should, leading to safety concerns. On the other hand, it may erroneously block certain suitable skin tones, preventing a portion of users from using the device.

An example is shown in <FIG>, which shows a prior art set-up for measurement of reflectance signal from skin. Light sources <NUM> and <NUM> emit light of central wavelengths λ<NUM> and λ<NUM>, respectively. Exemplary rays R<NUM> corresponding to light source <NUM>, and R<NUM> corresponding to light source <NUM>, are reflected off the skin and detected by detector <NUM>. In other words, incident ray I<NUM> is reflected from the skin as reflected ray R<NUM>, and incident ray I<NUM> is reflected from the skin as reflected ray R<NUM>. The effective sensing/detection area of detector <NUM> on the skin corresponds to an area that is illuminated by light sources <NUM><NUM> and <NUM>.

At the same time, incident ambient light Iamb from the surroundings also gets reflected off the skin towards the same detector <NUM> as reflectance signals Ramb. These signals manifest as noise in optical measurements and decrease the signal to noise ratio (S/N) of detection, leading to measurement inaccuracies.

At least for the problems mentioned above, it is an object of the invention to provide an improved method for determination of skin characteristics, e.g. a skin tone, and apparatus thereof, for effective use of the skin treatment device.

In more detail, it is an object of the invention to provide a method and an apparatus thereof which compensates for noise generated by ambient light in the signal measurements, and to improve the S/N (sensitivity) of detection of the skin characteristic.

It is another object of the invention to provide a signal measurement method which takes into account source and detector (and associated circuitry) behavior at different incident intensity levels.

It is yet another object of the invention to provide a signal measurement method which compensates for reflections of the incident light within the device giving rise to spurious signals.

It is yet another object of the invention to provide a signal measurement method and an apparatus thereof, which compensates for signal deviation generated by temperature of the environment.

The description herein is intended to overcome at least one of the problems mentioned above.

According to an aspect of the invention, an apparatus for measuring a skin characteristic is provided. The apparatus comprises a processor configured to receive, over a time period T, a first response indicating intensities of light of a first wavelength λ<NUM> reflected from skin, and a second response indicating intensities of light of a second wavelength λ<NUM> reflected from the skin, receive a first indication of a total intensity of light corresponding to the first response and a second indication of a total intensity of light corresponding to the second response, which are emitted to the skin over the time period T, determine a first variation curve (<NUM>) with the first response as a function of the first indication and a second variation curve (<NUM>) with the second response as a function of the second indication, determine a portion (<NUM>', <NUM>') of each variation curve, along which portion (<NUM>', <NUM>') the variation is substantially linear, calculate slopes at at least one point in said portion of the variation curves, and determine a value of the skin characteristic from a ratio of the slopes. The indication and response may include values of voltage, current or intensity. For example, in one aspect, the first/response and first/second indication comprise voltage or current values which are indicators of reflected and emitted intensities, respectively. In another aspect, these are values/levels of reflected and emitted intensities.

According to an aspect of the invention, the apparatus further comprises a signal generator configured to modulate an intensity of the emitted light by the first light source at a first frequency f1 and the second light source at a second frequency f2 across a plurality of intensities during the time period T. In cases where the emitted intensities are modulated, the processor is configured to receive over the time period T, a first modulated response and a second modulated response.

According to an aspect of the invention, the processor is further configured to adjust the first response and the second response based on an intensity of a part of the total emitted light reflected internally within the optical sensor wherein the intensity of said part is determined based on total light intensities emitted over the time period T and associated with said portion of the variation curve.

According to an aspect of the invention, the apparatus further comprises a temperature sensor, and wherein the processor is further configured to adjust the first response and/or the second response based on a measurement of the temperature sensor.

According to an aspect of the invention, the apparatus further comprises a memory to store a calibration correction for the first response and/or the second response, and wherein the processor is further configured to calibrate a reflection ratio S or a skin characteristic based on the calibration correction.

According to an aspect of the invention, a personal care device is provided, the device comprising the apparatus at least as described above, and/or wherein the apparatus is included in an attachment of the personal care device.

According to an aspect of the invention, a computer-implemented method for determining a skin characteristic is provided. The method comprises receiving, over a time period T, a first response indicating intensities of light of a first wavelength λ<NUM> reflected from skin, and a second response indicating intensities of light of a second wavelength λ<NUM> reflected from the skin, receiving a first indication of a total intensity of light corresponding to the first response and a second indication of a total intensity of light corresponding to the second response, which are emitted to the skin over the time period T, determining a first variation curve with the first response as a function of the first indication and a second variation curve with the second response as a function of the second indication, determining a portion of each variation curve, along which portion the variation is substantially linear, calculating (<NUM>) slopes at at least one point in said portion of the variation curves, and determining (<NUM>) a value of the skin characteristic from a ratio of the slopes.

According to an aspect of the invention, a computer program product or a computer-readable storage medium comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method. These and other aspects, and further advantages, will be apparent from and elucidated with reference to the embodiment(s) described herein.

The matters exemplified in this description are provided to assist in a comprehensive understanding of various exemplary embodiments of the present invention disclosed with reference to the accompanying figures.

Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the exemplary figures/embodiments described herein can be made without departing from the scope of the claimed invention. In particular, combinations of specific features of various aspects of the invention may be made. An aspect or embodiment of the invention may be further advantageously enhanced by adding a feature that was described in relation to another aspect or embodiment of the invention.

Further, the functionality associated with any particular means may be centralized or distributed, whether locally or remotely. It may be advantageous to set forth that the terms "include" and "comprise," as well as derivatives thereof, mean inclusion without limitation. In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

The expression "at least one of A, B and C" means "A, B, and/or C", and that it suffices if e.g. only B is present.

The term "reflected" used herein encompasses light reflected from the skin surface as well as that reflected after being scattered from the skin surface and/or interior.

The term "skin" used herein encompasses skin of a user while using the personal care device and a reference material mimicking skin.

<FIG> show a signal detection method according to an exemplary aspect of the present invention, and corresponds to amplitude (or intensity, which is expressed as a value proportional to square of amplitude) modulation. In principle, the method may be extended to any type of signal modulation as known in art (phase, polarization etc.). The method achieves a better reduction of noise due to ambient light.

Amplitude or intensity modulation techniques are those wherein an amplitude or intensity of a carrier signal is modified in time using a modulating signal having a modulation frequency f. The modulation yields a signal with a modulated amplitude or intensity, which is at least based on the modulation frequency f. <FIG> shows a general representation of amplitude/intensity modulation of a plurality of light waves [superposed] in time.

For example, a light source as used in the present invention can be arranged to emit signals of varying intensities, I_1 to I_i (i=<NUM>, in <FIG>). Preferably, the intensities emitted by the light source are varied stepwise, as shown in <FIG>, in + or - directions, for a certain duration or period T (called an intensity sweep). These intensities are modulated using a modulating frequency f. The peak intensities of the modulated signals are shown to correspond to I(f)_1, I(f)_2,. , I(f)_i, i> <NUM>, spanning an intensity range from low to high intensities.

In <FIG>, at times T_1, T_2 and T_3, the light source emits light with peak intensity I(f)_1, I(f)_2 and I(f)_3, respectively. Period or duration T=T_1+T_2+T_3, is otherwise called a duration of a measurement sweep.

Only light which is emitted by the light source is modulated and therefore linked to the modulating frequency f component. The light is reflected by the skin surface and detected by detector <NUM>. It can be demodulated and/or filtered out using suitable detection circuitry such as band pass filters, lock-in amplifiers or other signal demodulators. The ambient light remains unmodulated or may be modulated at a different frequency (e.g. frequency of the electricity grid (<NUM> or <NUM>) and harmonics thereof), and hence without association to the modulating frequency f. The detector <NUM> can be configured to detect and process signals (or transmit them for signal processing by control circuitry) which are associated with modulation frequency f, and disregard other detected signals, in this case, those which contribute to noise in a measurement.

Although the arrangement shows two light sources, from the above discussion, it is clear to the skilled person that it is sufficient to modulate intensities of a single light source to filter noise from ambient light. The use of two light sources is however desirable, as such measurement makes the method less prone to noise and decreases the number of additional steps (mathematical corrections, further measurements) otherwise required to compensate for this noise.

In order to accommodate reflectivities of different types of skin, in an embodiment, it is desirable to obtain skin characteristic information using a larger range of emitted intensities I(f), and consequently reflected intensities I'(f), which spans from low to high intensity values in an operating range of the light source and the detector. For example, the darker skin tones reflect too little light, and lighter skin tones too much.

<FIG> shows a signal detection method according to another exemplary embodiment of the present invention.

In <FIG>, values of a suitable indication, e.g. driving voltages or currents representing emitted intensities of light source <NUM> and light source <NUM> during time period T are plotted along the x axis. These intensities may be modulated as explained above if ambient light reduction is desired. As an example, I_m and I_n correspond to two intensity values emitted by source <NUM>.

Values of a suitable response (e.g. voltage or current values) parameter measured by the light detector <NUM> during the same time period T (for each emitted intensity), which indicate the reflected intensities, are plotted along the y axis. If the source intensities are modulated, the reflected intensities are also modulated. These signals are then demodulated by the detector <NUM> or associated signal processing circuitry, as mentioned above. I_m' and I_n' correspond to two intensity values of light reflected (as mentioned, directly from the skin surface or scattered internally and reflected) from the skin. I_m' and I_n' are reflected intensities corresponding to I_m and I_n, respectively.

Curves <NUM> and <NUM> represent the responsivity of detector <NUM> over time period T. A detector such as a photodiode outputs a current value (or any other parameter) based on the input light (e.g. photon count) it detects at a certain wavelength or a range of wavelengths, and the ratio of the output current to input current at the respective wavelength gives a measure of responsivity of the detector. The output current/voltage varies proportionally with the input current/power and gives a measure of the photon count registered by the detector. It is understood that the shape of the response curve may deviate from the exemplary variation shown in <FIG>.

Curves <NUM> and <NUM> shows the response of detector <NUM> corresponding to reflected intensities of wavelengths λ<NUM> by source <NUM> and λ<NUM> by source <NUM>, with respect to said indication of emitted intensities from the respective source. Assuming an LED to be a light source and a photodiode to be a light detector, the curve could e.g. depict a voltage-voltage relationship (voltage input to the LED driver - voltage measured at the output of the photodiode).

Irrespective of whether intensities of the emitted light (and therefore the reflected light) are modulated, when the emitted intensity of either light source <NUM> or <NUM> (or the driving voltage or current to the source) is below a certain minimum threshold, the intensity of the reflected light (and therefore the response registered at detector <NUM>) is correspondingly weak. In such cases, detector <NUM> and/or associated circuitry show a non-linear response characteristic, which typically results in noise in the measurement. This is termed as noise regime of a detector.

Above the minimum threshold intensity level/value I_0 (or the corresponding indication), the output (intensity/current/voltage) registered at detector <NUM> shows a substantially linear relationship with the input (intensity/current/voltage). In other words, the response variation is substantially linear. This is termed as the linear regime of a detector.

Correspondingly, detector <NUM> outputs a substantially linear response or a linear variation of the reflected intensity or the current/voltage with respect to the source intensity/current/voltage. Detector <NUM> can be configured to exhibit a linear response between a lower threshold I_0 and an upper threshold I_00. This linear regime is shown by substantially linear portions <NUM>' and <NUM>' of the detector response curves <NUM> and <NUM>, respectively. It is known to the skilled person that in practice, linear portions <NUM>' and <NUM>' of the response curves <NUM> and <NUM> may still have a degree of curvature.

Above the upper threshold I_00, said variation may not show a linear relationship, in this case, due to saturation of the detector. This means that the reflected intensity detected by detector <NUM> (or current or voltage) does not vary anymore, or minimally varies, with a corresponding variation in the emitted intensity (or current or voltage). This is termed as the detector saturation regime in art.

Thresholds 1_0 and I_00 may vary depending on the physical and electrical characteristics of the light source, light detector and associated circuitry.

The method and device according to the present invention takes into account the behavior characteristics of the sources, the detector and associated circuitry to determine skin characteristics of the user of the device by determining and processing information in the linear response regime of the detector.

The reflected intensity detected by the detector <NUM> is also affected by the reflectivity of the skin. For example, some skin tones reflect too little light, and others too much. If the skin reflectivity is low, the reflected intensities detected by detector <NUM> are weak, which could give rise to high noise relative to a low signal. In this case, obtaining signal information from the linear regime of the response curve provides optimum detection results.

If the skin reflectivity is high, the reflected intensities detected by detector <NUM> are very strong, which could give rise to detector saturation even at relatively low emitted intensities. In this case like above, obtaining signal information from the linear regime of the response curve provides optimum detection results.

It is desirable to modulate the intensities emitted from light sources <NUM> and/or <NUM> to achieve reduction in ambient noise in the reflectance signal measurements.

<FIG> shows a first arrangement for measuring skin characteristics according to an embodiment of an invention.

According to the embodiment, an apparatus <NUM> for measuring skin characteristic as mentioned above is part of a wireless communication network, e.g. may be located in a cloud computing/distributed server or in another processing device (e.g. a smartphone, a personal computer). Apparatus <NUM> comprises a processor <NUM> which is configured to receive, over a time period T, a first (detector) response comprising and/or indicating reflected intensities (from skin) of light of a first wavelength λ<NUM>. The intensities may be modulated at a frequency f1. It further receives, over the period T, a second (detector) response comprising and/or indicating reflected intensities (from skin) of light of a second wavelength λ<NUM>. The intensities may also be modulated at a frequency f2. The first and second response comprises reflected intensities and/or response parameters such as current or voltage values, which provide an indication of said reflected intensities. Processor <NUM> may be a general processor which is e.g. implemented in a user terminal such as a smartphone.

The first response and the second response are received from a client device <NUM>. The client device <NUM> may be the personal care (e.g. IPL) device for treatment of skin, or an optical sensor (or any other type of sensor) in the personal care device. In case, device <NUM> is the personal care device, the optical sensor may be comprised therein as one of its components, e.g., a device attachment. Transceiver <NUM> is configured to transmit and receive wireless signals over the wireless communication network via antenna <NUM>.

The client device <NUM> further comprises the light sources <NUM> and <NUM>, which are configured to emit light of wavelengths λ<NUM> and λ<NUM>, respectively, when device <NUM> is operated by a user by positioning device <NUM> on the skin. The light sources <NUM> and <NUM> (e.g. LEDs, laser diodes, surface emitting lasers) are configured to function independently, with respect to driving voltage/current and with respect to the time domain (can be turned on simultaneously or sequentially).

Intensities I_201_1, 1_201_2,. , I_201_i, i> <NUM> are emitted from light source <NUM> towards the skin over a time period T. These intensities are reflected from the skin, towards detector <NUM>. Detector <NUM> detects the reflected intensities over period T (the first response) and transmits the obtained reflected intensities to the signal processing circuitry <NUM>. The intensities may be transmitted in real time, or after period T. In an embodiment, a dummy or reference material mimicking skin is used for these intensity measurements.

In an embodiment, the signal processing circuitry may further comprise a signal generator which is configured to apply voltage or current to source <NUM> to modulate the emitted intensities, and hence the reflected intensities, during period T at a modulating frequency f1.

Similarly, intensities I_202_1, I_202_2,. I_202_i, i> <NUM> are emitted from light source <NUM> towards the skin over the same time period T. These intensities are also reflected from the skin, towards detector <NUM>. Detector <NUM> detects the reflected intensities over period T (the second response) and transmits the obtained reflected intensities to the signal processing circuitry <NUM>. As above, the intensities may be transmitted in real time, or after period T. As mentioned, in an embodiment, a dummy or reference material mimicking skin is used for these intensity measurements.

In <FIG>, these intensities are directed to the skin via guiding elements <NUM>. Mirrors <NUM> are shown for this purpose. It is clear to the skilled person that depending on the geometrical arrangement of the components (e.g. source <NUM>, <NUM>, detector <NUM>), these guiding elements may be omitted or be replaced with other suitable optical elements. The geometry of the guiding elements may be so chosen to maximize the geometrical overlap of the light emitted by the light sources <NUM> and <NUM>.

In an embodiment, light source <NUM> is configured to emit light of wavelength in the visible region of the electromagnetic spectrum, and light source <NUM>, in the infrared region. At these wavelengths, melanin has a relatively high absorption coefficient, so that the light is effectively absorbed by the pigment. Hence, the reflectance measurements and consequently the calculation of the skin tone are optimized. In an embodiment, light sources <NUM> and <NUM> may have peak emission wavelengths around <NUM> (visible, red) and <NUM> (Infrared), respectively. Alternately or additionally, light source <NUM> may have peak emission wavelength around <NUM> (visible, green). Light source <NUM> can be tailored to measure different skin properties or characteristics by choosing the respective wavelength. A tunable light source or a plurality of light sources each operating at the desired wavelength(s) may be used.

Detector <NUM> may be a photodiode, e.g. a silicon photodiode, having a detection sensitivity between <NUM> to <NUM>, and therefore shows a variation in electrical behavior as a function of the intensity of incident light for these wavelengths.

Although a single detector <NUM> is shown used to detect both the first response and the second response, separate detectors may also be configured to detect the respective response.

In an embodiment, the signal processing circuitry may further comprise a signal generator which is configured to apply voltage or current to source <NUM> to modulate the emitted intensities, and hence the reflected intensities, during period T at a modulating frequency f2.

As mentioned, the noise generating ambient light remains unmodulated, and therefore can be distinguished from the reflected signals.

In an embodiment, the modulating frequencies f1 and f2 may be the same or different.

In yet another embodiment, the intensities of each source <NUM> and <NUM> may be modulated, either sequentially or simultaneously, during period T or unmodulated.

Modulating light signals with different frequencies f1, f2 simultaneously allows measuring both colors or wavelengths at the same time with a single detector and without further optical filters.

Processor <NUM> further receives total intensities of light emitted over time period T by each light source <NUM> and <NUM> towards the skin (the incident light). The term total intensity herein refers to the intensity emitted by a light source. The response indicates an intensity which is reflected from the skin, after at least a part of this total intensity is incident on it. As will be clear from below, a part of this total intensity may be internally reflected in the device. These (total) intensities may emitted in a stepwise manner from a lower to an upper range.

Alternately or additionally, processor <NUM> may receive an indication such as a voltage or current for driving the light sources <NUM>/<NUM>, the values of which indicate the intensities emitted by the sources. The response (voltage, current) in this case provides an indication of the reflected intensities.

The first response hence is associated with an intensity of light which is reflected from the skin, from a first total intensity of the light which is incident on the skin over period T. Similarly, the second response is associated with an intensity of light which is reflected from the skin, from a second total intensity of the light which is incident on the skin over period T.

The client device <NUM> is configured to transmit each intensity value I_201_1, I_201_2,. , I_201_i, i> <NUM> and I_202_1, I_202_2,. , I_202_i, i> <NUM>, emitted by the light sources during time period T and/or an indication, e.g. voltage values V_201_1, V_201_2,. , V_201_i, i> <NUM> and V_202_1, V_202_2,. , V_202_i, i> <NUM> associated thereto and correspondingly, each value of the first response and second response to the processor <NUM> of apparatus <NUM>.

The client device <NUM> may further comprise further light detectors <NUM>, <NUM> configured to measure the intensity values 1_201_1, I_201_2,. , I_201_i, i> <NUM> and I_202_1,I_202_2,. , I_202_i, i> <NUM>, respectively, as these intensities are emitted by sources <NUM> and <NUM>. In <FIG>, detectors <NUM> and <NUM> are shown connected to signal processing circuitry <NUM>, which further transmits these values to processor <NUM> via the transceiver <NUM> and antenna <NUM>. Any alternate arrangement to transmit the emitted intensity values to processor <NUM> may also be used. It is also possible to use a single further detector to detect both wavelengths. The detector is preferably disposed near the light source, in order to capture the total intensity without losses to environment. Processor <NUM> is further configured to determine a first variation <NUM> of the first response (comprising the reflected intensities) with the first total intensity (comprising emitted intensities incident on skin). The variation is obtained as a curve, as discussed above. It is further configured to determine a second variation <NUM> of the second response (comprising the reflected intensities) with the second total intensity (comprising emitted intensities incident on skin).

Processor <NUM> further determines a portion <NUM>', <NUM>' of each variation curve, which corresponds to the aforementioned linear regime. Along portions <NUM>', <NUM>', the first variation curve <NUM> and the second variation curve <NUM> take a substantially linear shape. A certain degree of curvature may exist despite the linear relationship. It then determines a value of the skin characteristic by calculating a ratio of a slope at at least one point along portion <NUM>', α<NUM>, and a slope at at least one point along portion <NUM>', α<NUM>.

The ratio of the slopes <MAT> is proportional to the ratio of the reflected intensities S <NUM> and S2 corresponding to the two wavelengths (reflection ratio S∼S1/S2), dependent on the skin reflectivity at these wavelengths (explained below). The reflection ratio S can be mapped to the skin characteristic.

The advantage of using the linear portion of the curve is that one excludes those regions that are not useful/sufficiently accurate for skin characteristic measurement and uses only the optimal part of the response curve.

The mapping of the skin characteristic by extraction of the slope ensures that a plurality of intensity points, and hence a differential absorbance/reflectance is taken into account for calculation of the skin characteristic. Further, since a linear slope is calculated, the method ensures that data processing is performed using measured data points in said linear portion of the curve, and not e.g. in the non-linear or saturation regime. Apparatus <NUM> further comprises a memory <NUM> to store at least calibration data associated with the device <NUM>. These data may include corrections for temperature, internal reflection, skin characteristics mapping, information on the linear regime of the optical sensor etc..

Apparatus <NUM> may transmit the measured skin characteristic and/or calibration data to client device <NUM>. This may be done real-time, as client device <NUM> is used by the user.

In an embodiment, apparatus <NUM> may be used to measure the skin characteristic using reference/dummy skin data which are prior collected, for example, by client device <NUM> or another suitable device. This is particularly advantageous in a factory setting while manufacturing device <NUM>. The measured skin characteristic and/or calibration data may be transmitted to the client device <NUM> to be stored therein. The information can be processed by the client device <NUM> as reference data, to further optimize a skin characteristic which it measures real-time when used by the user. If information about the linear regime is then stored in the client device <NUM>, a further determination of the variation curves <NUM>, <NUM> and the linear regime may be omitted by the client device <NUM> in real-time measurements. This makes determination of the skin characteristic faster while using the client/personal care device. <FIG> shows a second arrangement for measuring skin characteristics according to an embodiment of an invention.

In this embodiment, processor <NUM> and memory <NUM> are comprised within the client device <NUM>. The signal processing circuitry <NUM> described in the embodiment of <FIG> may be disposed separately from processor <NUM> or be integrated therewith. Processor <NUM> is configured to perform steps described in connection with processor <NUM>. Similar to <FIG>, processor <NUM> may further optimize a skin characteristic, which is measured by the personal care/client device <NUM> when used by the user, based on prior measured skin characteristic and/or calibration data (as reference measurements). If memory <NUM> includes predetermined information on the linear regime of the optical sensor of the personal care/client device <NUM>, the personal care/client device may omit a further determination of the variation curves and the linear portion when the device is used by the user.

The remaining features of this embodiment, and their functions are the same as embodiment of <FIG> and shall not be repeated for sake of conciseness.

<FIG> shows a third arrangement for measuring skin characteristics according to an embodiment of an invention.

It shows an implementation without guiding elements <NUM> shown in <FIG> and <FIG>. As mentioned above, elements <NUM> may be omitted in either embodiment of <FIG> or <FIG>. The figure further shows how incident light is scattered from within the skin and reflected towards detector <NUM>.

<FIG> shows a method for measuring skin characteristics according to an embodiment of the invention. The method can be performed by apparatus <NUM> or <NUM> described above, according to either the embodiment of <FIG> or <FIG>.

Step <NUM> comprises receiving, over time period T, the first (detector) response indicating reflected intensities (from skin) of light of a first wavelength λ<NUM>. The intensities may be modulated at a frequency f1. Step <NUM> further comprises receiving, over the same period T, a second (detector) response which indicates reflected intensities (from skin) of light of a second wavelength λ<NUM>. The intensities may be modulated at a frequency f2 - f1 and f2 may be the same frequency, or different frequencies.

Step <NUM> comprises receiving a first indication and second indication, corresponding to total intensities of light emitted over time period T by each light source <NUM> and <NUM>, respectively, towards the skin (the incident light), i.e. the intensities I_201_1, I_201_2,. , I_201_i, i> <NUM> and I_202_1, I_202_2,. , I_202_i, i> <NUM> emitted at the first wavelength λ<NUM>, and the second wavelength λ<NUM>, respectively. The order of steps <NUM> and <NUM> is immaterial to the working of the method (or apparatus). As mentioned above, the term "total" herein is intended to emphasize the original emitted intensities without optical path losses, and in principle, is the emitted intensity of each source during T.

The intensities may be transmitted in real time, or after period T.

In step <NUM>, a variation <NUM>, <NUM>, is obtained, or a response curve is traced, with respect to each of the received first (detector) response vs indication of total intensity emitted by source <NUM> and the received second (detector) response vs indication of total intensity emitted by source <NUM>. Step <NUM> comprises identifying a substantially linear portion <NUM>', <NUM>', in each obtained variation <NUM>, <NUM>, where the detector response varies linearly with the emitted intensity of either light source. It is possible to combine these steps, or omit step <NUM>, and thus directly identifying the linear regime of the detector.

The method further comprises, in step <NUM>, obtaining the respective slope at at least a point of the substantially linear portion, i.e. in the linear regime of the detector. The skin characteristic is then calculated in step <NUM> as a function of the ratio of slopes. In other words, the ratio of the slopes is mapped to the skin characteristic.

As mentioned above, in an embodiment, light sources <NUM> and <NUM> are configured to emit intensities over a range of intensity values.

In another embodiment, the light source <NUM> and/or <NUM> are configured to emit intensities at selected values (e.g. <NUM> different intensities Im and In ) which lie in the linear regime of detector <NUM>.

In this case, the processor <NUM> or <NUM> is configured to receive the respective first response and the first total intensity comprising reflected intensities Im' and In' emitted intensities Im and In at the first wavelength λ<NUM> (i.e. corresponding to source <NUM>), respectively, and the second response and the second total intensity comprising reflected and emitted intensities at the second wavelength, respectively, and/or their indications, calculate the skin characteristic based on a ratio of slopes obtained as <MAT>.

Alternately or in addition, while using two or more selected intensities, the processor <NUM> or <NUM> is configured to calculate a reflection ratio S based on a ratio of a difference of light intensities at the first wavelength to a difference of light intensities at the second wavelength, wherein the light intensities e.g. In and Im, are emitted at any two instants in the time period T and associated with the linear portion of the response curve. Specifically, the reflection ratio S is calculated as <MAT>.

The ratio can also be calculated based on the respective responses received by the processor (equation <NUM>), which are a function of the above emitted intensities.

It then maps the reflection ratio S to the skin characteristic.

As mentioned, it is yet another object of the invention to provide a signal measurement method which compensates for reflection losses because of internal reflections in device or the optical sensor therein.

Not all light that is emitted by the light sources <NUM> and <NUM> will reach the skin. Some of the light (total emitted intensity) will be scattered on air-light guide interfaces, light guide imperfections, or (other) components inside the device <NUM>. A part of this scattered light will also reach detector <NUM> and be detected by detector <NUM> together with the light that is (back)scattered or reflected by the skin. This light that is generated by either light source <NUM>, <NUM> and detected by the detector <NUM>, without having interacted with the skin, is regarded as "internal reflection" for the purpose of this invention.

The device (processor <NUM> or <NUM>) and the method thereof calculates the skin characteristic which is corrected based on an intensity of a part of the total emitted light reflected internally within the optical sensor at each wavelength. The intensity of the internally reflected light is proportional to a difference of light intensities Im and In which are emitted at any two distinct instants in the time period T (or to detector responses e.g. voltages obtained for these intensities) and associated with the linear portion of the curve. This further improves the skin characteristic calculated as per the embodiment of <FIG>.

<FIG> shows a method according to an embodiment of the invention, detailing the above aspect of the invention, and can be performed by the apparatus <NUM> or <NUM> described in the above embodiments.

According to an embodiment, a method to reduce noise from internal reflection comprises in a first step <NUM>, measuring a reflectance signal detected by detector <NUM>, with light sources <NUM> and <NUM> emitting light of wavelengths λ<NUM>, λ<NUM> switched on at intensity I_n_201, I_n_202, respectively. Both intensities lie in the linear portion <NUM>', <NUM>' of the response curves <NUM>, <NUM>, i.e. they have values higher than 1_0_201 and I_0_202, respectively. The measured reflectance signals are denoted as Sg1_201 and Sg1_202. The signals can be measured simultaneously or sequentially.

Measurement signal Sg1 comprises contributions from the internal reflection, reflection of light from the skin and ambient light, as a function of the emitted intensities from light sources <NUM>, <NUM>. As mentioned above, at emitted intensity threshold I_0 of respective light source <NUM>, <NUM>, the detector response transitions from a non-linear to a linear regime. The contribution from internal reflection includes intensities which lie both below and above I_0. The reflection of ambient broadband (bb) light is also included. <MAT> is a measure of efficiency of detector <NUM> to convert broadband ambient light incident thereupon into an electric signal (response - current or voltage). Signal Sg1 can be expressed as: <MAT> <MAT>.

In a second step <NUM>, the method comprises measuring a reflectance signal detected by detector <NUM>, again with light source <NUM> and <NUM> switched on. The emitted intensity of each light source in this case is however only sufficiently high to lie in the linear portion <NUM>'/<NUM>' above I_0_201 and I_0_202, and have values lower than I_n_201 and 1_n_202. These intensities are denoted as I_m_201 and I_m_202, respectively. The measured signals are denoted as Sg2_201 and Sg2_202, and as above, a function of the emitted intensities from light sources <NUM>, <NUM>. Signal Sg2 can be expressed as: <MAT> <MAT>.

Measurement signal Sg2 comprises contributions from the internal reflection, reflection of light from the skin and ambient light.

Therefore, both signals Sg1 and Sg2 include contributions from internal reflections in the device. A part of this contribution lies in the non-linear regime of the response curves <NUM>, <NUM>, and another part, in the linear regime expressed by portions <NUM>' and <NUM>', respectively. The former is indicated by superscript nl in equations <NUM>-<NUM>.

In a third step <NUM>, signals Sg1 and Sg2 are processed for each light source <NUM> and <NUM>, to obtain a difference signal, Sg1-Sg2 as: <MAT> <MAT> Here,.

As seen above, although the contribution of internal reflection in the non-linear response range of detector <NUM> and the ambient light cancel out, the contribution from the internal reflection that is in the linear response range does not. ηPD · ηIntRefl · (In - Im) represents the reflected signal which arises due to scattering from the internally reflected light within the device <NUM>. It is desirable to remove this contribution from the detected reflectance signal.

Steps <NUM>-<NUM> may also be carried out as a calibration step while manufacturing apparatus <NUM> or <NUM>, i.e. as part of a manufacturing method. The magnitudes of ηPD · ηIntRefl · (In_201 - Im_201) and ηPD · ηIntRefl · (In_202 - Im_202) are stored as calibration data of device <NUM> and can be used by processor <NUM> or <NUM>, as mentioned above. They can further be stored in the memory <NUM> or <NUM>.

In step <NUM>, processor <NUM> or <NUM> is configured to retrieve the stored magnitudes during calculation of the skin characteristic and/or receive emitted intensitiesIn_201, Im_<NUM> and In_202, Im_<NUM>, n>m, relating to light source <NUM> and light source <NUM>, respectively from device <NUM>, as calibration data to calculate the internal reflection contribution. Im_<NUM> and Im_<NUM> may be equal to or near the thresholds I0_201 and I0_202.

In step <NUM>, once each difference signal (Sg1-Sg2) _201 ( or S1) and (Sg1-Sg2) _202(or S2) are corrected for internal reflection in device <NUM>, the processor is configured to calculate the reflection ratio <MAT>.

The above reflection ratio S essentially corresponds to <MAT>. In other words, the processor <NUM> or <NUM> adjusts the response obtained at each wavelength based on a measure of internal reflection inside the optical sensor.

In step <NUM>, the reflection ratio S calculated using the ratio of slopes as described in <FIG> is alternately or in addition adjusted using the calibration data obtained by the processor in step <NUM>.

As mentioned with reference to <FIG> and throughout the above description, instead of working directly with emitted and reflected intensities, the method and the apparatus of the present invention may use the driving voltage or current values of light sources <NUM> and <NUM>, and correspondingly, the voltage or current generated by detector <NUM> as the first and the second response. Since the magnitude for internal reflection is a function of the detector conversion efficiency (equation <NUM>), the calibration data may also be expressed in terms of the detector responses measured for the emitted intensities In_<NUM>, Im_<NUM> and In_<NUM>, Im_<NUM>. In this case, the reflection ratio S may be approximated in terms of the first response (corresponding to source <NUM>) and the second response (corresponding to source <NUM>) of detector <NUM>, for example, as <MAT> Here, [VPS,V<NUM>,n - VPS,V<NUM>,m]intr ref corresponds to a reference response received by processor <NUM> or <NUM>. It is a difference of voltage responses obtained at different driving voltages of light source <NUM> and when device <NUM> (optical sensor) is positioned facing an optical black box during a calibration step [as for above steps <NUM>-<NUM>]. This can be stored in memory <NUM> or <NUM> and retrieved by processor <NUM> or <NUM> for adjusting the first response. [VPS,V<NUM>,n - VPS,V<NUM>,m]int ref corresponds to a second reference response received by processor <NUM> or <NUM>. It is obtained in a manner similar to the first reference response, with light source <NUM>.

Equation <NUM> and <NUM> are similar, and therefore equation <NUM> can be used interchangeably with equation <NUM> together with the respective stored or measured voltage calibration data, to remove contribution of internal reflection from the detected response.

To obtain the above voltage responses and their difference, the driving voltages to the light sources <NUM> and <NUM> are chosen such that responses (VPS,V<NUM>,n, VPS,V<NUM>,m ) and (VPS,V<NUM>,n, VPS,V<NUM>,m) both lie in the linear portion of the curve.

Thus, the processor adjusts the first response and the second response based on an indicator or parameter (the intensity, voltage, current,. ) which represents a part of the total emitted light reflected internally within the optical sensor, wherein said indicator is determined based on the linear portion of the respective variation curve.

In an embodiment, device <NUM> further comprises a temperature sensor (not shown) to measure temperature of circuitry which control the operation of at least each of the light source <NUM>, <NUM>, and detector <NUM>. Processor <NUM> or <NUM> is configured to adjust the first response and/or the second response based on the temperature measurements of the sensor.

Processor <NUM> or <NUM> may be configured to receive measurements of the temperature sensor during the period T, and further, to offset the measured temperature using a reference temperature. This step is performed to compensate for temperature of the surrounding environment and to compensate for thermal influences on device <NUM> or <NUM>. The reflection ratio S, and hence the skin characteristic, are then calculated based on compensated temperature and hence temperature compensated light signals.

The reference temperature for performing temperature correction can be stored as calibration data in memory <NUM> or <NUM>.

According to the methods of the above-mentioned embodiments, once the reflectance ratio S is obtained, the ratio can be mapped to the skin characteristic using suitable mathematic functions.

In an embodiment, the determined skin characteristic is a skin tone. In another embodiment, it is skin color or a degree of redness. A plurality of skin characteristics may also be determined by device <NUM> or <NUM>, depending on the wavelengths of the light source used.

In yet another embodiment, a personal care device which comprises the apparatus described above, or which apparatus is suitable to perform any of the methods described above, is provided.

When apparatus <NUM> is an optical sensor, such sensor may be included in an attachment of the personal care device. This attachment may be calibrated in a factory setting as mentioned in any of the above embodiments and made available for use by the user in the client device <NUM>.

<FIG> shows an illustration of an exemplary personal care device <NUM> that can be used to apply light to skin for purposes of skin treatment. It will be appreciated that the device <NUM> in the figure is merely presented as an example of device <NUM> that the invention can be used with, and the device <NUM> is not limited to the form shown in <FIG> or to being a handheld treatment device. It may comprise additional components than those described below in connection with the figure. As noted above, the invention is likewise not limited to being implemented in or with device <NUM>, and in some embodiments the invention can be implemented in a remote server, or in another client apparatus that can be provided for the purpose of determining skin characteristics.

The device <NUM> is for use on a body of a subject (e.g. a person or an animal) and is to be held by a user during use. Device <NUM> is to perform some treatment operation to hairs on the body of the subject using one or more light pulses when it is in contact with a body part of the subject.

The exemplary personal care device <NUM> comprises a housing <NUM> that includes at least a handle portion <NUM> and a head portion <NUM>. The handle portion <NUM> is shaped to enable the user to hold device <NUM> with one hand. The head portion <NUM> has a head end <NUM> that is to be placed into contact with the subject in order for the treatment operation to be performed on the body or skin of the subject at the position that the head end <NUM> is in contact with the body or skin.

The device <NUM> is for performing a treatment operation using light pulses. Thus, in <FIG>, the head end <NUM> comprises an aperture <NUM> that is arranged in or on the housing <NUM> so that the aperture <NUM> can be placed adjacent to or on (i.e. in contact with) the skin of the subject. The personal care device <NUM> includes one or more light sources <NUM>, e.g. inside the head portion <NUM>, that are configured for generating light pulses to be applied to the skin of the subject via the aperture <NUM> and effect a treatment operation. The one or more light sources <NUM> are arranged in the housing <NUM> so that the light pulses are provided from the one or more light sources <NUM> through the aperture <NUM>.

The one or more light sources <NUM> can generate light pulses of any suitable or desired wavelength (or range of wavelengths) and/or intensities. The one or more light sources <NUM> are configured to provide pulses of light. In case of a single light source <NUM>, a plurality of optical filters may be used to separate the respective wavelength components. That is, the light source(s) <NUM> are configured to generate light at a high intensity for a short duration (e.g. less than <NUM> second). The intensity of the light pulse should be high enough to effect the treatment operation on the skin or body part adjacent the aperture <NUM>. For example, the light sources <NUM> can generate visible light, infra-red (IR) light and/or ultraviolet (UV) light. Each light source <NUM> can comprise any suitable type of light source, such as one or more light emitting diodes (LEDs), a (Xenon) flash lamp, a laser or lasers, etc. The light sources <NUM> can provide light pulses with spectral content in the <NUM>-<NUM> nanometre (nm) range.

In addition to the one or more light sources <NUM> for effecting the light-based treatment operation, the device also comprises an apparatus (sensor <NUM>) for determining a value of a skin characteristic, such as skin tone. Sensor <NUM> comprises one or more light source(s) and one or more light sensor(s). In <FIG>, the sensor <NUM> is shown disposed above aperture <NUM>, however, sensor <NUM> may be located at a different position relative to the aperture <NUM> on device <NUM>. In case of multiple sensors <NUM>, these may be positioned on either opposite side of aperture <NUM>. As mentioned above, the determination of the skin characteristic can be made by device <NUM> or the sensor <NUM> therein. It can also be determined by an apparatus <NUM> distributed from device <NUM>. The above embodiments detail the use of two light sources <NUM> and <NUM>, but it is understood that a skin reflectance and hence the skin characteristic, can be obtained using intensities of a single light source. In this case, the further step of obtaining ratio of slopes is omitted.

The device <NUM> also includes a user control <NUM> that can be operated by the user to activate the personal care device so that the required treatment operation is performed on the body of the subject (e.g. the generation of one or more light pulses by the one or more light source(s) <NUM>). The user control <NUM> may be in the form of a switch, a button, a touch pad, etc..

Light sources <NUM>, sensor <NUM> and user control <NUM> may be connected to processor <NUM> (not shown in <FIG> but in <FIG>), the latter can be implemented in numerous ways, with software and/or hardware, to perform the various functions described herein. For example, processor <NUM> may comprise one or more microprocessors or digital signal processors (DSPs) that may be programmed using software or computer program code to perform the required functions and/or to control components of the processor <NUM> to effect the required functions. The processor <NUM> may be implemented as a combination of dedicated hardware to perform some functions (e.g. amplifiers, pre-amplifiers, analog-to-digital convertors (ADCs) and/or digital-to-analog convertors (DACs)) and a processor (e.g., one or more programmed microprocessors, controllers, DSPs and associated circuitry) to perform other functions. Examples of components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, DSPs, application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), hardware for implementing a neural network and/or so-called artificial intelligence (AI) hardware accelerators (i.e. a processor(s) or other hardware specifically designed for AI applications that can be used alongside a main processor).

As mentioned, the processor <NUM> can comprise or be associated with a memory <NUM>. The memory <NUM> can store data, information and/or signals (including image(s)) for use by the processor <NUM> in controlling the operation of the device <NUM> and/or in executing or performing the methods described herein. In some implementations the memory <NUM> stores computer-readable code that can be executed by the processor <NUM> so that the processor <NUM> performs one or more functions, including the methods described herein. In particular embodiments, the program code can be in the form of an application for a smart phone, tablet, laptop, computer or server. The memory <NUM> can comprise any type of non-transitory machine-readable medium, such as cache or system memory including volatile and non-volatile computer memory such as random access memory (RAM), static RAM (SRAM), dynamic RAM (DRAM), read-only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM) and electrically erasable PROM (EEPROM), and the memory unit can be implemented in the form of a memory chip, an optical disk (such as a compact disc (CD), a digital versatile disc (DVD) or a Blu-Ray disc), a hard disk, a tape storage solution, or a solid state device, including a memory stick, a solid state drive (SSD), a memory card, etc. The details of processor <NUM> and memory <NUM> apply equally to processor <NUM> and <NUM> of apparatus <NUM>, which carries out the invention according to the above embodiments.

At least when the invention is carried out in another apparatus <NUM>, a transceiver <NUM> of device <NUM> enables a data connection to and/or data exchange with other devices <NUM>, including any one or more of servers, databases, user devices, and sensors. It can operate using WiFi, Bluetooth, Zigbee, or any cellular communication protocol (including but not limited to Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), LTE-Advanced, etc.). It may further comprise circuitry to control any suitable input component(s), including but not limited to a keyboard, keypad, one or more buttons, switches or dials, a mouse, a track pad, a touchscreen, a stylus, a camera, a microphone, etc., and the user interface can comprise any suitable output component(s), including but not limited to a display unit or display screen, one or more lights or light elements, one or more loudspeakers, a vibrating element, etc..

Claim 1:
An apparatus (<NUM>, <NUM>) for measuring a skin characteristic, the apparatus comprising:
a processor (<NUM>, <NUM>) configured to
- receive, over a time period T, a first response indicating intensities of light of a first wavelength λ<NUM> reflected from skin, and
a second response indicating intensities of light of a second wavelength λ<NUM> reflected from the skin;
- receive a first indication of a total intensity of light corresponding to the first response and a second indication of a total intensity of light corresponding to the second response, which are emitted to the skin over the time period T;
- determine a first variation curve (<NUM>) with the first response as a function of the first indication and a second variation curve (<NUM>) with the second response as a function of the second indication;
- determine a portion (<NUM>', <NUM>') of each variation curve, along which portion (<NUM>', <NUM>') the variation is substantially linear;
- calculate slopes at at least one point in said portion of the variation curves, and
- determine a value of the skin characteristic from a ratio of the slopes.