WO2024147003A1

WO2024147003A1 - Skin treatment apparatus

Info

Publication number: WO2024147003A1
Application number: PCT/GB2023/053319
Authority: WO
Inventors: Stuart Terry Jones
Original assignee: IPULSE Ltd
Current assignee: IPULSE Ltd
Priority date: 2023-01-05
Filing date: 2023-12-20
Publication date: 2024-07-11
Anticipated expiration: 2025-07-05
Also published as: GB2626012A; EP4646162A1; CN118285904A

Abstract

The present invention relates to a skin treatment device for delivery of light energy pulses to a subject's skin, the skin treatment device comprising: a light source for discharging light energy pulses to a subject's skin; at least one sensor for measuring a characteristic of the subject's skin; a control system for controlling discharge of the light energy pulses dependent upon the measured characteristic; wherein the control system is configured to operate according to the steps of permit emission of a first energy pulse; obtain multiple sensor measurements from the at least one sensor during a time period after emission of the first energy pulse; determine a difference between values of the sensor measurements; prevent emission of a second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold.

Description

Skin Treatment Apparatus

The present invention relates to a skin treatment device, preferably a skin treatment device for treating unwanted hair, and preferably comprising an Intense Pulsed Light (IPL) device.

Skin treatment devices are known in the art for treatment of, for example, cosmetic applications such as hair depilation, minimisation of skin blemishes or skin rejuvenation, as well as dermatological treatment of skin conditions such as acne or rosacea. The skin is exposed to dosages of radiation from a light source such as a flashlamp or laser where the radiation is targeted to the skin and the energy intensity and pulse duration is controlled. In hair depilation, the radiation source is targeted to cause heating of the hair root causing the hair root to die.

Safety of skin treatment devices is paramount and is particularly important for devices designed for home use. Home use devices require ‘good skin contact’ before allowing the device to emit radiation to the skin. Good contact can be defined as a condition where the light output area is sufficiently covered by skin that any stray optical radiation is below harmful levels. As such, safety features are implemented so that the device will not emit radiation unless the device is in contact with a user’s skin to minimise stray optical radiation from the device in operation which may be at unsafe levels for the eyes. This is typically achieved through the provision of multiple sensors adjacent each side of the output window (for example above, below and to either side of a rectangular output window) in the head of the device where a surface must be detected by each sensor as a requirement for radiation to be emitted. If one sensor does not measure a threshold value, then it is determined by the control system of the device that there is no skin contact and emission of a treatment energy pulse is prevented. This is to prevent the device firing when good contact with the skin is not achieved with the associated risk of the emission of potentially harmful levels of stray optical radiation. Stray optical radiation can be defined as light emitted by the device that is intended to be absorbed by the target skin area for treatment purposes, but either misses the intended target, is reflected or remitted from the target. . Various sensors may be utilised such as capacitive contact sensors (often referred to as ‘Skin Contact Sensors’) or proximity sensors from which skin tone can be determined (often referred to as ‘Skin Tone Sensors’). Devices typically comprise a housing having a handle portion shaped to be grasped by a user including a push button actuator for accepting a user input when a user requires the device to emit a light energy pulse. The housing has a forward end comprising an output window for emission of light energy pulses onto the skin. Positioned around the output window in the forward end of the housing are multiple sensors capable of detecting skin contact/proximity. Assuming there is a threshold measurement from each of the sensors and the user has depressed the actuator, then the control system in the device does not prevent emission of an energy pulse as it is deemed there is contact with the skin and emission of an energy pulse is safe.

Skin treatment devices can be operated in different ways which typically depends on the body areas undergoing treatment. For awkwardly shaped or smaller areas the device is placed upon the skin at the desired location, the actuator is depressed by the user and assuming there is good contact with the skin a pulse of light energy is emitted. The user can typically operate the device in two ways for small body areas. They can firstly depress and release the actuator to allow the pulse to be emitted, then remove the device from the skin, reposition the device and repeat. This can be referred to as ‘stamp mode’. Alternatively, the user may maintain the actuator depressed, remove the device from the skin and reposition to the next treatment location. This second mode of use is referred to as ‘stamp mode’ but with the actuator held.

For larger and typically less awkwardly shaped areas of the skin, the device can be operated in a similar manner to stamp mode with the actuator depressed where the actuator is maintained in a depressed configuration and the device kept in contact with the skin while the device is drawn across the skin. This increases usability as the device is drawn across the skin and light energy pulses are emitted at regular time intervals.

A problem exists in that it is generally undesirable for the same area of skin to receive multiple sequential energy pulses due to the risk of excessive heat build up leading to the possibility of burning. Such an issue can be termed ‘pulse stacking’. Accordingly, a problem exists as if the device is maintained in the same location on the user’s skin multiple pulses can be emitted, either by the user repeatedly pressing the actuator without moving the device between emission of energy pulses (which can occur in stamp mode) or by maintaining the actuator in an actuated configuration whilst maintaining the device in the same position on the skin (which can in stamp mode and glide mode) meaning multiple energy pulses are emitted to the same area of skin.

Aspects of the present invention aim to address the above-mentioned problems or at least provide a useful alternative.

According to the present invention there is a skin treatment device for delivery of light energy pulses to a subject’s skin, the skin treatment device comprising: a light source for discharging light energy pulses to a subject’s skin; at least one sensor for measuring a characteristic of the subject’s skin; a control system for controlling discharge of the light energy pulses dependent upon the measured characteristic; wherein the control system is configured to operate according to the steps of: permit emission of a first energy pulse; obtain multiple sensor measurements from the at least one sensor during a time period after emission of the first energy pulse; determine a difference between values of the sensor measurements; prevent emission of a second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold.

It has been determined that the outputs from one or more sensors typically present in skin treatment devices can be used to determine whether the device has moved between emitting energy pulses. Accordingly, it will be appreciated that meeting the predetermined difference is indicative that the device has moved to a different location on the skin meaning a second energy pulse can be emitted.

If the predetermined difference threshold is met, then this is indicative that the device has moved meaning a second energy pulse is no longer prevented. The characteristic of the subject’s skin is dependent upon the sensor utilised. For example, the characteristic may include a measured voltage if the sensor is a capacitive sensor or may include reflectance (if the sensor is what is commonly referred to as a proximity sensor or skin tone sensor).

The control system is preferably arranged such that the steps are repeated. The steps are beneficially cycled through in chronological order.

The control system is beneficially arranged to compare a difference between the measurements at least at two different times tl and t2 with the predetermined difference (that is beneficially stored in a memory of the control system) and prevents emission of the second energy pulse unless the predetermined difference is met. Meeting the predetermined difference means the predetermined difference is either reached or exceeded.

The control system determines a difference between values of sensor measurements. This difference is beneficially compared to the predetermined difference threshold. If the predetermined difference threshold is exceeded, then emission of the second energy pulse is no longer prevented. It will be understood that the measured difference may be a literal value between the two measurements or alternatively may be a value derived from the differences between multiple sensor measurements.

The control system is beneficially arranged to control operation of the at least one sensor to obtain sensor measurements. It is beneficial that the at least one sensor is controlled to repeatedly obtain sensor measurements at a repetition frequency. The frequency of obtaining sensor measurements may be in the order of eighty measurements per second. It will be understood that the difference in values between sensor measurements may be taken between any of the sensor measurements in a sequence of sensor measurements.

The control system is preferably configured to receive sensor measurements from the at least one sensor before emission of the first energy pulse and prevent emission of the first energy pulse unless a valid sensor measurement is received, where a valid sensor measurement is a sensor measurement value that falls with a predetermined first range.

The control system is preferably configured to terminate the cycle after emission of the first energy pulse in the event of receipt of a sensor measurement having a value that falls outside a predetermined second range of values.

This acts to provide a safety feature whereby emission of energy pulses (whether the first or second or any subsequent energy pulse) is only permitted if a valid sensor measurement is recorded by the control system. A valid sensor measurement may be indicative that the device is in contact with the skin (i.e. if a proximity sensor a predetermined reflectance is measured or if a capacitive sensor a predetermined voltage/current is measured).

The first and second ranges can be the same. The actual values of the first and second ranges depend on the sensor(s) utilised. For example, a valid proximity sensor measurement is derived from the reflectance - if the reflectance falls outside a predetermined range then emission of any energy pulse is prevented as it is possible that the device is not in close proximity to the skin.

The sensor measurements from the at least one sensor may be obtained at times tl and t2 during the time period after emission of the first energy pulse.

The sensor measurements from the at least one sensor are preferably consecutive sensor measurements.

The, or at least one of the sensors preferably comprises a reflectance measurement sensor. The reflectance measurement is an optical reflectance measurement. A benefit of measuring reflectance is that the difference between measured values of a stationary device compared to a moving device can be readily identified because the differences between measured values when the device is substantially stationary compared to measured values when the device is or has been moved vary by relatively large and measurable differences. This means that the possibility of the control system incorrectly determining that movement has not occurred thereby preventing emission of the second (or any subsequent) energy dose is reduced. This improves the usability of the device. It will be appreciated that the control system may use the reflectance from the skin to calculate a skin tone value for the skin and use this value as a basis for determining whether emission of the second or subsequent energy dose can be emitted.

The sensor can alternatively be a capacitive contact sensor however it has been determined that the difference between measured values from such a sensor when the device is moving compared to when the device is stationary is less than when utilising a proximity sensor meaning there is an increased possibility for a movement event to be characterised as a stationary event. As such, emission of an energy pulse may be prevented when it should be allowed.

A problem may occur in operation where emission of the second energy pulse is not prevented because a user ‘wobbles’ the device excessively meaning that the predetermined difference threshold is reached even though the device has not moved across the skin. In such a circumstance the control system may unlock the device to allow emission of the second energy dose even though the location of the device has not changed. It is therefore beneficial to include a further safety feature.

The control system is preferably further configured to determine first and second differences between the values of a first pair and a second pair of sensor measurements during the time period and prevent emission of the second energy pulse unless each of the first and second differences meet the predetermined difference threshold.

Again, meeting the predetermined difference threshold means reaching or exceeding the predetermined difference. The plurality of differences may be two differences or may be more. For example, the control system may be configured such that five separate differences between values of sensor measurements must exceed the predetermined difference threshold before prevention of emission of the second energy pulse is lifted. Furthermore, the differences may be derived from the values of consecutive sensor measurements. The control system may be configured to determine first and second differences between the values of a first pair and a second pair of sensor measurements during the time period and is further configured to determine an average difference from the first and second differences and prevent emission of the second energy pulse unless the average difference meets the predetermined difference threshold.

The control system may be arranged to prevent emission of the second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold and a predetermined time period has expired after meeting the predetermined difference threshold. This safety feature may be implemented to ensure that the device has sufficient time to travel to a subsequent treatment location before emission of the second energy pulse.

The at least one sensor preferably comprises a first sensor and a second sensor, wherein the control system is configured to obtain sensor measurements from both first and second sensors, determine a difference between the values of the first sensor measurements and between the values of the second sensor measurements, and prevent emission of the second energy pulse unless the difference between the values of the first sensor measurement and the difference between the values of the second sensor measurement meets the predetermined difference threshold. This configuration provides a further safety feature in that the chance of both first and second sensors measuring values that mean the difference meet the threshold from an event which is not actual movement of the device is reduced.

The device preferably further comprises a capacitor for discharging energy to the light source. The device preferably further comprises a user operable input for a user to input a signal indicative that one or more light energy pulses should be delivered to the skin. Delivery of the first energy pulse is permitted if one or more (and preferably all) requirements are met. Those requirements may be:

- the at least one sensor measures a value that meets (or exceeds) a predetermined threshold value; and/or

- the capacitor (if present) is charged to a sufficient voltage; and the user depresses the user operable actuator.

The device preferably comprises a first and second sensor, where the first sensor preferably comprises a proximity sensor and the second sensor preferably comprises a capacitive contact sensor. The control system is preferably configured to prevent emission of the first and second energy dose unless the measured value or a value derived from the measured value meets a predetermined threshold value. Even more preferably, the control system is configured to prevent emission of the first and second energy dose unless the measured value or value derived from the measured value falls within a predetermined range.

Also according to the present invention there is a method of operating a skin treatment device, the skin treatment device for delivery of light energy pulses to a subject’s skin, the skin treatment device comprising: a light source for discharging light energy pulses to a subject’s skin; at least one sensor for measuring a characteristic of the subject’s skin; a control system for controlling discharge of the light energy pulses dependent upon the measured characteristic; wherein the control system is operational according to the steps of: permitting emission of a first energy pulse; obtaining multiple sensor measurements from the at least one sensor during a time period after emission of the first energy pulse; determining a difference between values of the sensor measurements; preventing emission of a second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold.

Aspects of the present invention thus prevent multiple pulses being emitted without the device having been moved by an amount that causes a threshold difference in the value of the sensor measurement at two different times.

Aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure la-c are schematic representations of a device according to an illustrative embodiment of the present invention.

Figure 2 is a schematic illustration of a proximity sensor present in an illustrative embodiment of the present invention.

Figure 3 is a schematic illustration of a capacitive sensor present in an illustrative embodiment of the present invention.

Referring to Figure la-c presented is a skin treatment apparatus that may be used for treating skin disorders and conditions, and even more beneficially is suitable for cosmetic purposes such as hair depilation. The device comprises a housing (50) and a light source (22) accommodated by the housing such as a discharge lamp or flashlamp. The flashlamp is arranged to generate high intensity pulses of optical radiation. The housing (50) comprises a handle (52) meaning that the housing (50) can be manipulated to be positioned appropriately on a user and is particularly suitable for the home market. The device is beneficially handheld. As such, it is relatively small and portable, and can thus be used by an individual on themselves. The housing (50) includes a skin contact element (54) arranged to be positioned adjacent or preferably on a user’s skin. The skin contact element (54) includes a light transmission window (56) to enable the passage of high intensity pulses of optical radiation therethrough typically measuring 30mm in width and 10mm in height, where there is a light guide (55) defined between the light output aperture/transmission window (56) and the light emitting element (22). The cross-sectional area of the light output aperture/transmission/output window (56) is effectively the treatment area.

The skin contact element (54) further includes first, second, third and fourth sensors (58a, 58b, 58c, 58d) which will be described further below for providing associated sensing zones. A user operable actuator (62), for example in the form of a push button, is provided for the user to provide a request to the controller (28) to cause release of energy from the charge storage device such as a capacitor (20) to cause a pulse of optical radiation to the emitted from the flashlamp (22). This will occur assuming that certain safety requirements are met.

Referring to Figure lb, a transverse cross-section of the housing (50) is presented again showing the handle (52), light output aperture (56) and sensors (58c, 58d). Further shown is a fan (66) for cooling of the control circuit (28) on the main printed circuit board. Figure lb shows the lamp (22) secured in the housing (50). A filter (68) is provided to filter out ultra-violet light from transferring from the lamp (22) to the skin. A treatment light pulse generated by the lamp (22) passes through the filter, through the light output window (56) and onto the skin of a user.

Referring in particular to Figure 1c, a cross-sectional view is taken on an axis substantially perpendicular to the view of Figure lb. Represented in Figure 1c is the charger circuit (26), controller (28), lamp (22), filter (68) and light output window (56). Further shown is a reflector (70) for reflecting the pulse of optical radiation and accommodated within the handle portion (52) of the housing (50) is the energy storage device comprising a capacitor (20). The handle defines an opening (72) for mains power input.

The apparatus functions by the user providing an input to actuator (62) following which a determination is made as to whether the device can be safely operated, and if so the capacitor (20) discharges over the flashlamp (22). The safety features depend on the sensors utilised, however assuming the controller identifies a threshold response from the sensors (58) and the capacitor is charged then the capacitor can discharge.

The sensors may take different forms dependent upon the device in which it is utilised. For example, as shown in Figure 2 the sensors may simply comprise multiple proximity sensors in the form of capitative proximity/contact sensors each having a sensing zone where the control system requires a predetermined capacitance to be measured (90) from each sensing zone which is indicative of contact with a user’s skin. The sensing zone is provided by a metallic plate (92) retained behind the housing (50). The housing (50) contacts the skin (94) and capacitance can be measured. Assuming a threshold value is measured, then the control system enables firing of the flashlamp to emit a light energy pulse. However, one or more alternative or additional skin parameters may be sensed. For example, as shown in Figure 3 one or more sensors may comprise an optical sensor (58) often referred to as a skin tone sensor or sometimes again a proximity sensor and can be used in the alternative to or in tandem with one or more other sensor types such as capacitive sensors. Figure 3 is a schematic representation of a proximity sensor (58) comprising an LED (80), a photodiode (82) and transparent window (84).

The LED (80) transmits light through the sensor (84) onto the skin (94) to be treated and the light travels into the epidermis (96). The photodiode (82) is arranged to receive radiation reflected by the skin. Intensity of the received radiation is found to be representative of the tone of the skin, for example a light skin tone will reflect more than a dark skin tone. The intensity of the received radiation can be processed by the controller (28) using a processor provided thereby and compares the intensity with a calibrated set of intensity measurements to determine a sensed skin tone, which is then stored in a memory of the control circuit. The treatment light pulse energy then outputted to the skin can be controlled and is thus dependent on the sensed skin tone thus ensuring optimised treatment for the specific skin tone to be treated. The skin tone sensor can also be used as a safety feature meaning emission of an energy pulse is prevented if a valid measurement is not recorded. An invalid measurement may be one that equates to a derived skin tone that falls outside a predetermined range.

In the embodiment presented, there are two capacitive proximity sensors and two optical proximity sensors (or ‘skin tone sensors’ (58a)).

It will be appreciated that it is possible to utilise a single sensor having multiple sensing zones adjacent the output window (56). A single sensor may for example extend around the entirety of the output window (56), with sensing zones above, below and to either side of the output window (56). It is preferable however to provide multiple sensors adjacent the output window (56).

As indicated above, in an illustrative embodiment multiple individual sensors are disposed around the output window (56). There are typically four, the first (58a) positioned above, second (58b) positioned below and third (58c) and fourth (58d) on opposing sides of the output window (56) providing four individual sensing zones. It will be appreciated that in alternative embodiments there are different numbers of sensors. For example, a single sensor may comprise multiple sensing zones. In the illustrative embodiment there are two skin tone sensors and two capacitive touch sensors, where similar sensors are opposing one another.

Referring now to Figure 4 there is a graphical representation of values measured by two skin tone sensors and two capacitive sensors over time whilst the sensors are off the skin, static on the skin and moving along the skin. The left-hand axis presents measurement values for the skin tone sensor and the right-hand axis presents measurements for the capacitive sensors. Plots 1 and 2 are outputs from skin tone sensors and plots 3 and 4 for the capacitive sensors. Referring to plots 1 and 2, reflectance is low in zones (a) where there is no skin contact and as such the measured value is very low. This is indicative that there is poor or no contact with the skin, and thus the control system prevents emission of an energy pulse from the device. In zone (b) the sub-zones show the device being static and subsequently moving on the skin, and when static there is minimal variation (noise) in the measured reflectance during this time period. It will be appreciated that measurements are taken at high frequency (as an example the frequency may be 80 times per second) and the flat profile during the stationary period shows that the reflectance does not significantly change. When the device is moving however there can be seen to be significant variation in the values measured, and these variations can be used to determine that movement of the device across the skin has occurred.

The manner in which the variations can be used is for the control system to be programmed in a manner such that after emission of a first energy pulse, then a second energy pulse cannot be emitted unless a predetermined difference between at least a first and second sensor measurement is achieved. This difference will be indicative that the device has moved so treatment will therefore occur at the same skin location. As such, the control system is programmed to identify when a predetermined difference between two or more sensor measurements is met and then no longer prevent emission of another energy dose. The sensor measurements that may be analysed can depend on the specific system set up. An example of a suitable algorithm using a skin tone sensor is as follows:

1. Define the state "Movement Locked" = the device cannot emit an energy pulse until it has been "Movement Un-locked".

2. When a Valid Skin Tone is detected and the device has already emitted an energy pulse, Movement Lock the device. Note - the device should not be movement locked before the first flash has occurred.

3. When the device has been Movement Locked, start checking the step size between STS measurements, (i.e. define a variable STS STEP = for example the difference between the last 2 sequential STS measurements)

4. If STS STEP is greater than a predefined Movement Un-lock Threshold, then unlock the device and allow the next flash.

5. Immediately after the flash, repeat from step 2.

6. Stop the movement detection algorithm if the device is removed from skin indicated by the significant change in sensor value, and then re-start from (2) if skin valid skin tone is re-established.

The Movement Un-lock Threshold (referred to as the predetermined difference threshold) is dependent upon the sensitivity required and is determined based on the device itself. If this threshold is too big, then the differences identified during movement may fail to unlock the device causing user annoyance. If too small, then the device may be unlocked too easily, and energy pulse emission in the same location could be possible.

Additional control and functionality may be incorporated into the control system. For example, if there is significant noise in the measured values or the user accidentally wobbles the device significantly during use then the control system may detect the predetermined difference threshold and no longer prevent emission of an energy dose for that cycle. This could result in emission of multiple energy doses to the same skin location. As a secondary check, the control system may require the measured difference to exceed the predetermined difference threshold multiple times before unlocking. For example, three differences determined between measured sensor values that meet the predetermined difference threshold may be required. These differences may be required between consecutive measured values. These multiple differences reaching the threshold value may be utilised to approximate distance moved, meaning a predetermined number of differences that meet the threshold difference may infer a distance moved.

A further safety feature may be to prevent emission of the second energy pulse until a predetermined time has elapsed after the predetermined threshold difference has been identified to ensure that sufficient movement of the device has occurred before a further energy pulse is emitted.

A further safety feature may be for the control system to obtain sensor measurements from multiple sensors, determine a difference between the values of the first sensor measurements and between the values of the second sensor measurements, and prevent emission of the second energy pulse unless the difference between the values of the multiple sensor measurements meet the predetermined difference threshold. Accordingly, the two or more sensors much each provide measure values indicative that movement has occurred before the second pulse can be emitted.

Aspects of the present invention have been described by way of example only and it will be appreciated to the skilled addressee that modifications and variations may be made without departing from the scope of protection afforded by the appended claims.

Claims

1. A skin treatment device for delivery of light energy pulses to a subject’s skin, the skin treatment device comprising: a light source for discharging light energy pulses to a subject’s skin; at least one sensor for measuring a characteristic of the subject’s skin; a control system for controlling discharge of the light energy pulses dependent upon the measured characteristic; wherein the control system is configured to operate according to the steps of: permit emission of a first energy pulse; obtain multiple sensor measurements from the at least one sensor during a time period after emission of the first energy pulse; determine a difference between values of the sensor measurements; prevent emission of a second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold.

2. A skin treatment device according to claim 1 wherein the control system is configured to receive sensor measurements from the at least one sensor before emission of the first energy pulse and prevent emission of the first energy pulse unless a valid sensor measurement is received, where a valid sensor measurement is a sensor measurement value that falls with a predetermined first range.

3. A skin treatment device according to claim 1 wherein the control system is configured to terminate the cycle after emission of the first energy pulse in the event of receipt of a sensor measurement having a value that falls outside a predetermined second range of values.

4. A skin treatment device according to any preceding claim wherein the time period is between times tl and t2 after emission of the first energy pulse.

5. A skin treatment device according to any preceding claim wherein the sensor measurements from the at least one sensor are consecutive sensor measurements.

6. A skin treatment device according to any preceding claim wherein the, or at least one of the sensors comprises a reflectance measurement sensor.

7. A skin treatment device according to any preceding claim where the control system is further configured to determine first and second differences between the values of a first pair and a second pair of sensor measurements during the time period and prevent emission of the second energy pulse unless each of the first and second differences meet the predetermined difference threshold.

8. A skin treatment apparatus according to any preceding claim wherein the control system is configured to determine first and second differences between the values of a first pair and a second pair of sensor measurements during the time period and is further configured to determine an average difference from the first and second differences and prevent emission of the second energy pulse unless the average difference meets the predetermined difference threshold.

9. A skin treatment device according to any preceding claim wherein the control system is arranged to prevent emission of the second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold and a predetermined time period has expired after meeting the predetermined difference threshold.

10. A skin treatment apparatus according to any preceding claim wherein the at least one sensor comprises a first sensor and a second sensor, wherein the control system is configured to obtain sensor measurements from both first and second sensors, determine a difference between the values of the first sensor measurements and between the values of the second sensor measurements, and prevent emission of the second energy pulse unless the difference between the values of the first sensor measurement and the difference between the values of the second sensor measurement meets the predetermined difference threshold.

11. A method of operating a skin treatment device, the skin treatment device for delivery of light energy pulses to a subject’s skin, the skin treatment device comprising: a light source for discharging light energy pulses to a subject’s skin; - at least one sensor for measuring a characteristic of the subject’s skin; a control system for controlling discharge of the light energy pulses dependent upon the measured characteristic; wherein the control system is operational according to the steps of: permitting emission of a first energy pulse; - obtaining multiple sensor measurements from the at least one sensor during a time period after emission of the first energy pulse; determining a difference between values of the sensor measurements; preventing emission of a second energy pulse unless the difference between values of the sensor measurements meets a predetermined difference threshold.